@article{shaw_mahajan_hassan_2023, title={Critical Evaluation of a Novel Analysis Technique for Assessment of Printed Circuit Heat Exchangers in High-Temperature Nuclear Service}, volume={145}, ISSN={["1528-8978"]}, DOI={10.1115/1.4057061}, abstractNote={Abstract}, number={3}, journal={JOURNAL OF PRESSURE VESSEL TECHNOLOGY-TRANSACTIONS OF THE ASME}, author={Shaw, Avinash and Mahajan, Heramb P. and Hassan, Tasnim}, year={2023}, month={Jun} }
 @article{shaw_mahajan_hassan_2022, title={A Practical Analysis Framework for Assessment of Printed Circuit Heat Exchangers in High-Temperature Nuclear Service}, volume={144}, ISSN={["1528-8978"]}, DOI={10.1115/1.4052697}, abstractNote={Abstract}, number={4}, journal={JOURNAL OF PRESSURE VESSEL TECHNOLOGY-TRANSACTIONS OF THE ASME}, author={Shaw, Avinash and Mahajan, Heramb P. and Hassan, Tasnim}, year={2022}, month={Aug} }
 @article{islam_dewees_hassan_2022, title={Development of a unified constitutive model coupled with a continuum damage model for design and evaluation of high-temperature components}, volume={257}, ISSN={["1879-2146"]}, DOI={10.1016/j.ijsolstr.2022.111935}, abstractNote={A unified constitutive model (UCM) coupled with a continuum damage model (CDM) is developed to design and evaluate high-temperature components in the energy, aerospace, and petrochemical industries. While different constitutive models can predict certain aspects of thermomechanical creep-fatigue responses, a generally applicable model for both short and long-term responses, including stress relaxation and tertiary creep/damage, and strain softening has not been available. Hence, this study unifies strain-focused viscoplastic and creep-rupture-focused damage models to predict fatigue, creep, and creep-fatigue interactions using a single set of model parameters. Two CDMs, Kachanov and isotropic damage, are evaluated by coupling these with a modified Chaboche UCM. The strengths and limitations of the original Kachanov and isotropic damage models in predicting a broad set of low-cycle fatigue and creep responses for a commercially important material, modified Grade 91 steel, are determined. Based on the evaluations, a modified isotropic damage model is proposed. The proposed UCM-CDM is experimentally validated against a large set of modified Grade 91 steel responses, including cyclic softening, rate-dependence, short-term stress relaxation, long-term creep, thermomechanical fatigue, and creep-fatigue interaction at temperatures 400 to 625 °C. The UCM is further validated by simulating a set of modified Grade 91 steel notch specimen creep responses. The modified UCM is demonstrated to simulate the influence of stress triaxiality and prior fatigue on creep rupture life. Finally, the proposed UCM is evaluated by analyzing a thick cylinder under thermal transient loading to demonstrate the modified UCM’s applicability for the design and evaluation of high-temperature components.}, journal={INTERNATIONAL JOURNAL OF SOLIDS AND STRUCTURES}, author={Islam, Nazrul and Dewees, David J. and Hassan, Tasnim}, year={2022}, month={Dec} }
 @article{hassan_quayyum_2022, title={Influence of Weld Sequence on the Low-Cycle Fatigue Failure of WUF-B Connections}, volume={148}, ISSN={["1943-541X"]}, DOI={10.1061/(ASCE)ST.1943-541X.0003350}, abstractNote={Experiments on post-Northridge welded unreinforced flange bolted web (WUF-B) connections demonstrated a new low-cycle fatigue (LCF) crack initiation mechanism with final rupture occurring either near the weld access hole or the weld regions. Post-Northridge connection research reports and related commentary indicated that the weld and heat-affected zone (HAZ) conditions might be contributing factors in LCF-initiated failures of the modified WUF-B connections. The experimental study reported herein investigated the influence of weld sequence on the fatigue failure of the WUF-B connections. Two exterior WUF-B connections were fabricated using different weld sequences in laying the complete joint penetration welding between the beam and column flanges. These connections were tested under a constant-amplitude displacement-controlled loading protocol until crack initiation. Both specimens failed by cracking at the weld access hole, one in a brittle manner and the other in a ductile manner. Analysis of the recorded data demonstrated the influence of weld sequence on strain responses at the weld toe and weld access hole regions. Accumulation of strain with cycles, which is a phenomenon known as strain ratcheting, was observed near these locations in both tests. Recorded strain responses near the crack locations indicated the cause of earlier failure of one specimen compared with the other. Finally, future research needs in mitigating the influence of welding sequence on fatigue failure of welded steel moment connections are discussed.}, number={7}, journal={JOURNAL OF STRUCTURAL ENGINEERING}, author={Hassan, Tasnim and Quayyum, Shahriar}, year={2022}, month={Jul} }
 @article{mahajan_lima_hassan_2022, title={Mechanical and Microstructural Performance Evaluation of Diffusion Bonded Alloy 800H for Very High Temperature Nuclear Service}, volume={144}, ISSN={["1528-8889"]}, DOI={10.1115/1.4052825}, abstractNote={Abstract}, number={2}, journal={JOURNAL OF ENGINEERING MATERIALS AND TECHNOLOGY-TRANSACTIONS OF THE ASME}, author={Mahajan, Heramb P. and Lima, Lucas M. A. and Hassan, Tasnim}, year={2022}, month={Apr} }
 @article{mahajan_mckillop_keating_hassan_2022, title={Proposed Material Properties, Allowable Stresses, and Design Curves of Diffusion Bonded Alloy 800H for the ASME Code Section III Division 5}, volume={144}, ISSN={["1528-8978"]}, DOI={10.1115/1.4054073}, abstractNote={Abstract}, number={6}, journal={JOURNAL OF PRESSURE VESSEL TECHNOLOGY-TRANSACTIONS OF THE ASME}, author={Mahajan, Heramb P. and McKillop, Suzanne and Keating, Robert and Hassan, Tasnim}, year={2022}, month={Dec} }
 @article{barrett_takagi_islam_kuwabara_hassan_kinsey_knezevic_korkolis_2021, title={Material modeling and simulation of continuous-bending-under-tension of AA6022-T4}, volume={287}, ISSN={["1873-4774"]}, DOI={10.1016/j.jmatprotec.2020.116658}, abstractNote={In earlier contributions, we discussed continuous-bending-under-tension (CBT) experiments on AA6022-T4. We found that CBT significantly enhanced the elongation-to-fracture and strength, over uniaxial tension. In the present paper, our understanding of CBT is expanded beyond these experimental observations, with the aid of material modeling and numerical simulations of the process. Cyclic tension-compression experiments were performed on this material, using strain histories that are expected to replicate the loading during CBT, i.e., different combinations of constant strain amplitude and linearly increasing mean value, to failure. During these experiments, a limited but not negligible amount of kinematic hardening was discovered. Some of these experiments are used for calibration of a combined isotropic-kinematic hardening model, while the rest are used for experimental validation of the model. The modeling framework is based on a rate-independent, associated flow rule with the von Mises yield criterion as the plastic potential. Isotropic hardening is introduced by a simple, exponential-decay model of the growth of the yield surface with plastic deformation. Non-linear kinematic hardening is introduced by a 4-term, Chaboche-type model. The large strain hardening curve is identified by extrapolation, an approach that is validated later in the work and contrasted with alternative options. This material modeling framework is introduced in finite element models of the CBT process. The model is meshed with linear, reduced-integration elements, with 7 elements through the thickness. It is found that the numerical model reproduces the experimental force-displacement curve, including the succession of spikes and plateaus typical of CBT, very closely. The model also replicates the development of strain on the surface during CBT, and compares well with post-test strain measurements. After these validations, the model is used to probe the mechanics of the CBT process, e.g., the development of stress and strain through the thickness and per cycle, the location and onset of failure, as well as the failure angle, which in CBT differs from the localized neck angle found in a typical uniaxial tension experiment.}, journal={JOURNAL OF MATERIALS PROCESSING TECHNOLOGY}, author={Barrett, Timothy J. and Takagi, Shuhei and Islam, Nazrul and Kuwabara, Toshihiko and Hassan, Tasnim and Kinsey, Brad L. and Knezevic, Marko and Korkolis, Yannis P.}, year={2021}, month={Jan} }
 @article{rahman_ngaile_hassan_2021, title={Non-contact temperature control and stereo digital image correlation for high-temperature testing of miniature tubular specimens}, volume={92}, ISSN={["1089-7623"]}, url={https://doi.org/10.1063/5.0055718}, DOI={10.1063/5.0055718}, abstractNote={Component failures very often occur due to high temperature and multiaxial stress states arising at critical component locations. To imitate such loading conditions, a multiaxial miniature testing system (MMTS) with axial, torsional, and internal pressurization capabilities for high-temperature testing of miniature tubular specimens has been developed. Among many challenges of developing the MMTS, uniform heating, temperature measurement and control, and surface strain measurement on a miniature tubular specimen at high temperatures have significant difficulties. This paper addresses two significant challenges: first, the development of a non-contact temperature control system using infrared thermography to uniformly heat a miniature specimen of 1 mm outer diameter (OD), and second, the development of a stereo digital image correlation (stereo-DIC) setup for strain measurement on the miniature specimen subjected to high temperature. The developed control system maintains the test temperature through a closed feedback loop and employs a fail-safe mechanism to protect the MMTS load frame components against unanticipated temperature rises. The thermocouple wire-size effect on the measured temperature was examined for three different wire sizes: 0.05, 0.25, and 0.5 mm for accurate emissivity determination required for infrared thermography. Emissivities of the specimen surface at different high temperatures were experimentally determined. Inherent error analysis of the developed high-temperature stereo-DIC setup showed acceptable strain measurement uncertainty. The effectiveness of the developed non-contact temperature control system and high-temperature stereo-DIC setup has been verified by performing tensile testing of a 1 mm OD specimen at 500 °C.}, number={11}, journal={REVIEW OF SCIENTIFIC INSTRUMENTS}, author={Rahman, Farhan and Ngaile, Gracious and Hassan, Tasnim}, year={2021}, month={Nov} }
 @article{rahman_ngaile_hassan_2021, title={Optimized stereo digital image correlation setup for miniature round specimen: framework development and implementation}, volume={144}, ISSN={["1873-0302"]}, DOI={10.1016/j.optlaseng.2021.106555}, abstractNote={For the advancement of micro- and nano-technologies, multiaxial material testing at a small length scale is imperative. A novel multiaxial miniature testing system (MMTS) is under development for testing a tubular specimen of outer diameter (OD) as small as 1 mm. Because of the small specimen size of MMTS, stereo-Digital Image Correlation (DIC) is the preferred strain measurement technique. Although theoretically, stereo-DIC is length-scale independent, the implementation of stereo-DIC, particularly for miniature testing, faces experimental setup related challenges. For this reason, although stereo-DIC is strongly recommended over 2D DIC, researchers are sometimes compelled to use the latter. It is shown in the present study that the experimental setup related difficulties, particularly for miniature round specimen testing, can be overcome by a systematic development of a mathematical framework for stereo-DIC implementation. This framework addresses all setup decisions concerning stereo-DIC implementation, such as selections of stereo angle, speckle size, camera position, camera sensor size, lens focal length, dimensions of camera and lens bodies, calibration grid size, etc., as well as stereo-DIC analysis parameters, such as subset and step size. Besides serving the need of building a stereo-DIC setup for MMTS, since the developed mathematical framework treats all stereo-DIC setup decisions as variables, it can be used to develop an optimized stereo-DIC setup for any application. Examples of two general cases are reported. Since this general framework serves as a tool to solve the stereo-DIC experimental setup related challenges, the developed framework will contribute to the wider adoption of stereo-DIC over 2D DIC.}, journal={OPTICS AND LASERS IN ENGINEERING}, author={Rahman, Farhan and Ngaile, Gracious and Hassan, Tasnim}, year={2021}, month={Sep} }
 @article{das_khutia_dey_arora_gupta_2021, title={Ratcheting and stress relaxation of SA333 Gr. 6 carbon steel samples under uniaxial multistep strain-controlled condition}, volume={43}, ISSN={["1806-3691"]}, DOI={10.1016/j.mechmat.2015.05.011}, abstractNote={An existing cyclic plasticity constitutive model is enhanced to simulate low-cycle fatigue and ratcheting responses of 304 stainless steel (SS) under proportional and various nonproportional loading cycles. Nonproportional loading and multiaxial ratcheting parameters, and strain range dependent cyclic hardening/softening modeling features are incorporated into a modified Ohno–Wang model to enhance its uniaxial and multiaxial loading responses. The improved constitutive model is incorporated in the commercial Finite Element Code ABAQUS through its user defined subroutine UMAT and the responses of 304 SS tubular specimen from literature have been simulated. The proposed model has demonstrated good correlation with uniaxial and different types of multiaxial fatigue and ratcheting responses. Two types of multiaxial loading cycles are studied; the first included axial and torsion cycles along different loading paths, and the second included steady internal pressure and axial strain or stress cycles. The axial–torsional loading cycles demonstrated axial and/or shear strain ratcheting, whereas the internal pressure-axial cycles demonstrated axial and/or circumferential strain ratcheting. Complex interactions between ratcheting strains in different directions along with the rate of ratcheting are simulated well by the improved Ohno–Wang model.}, number={6}, journal={JOURNAL OF THE BRAZILIAN SOCIETY OF MECHANICAL SCIENCES AND ENGINEERING}, author={Das, P. and Khutia, N. and Dey, P. P. and Arora, Punit and Gupta, Suneel K.}, year={2021}, month={Jun} }
 @article{li_chan_ngaile_hassan_2020, title={A novel gripper for multiaxial mechanical testing of microtubes at elevated temperatures}, volume={91}, ISSN={["1089-7623"]}, DOI={10.1063/5.0007150}, abstractNote={The success of a microtube hydroforming (μTHF) process heavily depends on the material properties of microtubes, which can reveal the material response under multiaxial stress and influence the formability of hydroformed products. However, these material properties are not well understood because of the limited availability of material testing apparatus that would permit control of axial force and internal pressure simultaneously to mimic realistic μTHF loading. The main purpose of this study is to develop a set of grippers that can transfer required testing loads under fully coupled combinations of axial force and internal pressure. The grippers are designed so that they may be kept at the safe working temperature even when tests are carried out at higher temperatures. The grippers are also designed to fit in a load frame that is integrated in a scanning electron microscope for in situ material testing. The capabilities of the grippers are demonstrated by performing uniaxial and multiaxial material tests on SS304 microtubes with 1 mm outside diameter and 0.15 mm nominal tube wall thickness. The finite element simulations and experimental results show that the designed grippers can firmly hold the specimen and thus enable tensile, compression, torsion, and microtube bulge material tests to be accurately performed.}, number={5}, journal={REVIEW OF SCIENTIFIC INSTRUMENTS}, author={Li, Lin and Chan, Yu-Chin and Ngaile, Gracious and Hassan, Tasnim}, year={2020}, month={May} }
 @article{barrett_hassan_2020, title={A unified constitutive model in simulating creep strains in addition to fatigue responses of Haynes 230}, volume={185}, ISSN={["1879-2146"]}, DOI={10.1016/j.ijsolstr.2019.09.001}, abstractNote={• Evaluation of modeling features in simulating creep and fatigue responses. • Need of coupling continuum damage model to unified constitutive model (UCM). • Issues in simulating creep strains at low temperatures by the modified UCM. • Issues in simulating fatigue responses at high temperatures by the modified UCM. • Challenges of UCM in simulating elevated temperature creep and fatigue responses. A unified constitutive model (UCM) specifies that its flow rule for inelasticity computes both the plastic and creep strains as a single state variable. A Chaboche framework based UCM with the modeling features of strain range-dependence, strain rate-dependence, static recovery and mean stress evolution was developed and experimentally validated against a broad set of fatigue and fatigue-creep responses of Haynes 230 (HA 230) under isothermal and anisothermal temperature conditions. This article demonstrates that this advanced Chaboche-based UCM can simulate the secondary minimum creep strain rates reasonably, but is unable to predict the tertiary creep strain responses. To simulate the tertiary creep strain responses a continuum damage model is needed to be coupled to the UCM. This study also evaluated three different unified flow rules, Norton's power law, exponential Norton and sine-hyperbolic Norton for calculating the inelastic strain rates. It is found that the choice of flow rule is important in simulating the stress amplitude saturation rate of fatigue responses, but has minimal effect in simulating the tertiary creep strains. However, the damage coupled UCM independent to the unified flow rules listed above can adequately simulate fatigue, fatigue-creep including the stress relaxation during strain dwell, and creep strain up to the tertiary range for HA 230. The drawbacks of the damage coupled UCM are the hysteresis loop softening at very high temperatures and asymptotic simulation at low creep temperatures, which are identified as challenges to be overcome towards developing a universal UCM for robust design and analysis of high temperature components.}, journal={INTERNATIONAL JOURNAL OF SOLIDS AND STRUCTURES}, author={Barrett, Paul R. and Hassan, Tasnim}, year={2020}, month={Mar}, pages={394–409} }
 @article{zhang_wang_zhang_hassan_gong_2020, title={Fatigue-Creep Interaction of P92 Steel and Modified Constitutive Modelling for Simulation of the Responses}, volume={10}, ISSN={["2075-4701"]}, DOI={10.3390/met10030307}, abstractNote={Fatigue–creep interaction (FCI) responses of P92 steel are investigated experimentally and numerically. A series of isothermal FCI experiments with tensile dwell time ranging from 60 to 600 s were conducted at two temperatures under strain-controlled trapezoidal waveform. The experimental responses demonstrate that the peak stress is influenced by temperature and dwell time. In other words, creep-mechanism-influenced stress relaxation during dwell time influences the peak stress and fatigue life (Nf). In addition, effects of strain range on peak stress and fatigue life under fatigue–creep loading are evaluated. Towards developing a simulation-based design methodology for high temperature components, first a conventional unified constitutive model is evaluated against the P92 steel experimental responses. Based on the simulation deficiency of the conventional model, a modified static recovery term incorporated in the kinematic hardening rule is proposed and satisfactory simulations of the P92 steel FCI responses are demonstrated. The experimental responses of P92 steel and strengths and deficiencies of the conventional and modified Chaboche models are elaborated identifying the important FCI phenomena and progress in constitutive model development for FCI response simulation.}, number={3}, journal={METALS}, author={Zhang, Tianyu and Wang, Xiaowei and Zhang, Wei and Hassan, Tasnim and Gong, Jianming}, year={2020}, month={Mar} }
 @article{zhang_wang_ji_zhang_hassan_gong_2020, title={P92 steel creep-fatigue interaction responses under hybrid stress-strain controlled loading and a life prediction model}, volume={140}, ISSN={["1879-3452"]}, DOI={10.1016/j.ijfatigue.2020.105837}, abstractNote={In contrast to conventional strain-controlled creep-fatigue interaction (CCFI) loadings, a novel hybrid stress- and strain-controlled creep-fatigue interaction (HCFI) loadings were developed on P92 steel. Dwell stresses ranging from 140 MPa to 170 MPa, and dwell periods of 300 s, 600 s and 1800 s were employed at 625 °C. The test responses demonstrate that cyclic softening and hardening effects lead to complicated cyclic responses. In addition, the failure life under HCFI loading is observed to be reduced with the dwell stress and dwell period. Finally, a viscosity-based model is proposed to predict the lifetime of both HCFI and CCFI tests.}, journal={INTERNATIONAL JOURNAL OF FATIGUE}, author={Zhang, Tianyu and Wang, Xiaowei and Ji, Yunnan and Zhang, Wei and Hassan, Tasnim and Gong, Jianming}, year={2020}, month={Nov} }
 @article{morrison_schweizer_quayyum_hassan_2019, title={An Unstiffened Eight-Bolt Extended End-Plate Moment Connection for Special and Intermediate Moment Frames}, volume={145}, ISBN={1943-541X}, DOI={10.1061/(ASCE)ST.1943-541X.0002332}, abstractNote={AbstractBolted extended end-plate (BEEP) moment-resisting connections are prequalified for use in special and intermediate moment frames. The current limits of this prequalification dictate that ro...}, number={7}, journal={JOURNAL OF STRUCTURAL ENGINEERING}, author={Morrison, Machel L. and Schweizer, Doug Q. and Quayyum, Shahriar and Hassan, Tasnim}, year={2019} }
 @article{pec_sebek_zapletal_petruska_hassan_2019, title={Automated calibration of advanced cyclic plasticity model parameters with sensitivity analysis for aluminium alloy 2024-T351}, volume={11}, ISSN={["1687-8140"]}, DOI={10.1177/1687814019829982}, abstractNote={ The plasticity models in finite element codes are often not able to describe the cyclic plasticity phenomena satisfactorily. Developing a user-defined material model is a demanding process, challenging especially for industry. Open-source Code_Aster is a rapidly expanding and evolving software, capable of overcoming the above-mentioned problem with material model implementation. In this article, Chaboche-type material model with kinematic hardening evolution rules and non-proportional as well as strain memory effects was studied through the calibration of the aluminium alloy 2024-T351. The sensitivity analysis was performed prior to the model calibration to find out whether all the material model parameters were important. The utilization of built-in routines allows the calibration of material constants without the necessity to write the optimization scripts, which is time consuming. Obtaining the parameters using the built-in routines is therefore easier and allows using the advanced modelling for practical use. Three sets of material model parameters were obtained using the built-in routines and results were compared to experiments. Quality of the calibration was highlighted and drawbacks were described. Usage of material model implemented in Code_Aster provided good simulations in a relatively simple way through the use of an advanced cyclic plasticity model via built-in auxiliary functions. }, number={3}, journal={ADVANCES IN MECHANICAL ENGINEERING}, author={Pec, Michal and Sebek, Frantisek and Zapletal, Josef and Petruska, Jindrich and Hassan, Tasnim}, year={2019}, month={Mar} }
 @article{islam_hassan_2019, title={Development of a novel constitutive model for improved structural integrity analysis of piping components}, volume={177}, ISSN={["1879-3541"]}, DOI={10.1016/j.ijpvp.2019.103989}, abstractNote={Elbows are critical components of piping systems in the nuclear power industry, however, existing constitutive models are unable to simulate the low-cycle fatigue and ratcheting responses of this component. This study developed a constitutive model, incorporating a novel and various advanced uniaxial and multiaxial modeling features for successful response simulations of stainless steel (SS) 304 short and long radius elbows subjected to internal pressure and opening-closing displacement-controlled cycles. Simulated results demonstrate that if an existing advanced constitutive model is calibrated solely based on the material level responses, it is not able to simulate the elbow responses with acceptable accuracy. This drawback is primarily attributed to the fact that the prior loading and loading histories at different locations in an elbow are different and not represented by the loading histories of the material experiments performed for model parameter determination. Hence, model development and simultaneous experimental verification at the material and component levels trace the drawbacks of the constitutive modeling features effectively. Such evaluation of the simulated responses at two levels provided a novel modeling concept in improving the elbow response simulations quite satisfactorily. The implemented modeling features and response simulations at both levels are presented and critically analyzed for providing insights in developing robust constitutive models for structural integrity analysis.}, journal={INTERNATIONAL JOURNAL OF PRESSURE VESSELS AND PIPING}, author={Islam, Nazrul and Hassan, Tasnim}, year={2019}, month={Nov} }
 @article{rahman_ngaile_hassan_2019, title={Development of scanning electron microscope-compatible multiaxial miniature testing system}, volume={30}, ISSN={["1361-6501"]}, url={https://doi.org/10.1088/1361-6501/ab1ca6}, DOI={10.1088/1361-6501/ab1ca6}, abstractNote={Knowledge of deformation and failure mechanisms at micro- to nano-length scales is important for the prediction of material behavior as well as the development of new materials with desired properties. In situ multiaxial testing with scanning electron microscopes (SEM) can reveal physical deformation mechanisms under realistic multiaxial loading conditions. Although in situ SEM testing has gained traction in recent years, there is currently no multiaxial in situ SEM testing stage available with axial-torsional loading capabilities which can generally be used in any SEM. In this study, we report the development of a multiaxial miniature testing system (MMTS) with a unique capability for performing axial-torsional testing of a tubular specimen with a 1–2 mm outer diameter, inside most SEMs. The different challenges of developing a multiaxial in situ SEM testing stage, such as small load frame size, appropriate specimen position, high vacuum compatibility of MMTS load frame components, as well as the development of installation accessories, were addressed. A custom SEM stage door was developed for the MMTS load frame. Verification tests have confirmed the successful development of the MMTS for in situ SEM testing. In addition, digital image correlation was used with recorded SEM images during the test to determine the surface strain.}, number={10}, journal={MEASUREMENT SCIENCE AND TECHNOLOGY}, publisher={IOP Publishing}, author={Rahman, Farhan and Ngaile, Gracious and Hassan, Tasnim}, year={2019}, month={Oct} }
 @article{morrison_gould_charit_hassan_2019, title={Performance Evaluation of Surface-Activated Solid-State Welding for ASTM A992 Structural Steel}, volume={31}, ISBN={1943-5533}, DOI={10.1061/(ASCE)MT.1943-5533.0002805}, abstractNote={AbstractThis paper presents the results of a pilot study to evaluate a solid-state welding technology, called surface activated solid-state (SASS) welding, for joining structural steel members. SAS...}, number={8}, journal={JOURNAL OF MATERIALS IN CIVIL ENGINEERING}, author={Morrison, Machel L. and Gould, Jerry and Charit, Indrajit and Hassan, Tasnim}, year={2019} }
 @article{quayyum_hassan_2018, title={Seismic Performance of a Fire-Exposed Moment-Resisting Frame}, volume={144}, ISSN={["1943-541X"]}, DOI={10.1061/(ASCE)ST.1943-541X.0002201}, abstractNote={AbstractMajor earthquakes in urban areas often lead to building fires. Such earthquakes frequently damage buildings’ water sprinkler systems and diminish or strain firefighting capabilities. In suc...}, number={11}, journal={JOURNAL OF STRUCTURAL ENGINEERING}, author={Quayyum, Shahriar and Hassan, Tasnim}, year={2018}, month={Nov} }
 @article{li_ngaile_hassan_2017, title={A Novel Hybrid Heating Method for Mechanical Testing of Miniature Specimens at Elevated Temperature}, volume={5}, ISSN={["2166-0476"]}, DOI={10.1115/1.4035954}, abstractNote={A novel hybrid heating method which combines the conventional electric-resistance specimen heating with microcoil heating of specimen ends to achieve uniform heating over the gauge length is presented. Resistive heating of a miniature specimen develops a parabolic temperature profile with lowest temperature at the grip ends because of the heat loss to the gripper. Coil heating at the specimen ends compensates for this heat loss resulting in uniform temperature distribution over the central segment of the specimen. Thermo-electric finite element simulations were carried out to analyze the transient and steady temperature distribution in miniature specimens followed by experimental validation.}, number={2}, journal={JOURNAL OF MICRO AND NANO-MANUFACTURING}, author={Li, Lin and Ngaile, Gracious and Hassan, Tasnim}, year={2017}, month={Jun} }
 @article{ahmed_hassan_2017, title={Constitutive modeling for thermo-mechanical low-cycle fatigue-creep stress-strain responses of Haynes 230}, volume={126}, ISSN={["1879-2146"]}, DOI={10.1016/j.ijsolstr.2017.07.031}, abstractNote={• Unified constitutive modeling for thermomechanical fatigue-creep (TMFC) responses. • Illustration of the need of advancing constitutive modeling for TMFC responses. • Experimental validation of the modified model against a broad set of TMFC responses. • Discussion and performance illustration of the modified modeling features. • Demonstration of significant progress in unified constitutive modeling. Haynes 230 (HA 230), a Nickel-based superalloy, is the primary material of combustor liners in airplane gas turbine engines. This component operates in the temperature range between ambient to as high as 1000 °C. Such thermal cycles together with the resulting strain cycles in a combustor liner may induce thermo-mechanical fatigue-creep (TMFC) damage and initiate cracks earlier than the estimated life. Use of a robust unified constitutive model (UCM) for nonlinear analysis based design may improve fatigue life estimation of high temperature components. Available UCMs in the literature or commercial software packages are unable to simulate the TMFC responses reasonably. Hence, this study developed a UCM incorporating the modeling features of rate and temperature dependence, static-recovery, various kinematic hardening evolutions, and strain-range dependence, and validated the model against a broad set of TMFC experimental responses of HA 230. This modified UCM is capable of simulating the mean-stress evolution under both the out-of-phase and in-phase TMFC loading cycles. The modified UCM can adequately simulate most of the characteristic cyclic phenomena of HA 230 including the influence of maximum temperature on the out-of-phase and in-phase TMFC hysteretic responses, stress amplitude, and stress relaxation during strain-dwell. The time-derivative of the elastic modulus is an essential modeling feature for accurately simulating the inelastic strains under TMFC loading. These simulations demonstrate the progresses made in UCM.}, journal={INTERNATIONAL JOURNAL OF SOLIDS AND STRUCTURES}, author={Ahmed, Raasheduddin and Hassan, Tasnim}, year={2017}, month={Nov}, pages={122–139} }
 @inproceedings{islam_dewees_cooch_hassan_2017, title={Creep-fatigue damage evaluation of modified grade 91 headers using damage coupled unified viscoplastic model}, DOI={10.1115/pvp2017-65851}, abstractNote={A case study for life prediction of Grade 91 heat recovery steam generator (HRSG) superheater outlet header of typical combined cycle power plants (CCPP) is presented in this paper. The effect of high cycling and fast startup along with elevated design temperature and pressure on the creep life is studied. A consistent material model based on MPC Omega is used to evaluate the creep damage of HRSG header components. In addition, a robust unified constitutive model (UCM) based on continuum damage mechanics (CDM) (see [1]) is used for creep-fatigue damage evaluation of the header components. The performance of the UCM is compared against creep and damage focused models in predicting the life of HRSG header components subjected to steady operation condition with low cycle fatigue scenario.}, booktitle={Proceedings of the ASME Pressure Vessels and Piping Conference, 2017, Vol 3B}, author={Islam, N. and Dewees, D. J. and Cooch, M. and Hassan, T.}, year={2017} }
 @inproceedings{islam_hassan_2017, title={Improving simulations for low cycle fatigue and ratcheting responses of elbows}, booktitle={Proceedings of the ASME Pressure Vessels and piping conference, 2016, vol 5}, author={Islam, N. and Hassan, T.}, year={2017} }
 @inproceedings{islam_hassan_2017, title={Influence of initial and welding residual stresses on low cycle fatigue and ratcheting response simulations of elbows}, DOI={10.1115/pvp2017-65847}, abstractNote={Earlier studies [1] showed that the ANSYS software package customized with an advanced rate-independent constitutive model was unable to simulate some of the low-cycle fatigue responses of elbow components. Hence, simulations are performed to investigate the influence of manufacturing and welding residual stresses on elbow low-cycle fatigue responses. The sequentially coupled thermo-mechanical finite element analysis is performed to determine the initial residual stress states in elbows due to the elbow manufacturing processes and welding of elbows to straight pipes. Real-time girth-welding processes are taken into account to simulate the welding induced residual stress field. Incorporating these initial residual stresses in the computations, low-cycle fatigue and strain ratcheting responses are simulated by ANSYS. The simulation responses demonstrate that the influence of manufacturing and welding residual stresses in elbows on its low-cycle fatigue responses is negligible. Hence, the question remains what is missing in the simulation models that some of the elbow low-cycle fatigue responses cannot be simulated.}, booktitle={Proceedings of the ASME Pressure Vessels and Piping Conference, 2017, vol 8}, author={Islam, N. and Hassan, T.}, year={2017} }
 @inproceedings{robert_morrison_hassan_2017, title={Influence of notch geometry on the notch vicinity stress and strain responses}, DOI={10.1115/pvp2016-63287}, abstractNote={The most common method used to determine the crack initiation life of a component containing a stress raiser in the low cycle fatigue regime is to calculate the maximum strain and then to use a strain-life curve. General practice is to base fatigue life estimates on the stabilized strain amplitude and to neglect the effects of transient behavior due to cyclic hardening or softening and ratcheting. For certain structures in which the accumulation of plastic strains may be significant, a separate check may be performed to ensure that these strains remain below a specified level. An objective of this research is to understand the notch tip local strain ratcheting and shakedown through finite element analyses and physical experiments. Towards planning a set of notched flat coupon experiments, this study performed analyses of various notched coupons under force-controlled cyclic loading. A question that will be addressed, what is the notch tip failure mechanism under a force-controlled load cycle with a non-zero mean force? Smooth specimens under such a force-controlled load cycle normally results in strain ratcheting. It is investigated whether notch tip strain responds in a similar manner under a force controlled loading cycle. The analysis results show that the strain ratcheting rate at the notch tip depends on the sharpness of the notch. In case of semi-circular and blunt elliptical notches shakedown of strain ratcheting within 25 cycles is observed, whereas for the sharp elliptical notch strain ratcheting doesn’t shakedown after 300 cycles. A novel observation made from the analysis results is that the mean stress at the notch tip gradually decreases with inelastic cycle while the stress amplitude remains unchanged. These result and future experimental plan on notch specimens are presented in this article.}, booktitle={Proceedings of the ASME Pressure Vessels and Piping Conference, 2016, Vol 1A}, author={Robert, A. and Morrison, M. L. and Hassan, T.}, year={2017} }
 @article{quayyum_hassan_2017, title={Initial Residual Stresses in Hot-Rolled Wide-Flange Shapes: A Computational Technique and Influence on Structural Performances}, volume={143}, ISSN={["1943-541X"]}, DOI={10.1061/(asce)st.1943-541x.0001739}, abstractNote={AbstractWide-flange members are hot-rolled to their final shapes and subsequently air-cooled, during which residual stresses are developed due to nonuniform cooling. Both global and local flange-bu...}, number={5}, journal={JOURNAL OF STRUCTURAL ENGINEERING}, author={Quayyum, Shahriar and Hassan, Tasnim}, year={2017}, month={May} }
 @article{morrison_quayyum_hassan_2017, title={Performance enhancement of eight bolt extended end-plate moment connections under simulated seismic loading}, volume={151}, ISSN={["1873-7323"]}, DOI={10.1016/j.engstruct.2017.08.040}, abstractNote={Extended end-plate (EEP) moment resisting connections provide the advantage of eliminating field welding and by virtue of this, facilitate fast field erection of building frames. The eight bolt stiffened (8ES) EEP connection is one of the prequalified moment connections in the AISC 358 standard for special moment frames (SMFs) in seismic regions. In this connection, a stiffener plate is welded between the end plate and the beam flanges to strengthen the extended portion of the end plate. This stiffener reduces prying action and more uniformly distributes flange forces among the bolt group. In experimental studies, the 8ES connection has shown ductile response to simulated seismic loading with test specimens typically failing due to beam buckling and gradual strength degradation. However, cracks initiating at the toe of the stiffener leading to brittle fracture of the beam flange has also been observed due to the high stress concentration in this region. The study reported herein proposes an eight-bolt EEP connection in which the end plate stiffener is removed and the bolt arrangement is modified to promote uniform distribution of flange forces among the bolt group. The proposed connection was developed through detailed finite element analysis in which various bolt arrangements for stiffened and unstiffened eight-bolt EEP connections were considered. The proposed connection displayed reduced beam flange stress and strain concentrations, delayed or reduced rate of strength degradation from local buckling and more uniform distribution of bolt forces when compared to the alternatives. Furthermore, when compared to the currently prequalified 8ES connection, despite requiring thicker end plates, the proposed connection is anticipated to result in cost savings from the removal of the end plate stiffener. Future analytical and experimental needs for further development of the proposed connection are discussed.}, journal={ENGINEERING STRUCTURES}, author={Morrison, Machel and Quayyum, Shahriar and Hassan, Tasnim}, year={2017}, month={Nov}, pages={444–458} }
 @article{ahmed_barrett_menon_hassan_2017, title={Thermo-mechanical low-cycle fatigue-creep of Haynes 230}, volume={126}, ISSN={["1879-2146"]}, DOI={10.1016/j.ijsolstr.2017.07.033}, abstractNote={• Developed a broad set of thermo-mechanical fatigue-creep (TMFC) responses of a superalloy. • Demonstration of the influence of TMFC loading on mean stress evolution. • Demonstration of the influence of TMFC loading on the elastic modulus rate change. • Discussion on the challenges in constitutive model development for TMFC responses. • Comparison of isothermal and thermo-mechanical fatigue-creep lives. Combustor liners of airplane gas turbine engines experience premature thermo-mechanical fatigue-creep (TMFC) failure under operational loading conditions. The loading history of combustor liners encompass temperature fluctuation between ambient to as high as 1000 °C, and concurrent strain or stress fluctuation with load peak dwell-periods of 30 min to as long as 16 h of flying time. Repetition of such an anisothermal loading history leads to crack initiation in components via TMFC damage accumulation processes. In an effort to investigate such TMFC failures, a set of anisothermal experiments, both in-phase and out-of-phase, with peak dwell periods, were carried out for Haynes 230, a nickel-based superalloy used in constructing combustor liners. Analysis of the responses from out-of-phase experiments with compression dwells show mean-stress evolution in the tensile direction, while that from in-phase experiments with tensile dwells show mean-stress evolution in the compression direction. The total stress relaxation during peak strain dwell in the TMFC loading in general decreases with cycle, whereas that in the isothermal low-cycle fatigue-creep (LCFC) increases with cycle. The cyclic hardening-softening response is found to depend on the maximum temperature in the TMFC loading cycle. While calculating the inelastic strain in the experiments, it was found that the time derivative of the elastic modulus needed to be considered to prevent anomalous shifting of the hysteresis loops with cycles. The fatigue lives in the TMFC experiments are adversely affected by higher maximum temperatures and longer dwell-periods. These experimental responses are presented and analyzed, and challenges in developing a unified constitutive model for simulation of these responses are identified.}, journal={INTERNATIONAL JOURNAL OF SOLIDS AND STRUCTURES}, author={Ahmed, Raasheduddin and Barrett, Paul Ryan and Menon, Mamballykalathil and Hassan, Tasnim}, year={2017}, month={Nov}, pages={90–104} }
 @inproceedings{morrison_ahmed_hassan_2017, title={Thermomechanical fatigue response and constitutive modeling for Haynes 230}, DOI={10.1115/pvp2016-63283}, abstractNote={Design by analysis is usually performed by commercially available finite element analysis (FEA) software. Constitutive models available in the FEA software are developed and validated using limited experimental data. Hence, a broad set of thermomechanical fatigue experiments with strain dwell at compressive peaks are performed to understand local fatigue failure responses of high temperature components. This study developed a unified viscoplastic model based on nonlinear kinematic hardening of Chaboche type with added features of strain range dependence, rate dependence, temperature dependence, static recovery, and mean stress evolution. The robustness of the constitutive model is demonstrated by comparing its simulations against the experimental responses.}, booktitle={Proceedings of the ASME Pressure Vessels and piping conference, 2016, vol 5}, author={Morrison, M. and Ahmed, R. and Hassan, T.}, year={2017} }
 @inproceedings{islam_dewees_hassan_2017, title={Unified viscoplastic model coupled with damage for evaluation of creep-fatigue of grade 91 steel}, DOI={10.1115/pvp2017-65849}, abstractNote={A continuum damage mechanics (CDM) coupled unified viscoplasticity model has been developed to predict the creep-fatigue life of modified Grade 91 steel. A tertiary creep model termed MPC-Omega codified in Part 10 of API (and also implemented in the ASME BP&V Code for Grade 22V and more recently Grade 91 Steel) is also employed for creep damage evaluation. As MPC-Omega has a direct relationship with Larson-Miller parameter (LMP) coefficients, creep damage coefficients in the unified constitutive model (UCM) are tied with MPC-Omega coefficients in order to utilize WRC and API 579-1 Grade 91 creep rupture database. The model is validated against long-term creep, LCF, creep-fatigue and TMF experimental responses at T = 20–600°C.}, booktitle={Proceedings of the ASME pressure vessels and piping conference, 2017, vol 6a}, author={Islam, N. and Dewees, D. J. and Hassan, T.}, year={2017} }
 @inproceedings{islam_dewees_hassan_2017, title={Unified viscoplasticity modeling features needed for simulation of grade 91 creep and fatigue responses}, DOI={10.1115/pvp2016-63578}, abstractNote={Chaboche unified viscoplasticity model and uncoupled plasticity and creep models (nonunified) are evaluated for their capability in simulating low-cycle fatigue, creep and creep-fatigue responses of Grade 91 steel. The primary objective of this study is to develop a constitutive model incorporating various advanced modeling features for design-by-analysis of elevated temperature power plant components. For validation of the model a broad set of experimental responses of Grade 91 in the temperature range 20–600°C are collected from literature. Performance of the models is demonstrated against simulating these experimental responses. It is demonstrated that the unified Chaboche model simulation capability can be improved through implementing strain range dependence, cyclic hardening through kinematic hardening rule and static recovery modeling features.}, booktitle={Proceedings of the ASME Pressure Vessels and Piping Conference, 2016, Vol 6a}, author={Islam, N. and Dewees, D. and Hassan, T.}, year={2017} }
 @inproceedings{li_ngaile_hassan_2016, title={A novel hybrid heating method for elevated temperature mechanical testing of miniature specimens}, DOI={10.1115/msec2016-8852}, abstractNote={The lack of robust testing systems to generate uniform elevated temperatures on specimens in material tests is hindering the advancement of small specimen testing technology (SSTT). The purpose of this study is to develop a novel hybrid heating method combining coil heating and electric-resistance specimen heating to uniformly heat micro specimens in material tests. In a hybrid heating process, two heating coils are used to heat the local temperatures on the specimen ends, and electric current is conducted through the specimen to generate Joule heat and compensate the heat transfer effects of natural convection and radiation around the specimen center area. In this way, a highly uniform temperature distribution can be generated on the specimen between the heating coils. In this study, Thermal-Electrical and Transient Thermal FEA simulations are applied to analyze the temperature distributions and preheating times on the micro specimens under coil heating, electric-resistance specimen heating, and hybrid heating respectively. According to the simulation results, it can be concluded that hybrid heating method can provide the ability to generate highly uniform elevated temperature conditions on different micro tubular specimens with short preheating times.}, booktitle={Proceedings of the ASME 11th International Manufacturing Science and Engineering Conference, 2016, vol 1}, author={Li, L. and Ngaile, G. and Hassan, T.}, year={2016} }
 @article{barrett_ahmed_menon_hassan_2016, title={Isothermal low-cycle fatigue and fatigue-creep of Haynes 230}, volume={88-89}, ISSN={["1879-2146"]}, DOI={10.1016/j.ijsolstr.2016.03.011}, abstractNote={Service temperature of airplane gas turbine engine combustors fluctuates between ambient to as high as 982 °C, during which structural constraints induce cyclic stresses and strains resulting in thermo-mechanical fatigue damage accumulation in the combustor liner. In order to substantially improve the current design methodologies or low-cycle fatigue (LCF) life predictions of such high-temperature components, it is essential to develop an experimentally validated advanced constitutive model. This requires a broad set of fatigue data of the combustor liner material, Haynes 230 (HA 230) – a nickel-based superalloy, to characterize its fatigue failure responses. Hence, a systematic set of isothermal experiments are conducted prescribing uniaxial strain-controlled loading cycles, with and without a compression peak strain-dwell, with and without a mean strain, at seven different temperatures in the range of 24–982 °C and at three strain rates. The experimental responses are critically examined to explore various fatigue failure responses of HA230, which is a complex material showing unique fatigue-creep, strain rate sensitivity, strain range dependence, temperature dependence and dynamic strain aging (DSA) properties. DSA is found to occur in the temperature domain 427–760 °C. Isothermal experimental responses at different strain rates show that HA 230 can be considered rate-independent at and below 760 °C. However, stress relaxation is observed at lower temperatures up to 649 °C during the peak strain-dwell period. Finally, fatigue lives of HA 230 from the isothermal experiments are found to decrease with increase in temperature. These experimental responses are presented and challenges in constitutive model development are discussed.}, journal={INTERNATIONAL JOURNAL OF SOLIDS AND STRUCTURES}, author={Barrett, Paul R. and Ahmed, Raasheduddin and Menon, Mamballykalathil and Hassan, Tasnim}, year={2016}, month={Jun}, pages={146–164} }
 @article{ben naceur_sai_hassan_cailletaud_2016, title={Micromechanical Modeling of the Ratcheting Behavior of 304 Stainless Steel}, volume={138}, ISSN={["1528-8889"]}, DOI={10.1115/1.4032154}, abstractNote={Numerical simulations of 304 austenitic stainless steel (SS304) cyclic and ratcheting responses are performed using polycrystalline plasticity models. On the basis of the polycrystalline model of Cailletaud and Pilvin (1994, “Utilisation de modèles polycristallins pour le calcul par éléments finis,” Rev. Eur. Élém. Finis, 3, pp. 515–541), a modification of the β rule that operates the transition between the macroscopic level and the grain level is proposed. The improvement of the transition rule is obtained by introducing a “memory variable” at the grain level, so that a better description of the local stress–strain behavior is provided. This new feature is calibrated by means of previous simulations using finite element (FE) aggregate models. The results of the updated polycrystalline plasticity model are in good agreement with the macroscopic responses.}, number={2}, journal={JOURNAL OF ENGINEERING MATERIALS AND TECHNOLOGY-TRANSACTIONS OF THE ASME}, author={Ben Naceur, I. and Sai, K. and Hassan, T. and Cailletaud, G.}, year={2016}, month={Apr} }
 @article{morrison_hassan_2016, title={Resilient welded steel moment connections by enhanced beam buckling resistance}, volume={127}, ISSN={["1873-5983"]}, DOI={10.1016/j.jcsr.2016.07.012}, abstractNote={This study develops two (2) simple but effective techniques for enhancing buckling resistance of welded steel moment connections (WSMCs). The ANSI/AISC 358-10 prequalified connections satisfy the 4% interstory drift requirement, however experimental studies have shown that their strength degradation may initiate as early as 3% drift. This strength degradation has been observed to be initiated by buckling of the beam web which is followed by buckling of the beam flange and twisting of the beam. Consequently, buildings with the prequalified connections may sustain significant buckling damages under severe earthquakes and it is questionable as to whether these connections are capable of resisting gravity loads or lateral loads from strong aftershocks following a severe earthquake. To improve upon these shortcomings, two (2) performance enhancing techniques are proposed and investigated through finite element analysis (FEA). The more promising of the two involves reinforcing the beam web in the expected plastic hinge with a web reinforcement plate. Finite element analysis demonstrated that this reinforcement enhances the beam buckling resistance of WSMCs and thereby significantly reduces the beam buckling damages even at 5% interstory drift. The potential of this technique is analytically and experimentally demonstrated for the recently developed heat-treated beam section (HBS) WSMC. Test results confirm that the web reinforcement plate was effective in reducing local buckling damage and associated strength degradation, thereby improving the performance of HBS WSMCs. Areas for application and future development of the proposed techniques are identified.}, journal={JOURNAL OF CONSTRUCTIONAL STEEL RESEARCH}, author={Morrison, Machel L. and Hassan, Tasnim}, year={2016}, month={Dec}, pages={77–91} }
 @article{morrison_schweizer_hassan_2016, title={Seismic Enhancement of Welded Unreinforced Flange-Bolted Web Steel Moment Connections}, volume={142}, ISSN={["1943-541X"]}, DOI={10.1061/(asce)st.1943-541x.0001575}, abstractNote={AbstractWidespread damage to welded unreinforced flange-bolted web (WUF-B) steel moment connections during the 1994 Northridge earthquake led to intensive research study of this connection. Despite the improvements to weld metal and connection details, the post-Northridge WUF-B connection was unable to attain sufficient ductility for use in special moment frames (SMFs). This study presents detailed finite element (FE) analysis of post-Northridge WUF-B connections to better understand the mechanisms which limited connection ductility in laboratory tests. Observations made from the FE analysis led to the development and numerical study of a modified WUF-B connection that combines a new bolted web design with a recently validated technique to promote plastic hinging of the beam away from the connection joint. The proposed connection provides the benefit of reduced field welding and UT inspection without sacrificing connection ductility and seismic performance. Finally, the proposed connection is experimental...}, number={11}, journal={JOURNAL OF STRUCTURAL ENGINEERING}, author={Morrison, Machel Leigh and Schweizer, Douglas Quinn and Hassan, Tasnim}, year={2016}, month={Nov} }
 @article{ahmed_barrett_hassan_2016, title={Unified viscoplasticity modeling for isothermal low-cycle fatigue and fatigue-creep stress-strain responses of Haynes 230}, volume={88-89}, ISSN={["1879-2146"]}, DOI={10.1016/j.ijsolstr.2016.03.012}, abstractNote={A robust cyclic viscoplasticity model is developed for simulating a broad set of isothermal, low-cycle fatigue and fatigue-creep responses of Haynes 230 (HA 230) under uniaxial loading. High temperature components experiencing thermo-mechanical fatigue failures can be designed considering their failure responses such that their fatigue life is predictable. Hence, design of high temperature components in aerospace, automobile, nuclear power, and chemical industries should be based on viscoplastic nonlinear analysis using a robust constitutive model. A unified viscoplasticity model based on the nonlinear kinematic hardening rule of Chaboche with several added features for strain-range dependence, rate-dependence, static recovery, and mean stress evolution is developed and evaluated against a broad set of HA 230 responses. Robustness of the constitutive model is demonstrated against predicting fatigue and dwell period stress relaxation responses under uniaxial strain-controlled loading for a broad temperature range of 25–982 °C and strain rate range of 1.1×10−2 to 2.6×10−5/s. Parameter determination of such an advanced model is discussed showing the importance of a well thought out experimental database and thereby providing physical meaning to model parameters.}, journal={INTERNATIONAL JOURNAL OF SOLIDS AND STRUCTURES}, author={Ahmed, Raasheduddin and Barrett, Paul R. and Hassan, Tasnim}, year={2016}, month={Jun}, pages={131–145} }
 @inproceedings{barrett_hassan_2015, title={A unified viscoplastic model for creep and fatigue-creep response simulation of Haynes 230}, DOI={10.1115/pvp2015-45671}, abstractNote={A Chaboche-based unified viscoplastic constitutive model, including features of strain range dependence, strain rate-dependence, static recovery, and mean stress evolution is developed and evaluated for simulating fatigue-creep and creep responses of Haynes 230. In other words, this constitutive model attempt to simulate not only strain-controlled fatigue and fatigue-creep responses of Haynes 230, but also stress-controlled creep responses. After investigating various flow rules and kinematic hardening rules, a unified viscoplastic constitutive model is developed for simulating both the fatigue-creep and creep responses. The parameter determination for this constitutive model, however, requires a robust optimization algorithm. The proposed unified constitutive model can adequately simulate fatigue-creep responses, and creep responses up to the secondary creep regimes. However, with the introduction of damage modeling features the constitutive model can simulate the tertiary creep regime responses, but with some limitations in simulating fatigue-creep responses. Nonetheless, the unified viscoplastic constitutive model with or without damage modeling features has shown to be able to capture the stress-controlled creep responses while still maintaining high fidelity in capturing the strain-controlled fatigue and fatigue-creep responses.}, booktitle={ASME Pressure Vessels and Piping Conference - 2015, vol 3}, author={Barrett, P. R. and Hassan, T.}, year={2015} }
 @article{morrison_schweizer_hassan_2015, title={An innovative seismic performance enhancement technique for steel building moment resisting connections}, volume={109}, ISSN={["1873-5983"]}, DOI={10.1016/j.jcsr.2015.02.010}, abstractNote={This study develops and experimentally validates an innovative technique for enhancing the seismic performance of steel beam to column moment connections. The technique involves reducing the strength of specified regions of the beam flanges by exposing them to high temperatures followed by slow cooling. Analogous to the reduced beam section (RBS) connection, yielding and plastic hinge formation is promoted in the heat-treated beam section (HBS). Moreover, because the elastic and inelastic modulus of the steel is unmodified by the heat-treatment and the beam cross section is not altered, an HBS connection does not sacrifice elastic stiffness or buckling resistance as does the RBS. Design of the HBS connection was performed through detailed finite element analysis and material testing. Two large scale connections modified with the HBS technique were tested in this study. The test program showed that the proposed heat-treatment technique was successful in the promotion of yielding and plastic hinge development in the heat-treated regions with specimens attaining interstory drifts as high as 6% without weld or near weld fracture. Strength degradation due to beam buckling within the HBS was the observed failure mechanism in both specimens. Detail analyses of strain and beam deformation data are presented to explain the HBS connection plastic hinge formation and gradual strength degradation. Broader applications of the technique to other structural components are identified.}, journal={JOURNAL OF CONSTRUCTIONAL STEEL RESEARCH}, author={Morrison, Machel and Schweizer, Doug and Hassan, Tasnim}, year={2015}, month={Jun}, pages={34–46} }
 @article{hassan_rahman_2015, title={Constitutive Models in Simulating Low-Cycle Fatigue and Ratcheting Responses of Elbow}, volume={137}, ISSN={["1528-8978"]}, DOI={10.1115/1.4029069}, abstractNote={As stated in the sister article that the objective of this study was to explore the low-cycle fatigue and ratcheting failure responses of elbow components through experimental and analytical studies. Low-cycle fatigue and ratcheting damage accumulation in piping components may occur under load reversals induced by earthquakes or thermomechanical operations. Ratcheting damage accumulation can cause failure of components through cracking or plastic buckling. Hence, design by analysis of piping components against ratcheting failure will require simulation of this response with reasonable accuracy. In developing a constitutive model that can simulate ratcheting responses of piping components, a systematic set of elbow experiments involving deformation and strain ratcheting were conducted and reported in the sister article. This article will critically evaluate seven different constitutive models against their elbow response simulation capabilities. The widely used bilinear, multilinear, and Chaboche models in ansys are first evaluated. This is followed by evaluation of the modified Chaboche, Ohno–Wang, modified Ohno–Wang, and Abdel Karim–Ohno models. Results from this simulation study are presented to demonstrate that all the seven models can simulate the elbow force response reasonably. The bilinear and multilinear models can simulate the initial elbow diameter change or strain accumulation, but always simulate shakedown during the subsequent cycles when for some of the cases the experimental trends are ratcheting. Advanced constitutive models like Chaboche, modified Chaboche, Ohno–Wang, modified Ohno–Wang, and Abdel Karim–Ohno can simulate many of the elbow ratcheting responses well, but for some of the strain responses, these models simulate negative ratcheting, which is opposite to the experimental trend. Finally, implications of negative ratcheting simulation are discussed and suggestions are made for improving constitutive models ratcheting response simulation.}, number={3}, journal={JOURNAL OF PRESSURE VESSEL TECHNOLOGY-TRANSACTIONS OF THE ASME}, author={Hassan, T. and Rahman, M.}, year={2015}, month={Jun} }
 @inproceedings{islam_fenton_hassan_2015, title={Long and short radius elbow experiments and evaluation of advanced constitutive models to simulate the responses}, DOI={10.1115/pvp2015-45688}, abstractNote={Low-cycle fatigue (LCF) and strain ratcheting responses of long and short radius elbows are studied experimentally and analytically. Elbow piping components are widely used in piping systems, however, the prediction of their low-cycle fatigue and ratcheting responses remain a challenge. Hence, a systematic set of short and long radius elbow LCF responses are developed by prescribing displacement-controlled loading cycles with or without internal pressure. A setup comprised of four LVDTs was utilized to measure diameter change during cyclic loading. In order to evaluate the accuracy of the strain gage data, strains are also acquired using the digital image correlation (DIC) technique. Recorded fatigue responses are analyzed in understanding the differences in LCF lives between the long and short radius elbows. The Chaboche nonlinear kinematic hardening constitutive model in ANSYS and a modified version of this model are evaluated for their simulation capability against the recorded elbow responses. The experimental and finite element simulation responses are presented in this article.}, booktitle={Asme Pressure Vessels and Piping Conference - 2015, vol 8}, author={Islam, N. and Fenton, M. and Hassan, T.}, year={2015} }
 @article{hassan_rahman_bari_2015, title={Low-Cycle Fatigue and Ratcheting Responses of Elbow Piping Components}, volume={137}, ISSN={["1528-8978"]}, DOI={10.1115/1.4029068}, abstractNote={The objective of this study was to investigate low-cycle fatigue and ratcheting responses of elbows through experimental and analytical studies. Low-cycle fatigue and ratcheting damage accumulation in piping components may occur under repeated reversals of loading induced by earthquake and/or thermomechanical operation. Ratcheting and fatigue damage accumulation can cause failure of piping systems through fatigue cracks or plastic buckling. However, the ratcheting damage induced failures are yet to be understood clearly; consequently, ASME Code design provisions against ratcheting failure continue to be a controversial issue over the last two decades. A systematic set of piping component experimental responses involving ratcheting and a computational tool to simulate these responses will be essential to rationally address the issue. Development of a constitutive model for simulating component ratcheting responses remains to be a challenging problem. In order to develop an experimentally validated constitutive model, a set of elbow experiments was conducted. The loading prescribed in the experiments involved displacement-controlled or force-controlled in-plane cyclic bending with or without internal pressure. Force, displacement, internal pressure, elbow diameter change, and strains at four locations of the elbow specimens were recorded. This article presents and discusses the results from the experimental study. A sister article evaluates seven different constitutive models against simulating these elbow ratcheting and fatigue responses.}, number={3}, journal={JOURNAL OF PRESSURE VESSEL TECHNOLOGY-TRANSACTIONS OF THE ASME}, author={Hassan, T. and Rahman, M. and Bari, S.}, year={2015}, month={Jun} }
 @article{asmaz_colak_hassan_2014, title={Biaxial Ratcheting of Ultra High Molecular Weight Polyethylene: Experiments and Constitutive Modeling}, volume={42}, ISSN={["1945-7553"]}, DOI={10.1520/jte20130131}, abstractNote={Abstract}, number={6}, journal={JOURNAL OF TESTING AND EVALUATION}, author={Asmaz, Kerem and Colak, Ozgenu U. and Hassan, Tasnim}, year={2014}, month={Nov}, pages={1486–1492} }
 @inproceedings{ahmed_barrett_hassan_2014, title={Constitutive modeling of Haynes 230 for anisothermal thermo-mechanical fatigue and multiaxial creep-ratcheting responses}, DOI={10.1115/pvp2013-97248}, abstractNote={Service life analysis and design of high temperature components, such as turbine engines, needs accurate estimation of stresses and strains at failure locations. The structural integrity under these high temperature environments can be evaluated through finite element structural analysis. This requires a robust constitutive model to predict local stresses and strains. A unified viscoplastic constitutive model based on the Chaboche type nonlinear kinematic hardening rule was developed including the added features of strain range dependence, rate dependence, temperature dependence, static recovery, and a mean stress evolution. The new constitutive model was validated through critical evaluation of the simulation of a broad set of stress and strain responses of a nickel-base superalloy Haynes 230. The experimental database encompasses uniaxial strain-controlled loading histories which include isothermal low cycle creep-fatigue and anisothermal thermo-mechanical fatigue experiments at temperatures ranging from 75°F to 1800°F. Simulations from the modified model are presented to demonstrate its strengths and weaknesses, and future work is needed for developing a robust constitutive model.}, booktitle={Proceedings of the ASME Pressure Vessels and Piping Conference - 2013, vol 6B: Materials and Fabrication}, author={Ahmed, R. and Barrett, P. R. and Hassan, T.}, year={2014} }
 @inproceedings{pritchard_carroll_hassan_2014, title={Constitutive modeling of high temperature uniaxial creep-fatigue and creep-ratcheting responses of alloy 617}, DOI={10.1115/pvp2013-97251}, abstractNote={Inconel Alloy 617 is a high temperature creep and corrosion resistant alloy and is a leading candidate for use in Intermediate Heat Exchangers (IHX) of the Next Generation Nuclear Plants (NGNP). The IHX of the NGNP is expected to experience operating temperatures in the range of 800°–950°C, which is in the creep regime of Alloy 617. A broad set of uniaxial, low-cycle fatigue, fatigue-creep, ratcheting, and ratcheting-creep experiments are conducted in order to study the fatigue and ratcheting responses, and their interactions with the creep response at high temperatures. A unified constitutive model developed at North Carolina State University is used to simulate these experimental responses. The model is developed based on the Chaboche viscoplastic model framework. It includes cyclic hardening/softening, strain rate dependence, strain range dependence, static and dynamic recovery modeling features. For simulation of the alloy 617 responses, new techniques of model parameter determination are developed for optimized simulations. This paper compares the experimental responses and model simulations for demonstrating the strengths and shortcomings of the model.}, booktitle={Proceedings of the ASME Pressure Vessels and Piping Conference - 2013, vol 5}, author={Pritchard, P. G. and Carroll, L. and Hassan, T.}, year={2014} }
 @inproceedings{quayyum_sengupta_choi_lissenden_hassan_2014, title={Fatigue and ratcheting experimental responses of alloy 617 under high temperature multiaxial loading}, DOI={10.1115/pvp2013-97252}, abstractNote={Nickel based Alloy 617 is one of the leading candidate materials for intermediate heat exchanger (IHX) of the next generation nuclear plant (NGNP). The IHX is anticipated to operate at temperatures between 800–950°C, which is in the creep regime. In addition, system start-ups and shut-downs will induce low cycle fatigue (LCF) damages in the IHX components. Hence, designing IHX using Alloy 617 for NGNP construction will require a detailed understanding of the creep-fatigue and ratcheting responses. In this study, a broad set of multiaxial creep-fatigue and ratcheting experiments are performed and the results are critically evaluated. Experiments are conducted by prescribing multiaxial loading histories in axial and shear directions at 850°C and 950°C with different strain rates and strain amplitudes. Experimental results revealed that the axial strain ratcheting and cyclic hardening/softening responses of Alloy 617 vary significantly with temperature levels, strain rates and strain amplitudes indicating the dependence of creep-fatigue and ratcheting responses on these parameters. This necessitates the incorporation of strain rate and strain amplitude dependence and effect of loading non-proportionality and temperature in the unified constitutive modeling (UCM) for a better prediction of the material behavior. Development of a UCM is underway based on the experimental results developed.}, booktitle={Proceedings of the ASME Pressure Vessels and Piping Conference - 2013, vol 3: Design and Analysis}, author={Quayyum, S. and Sengupta, M. and Choi, G. and Lissenden, C. J. and Hassan, T.}, year={2014} }
 @inbook{ahmed_menon_hassan_2014, place={Cham, Switzerland}, series={Conference Proceedings of the Society for Experimental Mechanics Series}, title={Haynes 230 High Temperature Thermo-Mechanical Fatigue Constitutive Model Development}, volume={2}, ISBN={9783319008516 9783319008523}, ISSN={2191-5644 2191-5652}, url={http://dx.doi.org/10.1007/978-3-319-00852-3_17}, DOI={10.1007/978-3-319-00852-3_17}, abstractNote={Service temperatures of propulsion turbine engine combustor components can be as high as 1,800 °F. This induces a thermo-mechanical fatigue (TMF) loading which, as a result of dwell periods and cyclic loadings, eventually leads to failure of the components via creep-fatigue processes. A large set of isothermal and anisothermal experiments have been carried out on Haynes 230, in an effort to understand its high temperature fatigue constitutive response. Isothermal experiments at different loading strain rates show that the material can be considered to be rate-independent below and at 1,400 °F. However, isothermal strain hold experiments show stress relaxations below and at 1,400 °F. The out-of-phase strain-controlled TMF experiments show a mean stress response. A Chaboche based viscoplastic constitutive model with various features is under development with the final objective of predicting the strains in an actual combustor liner in service through finite element simulation for fatigue lifing. Temperature rate terms have been found to improve hysteresis loop shape simulations and static recovery terms are essential in modeling stress relaxation at temperatures where the behavior is overall rate-independent. It is anticipated that the new modeling feature of mean stress evolution will model the experimentally observed thermo-mechanical mean stress evolution.}, booktitle={Challenges In Mechanics of Time-Dependent Materials and Processes in Conventional and Multifunctional Materials}, publisher={Springer International Publishing}, author={Ahmed, Raasheduddin and Menon, M. and Hassan, Tasnim}, editor={Antoun, Bonnie and Qi, H. Jerry and Hall, Richard and Tandon, G.P. and Lu, Hongbing and Lu, Charles and Furmanski, Jevan and Amirkhizi, AlirezaEditors}, year={2014}, pages={151–160}, collection={Conference Proceedings of the Society for Experimental Mechanics Series} }
 @inbook{quayyum_sengupta_choi_lissenden_hassan_2014, series={Conference Proceedings of the Society for Experimental Mechanics Series}, title={High Temperature Multiaxial Creep-Fatigue and Creep-Ratcheting Behavior of Alloy 617}, volume={2}, ISBN={9783319008516 9783319008523}, ISSN={2191-5644 2191-5652}, url={http://dx.doi.org/10.1007/978-3-319-00852-3_10}, DOI={10.1007/978-3-319-00852-3_10}, abstractNote={Nickel based Alloy 617 is one of the leading candidate materials for intermediate heat exchanger (IHX) of the next generation nuclear plant (NGNP). The IHX is anticipated to operate at temperatures between 800 °C and 950 °C, which is in the creep regime. In addition, system start-ups and shut-downs will induce low cycle fatigue (LCF) damages in the IHX components. Hence, designing IHX using Alloy 617 for NGNP construction will require a detailed understanding of the creep-fatigue and ratcheting responses. In this study, a broad set of multiaxial creep-fatigue and ratcheting experiments are performed and the results are critically evaluated. Experiments are conducted by prescribing multiaxial loading histories in axial and shear, stress and strain space at 850 °C and 950 °C with different strain rates and strain amplitudes. Experimental results revealed that the axial strain ratcheting and cyclic hardening/softening responses of Alloy 617 vary significantly with temperature levels, strain rates and strain amplitudes indicating the dependence of creep-fatigue and ratcheting responses on these parameters. A unified constitutive model (UCM) based on the Chaboche framework is developed and validated against the multiaxial experimental responses. UCM simulated responses are compared against the experimental responses for determining the current state of material modeling and if modeling improvement are needed for IHX design applications.}, booktitle={Challenges In Mechanics of Time-Dependent Materials and Processes in Conventional and Multifunctional Materials}, publisher={Springer International Publishing}, author={Quayyum, Shahriar and Sengupta, Mainak and Choi, Gloria and Lissenden, Clifford J. and Hassan, Tasnim}, editor={Antoun, Bonnie and Qi, H. Jerry and Hall, Richard and Tandon, G.P. and Lu, Hongbing and Lu, Charles and Furmanski, Jevan and Amirkhizi, AlirezaEditors}, year={2014}, pages={83–97}, collection={Conference Proceedings of the Society for Experimental Mechanics Series} }
 @inbook{barrett_menon_hassan_2014, place={Cham, Switzerland}, series={Conference Proceedings of the Society for Experimental Mechanics Series}, title={Unified Constitutive Modeling of Haynes 230 for Isothermal Creep-Fatigue Responses}, volume={2}, ISBN={9783319008516 9783319008523}, ISSN={2191-5644 2191-5652}, url={http://dx.doi.org/10.1007/978-3-319-00852-3_20}, DOI={10.1007/978-3-319-00852-3_20}, booktitle={Challenges In Mechanics of Time-Dependent Materials and Processes in Conventional and Multifunctional Materials}, publisher={Springer International Publishing}, author={Barrett, Paul Ryan and Menon, Mamballykalathil and Hassan, Tasnim}, editor={Antoun, Bonnie and Qi, H. Jerry and Hall, Richard and Tandon, G.P. and Lu, Hongbing and Lu, Charles and Furmanski, Jevan and Amirkhizi, AlirezaEditors}, year={2014}, pages={175–185}, collection={Conference Proceedings of the Society for Experimental Mechanics Series} }
 @inproceedings{ahmed_menon_hassan_2012, title={Constitutive model development for thermo-mechanical fatigue response simulation of Haynes 230}, DOI={10.1115/pvp2012-78221}, abstractNote={Turbine engine combustor components are subject to thermo-mechanical fatigue (TMF) during service. The combustor liner temperatures can sometimes reach as high as 1800°F. An accurate estimate of the strains at critical locations in the combustor liner is required for reliable lifing predictions. This demands the need for a detailed analysis of the TMF responses and a robust constitutive model capable of predicting the same. A large set of experiments have been carried out on the liner material, a nickel based alloy, HA 230, in an effort to understand its thermo-mechanical fatigue constitutive response. The out-of-phase strain-controlled TMF experiments with a negative mean strain show a positive mean stress response, while the in-phase TMF experiments with a positive mean strain show a negative mean stress response. A Chaboche based viscoplastic constitutive model is under development. It will have several essential features such as nonlinear kinematic hardening, isotropic hardening, strain range dependence, rate dependence, temperature dependence and static recovery. The constitutive model being developed for accurately calculating the stress-strain response is being carried out with the final objective of predicting the strains in an actual combustor liner in service through finite element simulation for fatigue lifing.}, booktitle={Proceedings of the ASME Pressure Vessels and Piping Conference 2012, PVP 2012, vol 9}, author={Ahmed, R. and Menon, M. and Hassan, T.}, year={2012}, pages={171–179} }
 @inproceedings{barrett_menon_hassan_2012, title={Isothermal fatigue responses and constitutive modeling of Haynes 230}, DOI={10.1115/pvp2012-78342}, abstractNote={Constitutive models are an integral part of a lifing system because it allows for accurate estimation of stresses and strains at failure locations of interest. Constitutive models can be properly defined in a material subroutine of a finite element code. The computational capabilities of today are far higher, allowing for more comprehensive models that can provide more accurate results. Macroscopic models that are physically based, phenomenological models characterize the material behavior on a larger scale that provides invaluable insights even at such length scales which are compatible for industrial application. A unified viscoplastic model based on nonlinear kinematic hardening (Chaboche type) with several added features such as nonproportionality, multiaxiality, strain range dependence, and thermal recovery is being implemented in ANSYS through the User Programmable Features. The simulation capability of the model will be experimentally validated on a nickel based superalloy, HA230. The experimental database encompasses a broad set of low cycle fatigue, symmetric, uniaxial strain-controlled loading histories which include isothermal with and without hold times, with and without a mean strain, at temperatures ranging from 75°F to 1800°F. Simulations from the modified model compared to the experimental responses will be presented to demonstrate the strengths and weaknesses.}, booktitle={Proceedings of the ASME Pressure Vessels and Piping Conference 2012, PVP 2012, vol 9}, author={Barrett, P. R. and Menon, M. and Hassan, T.}, year={2012}, pages={71–79} }
 @article{fulmer_kowalsky_nau_hassan_2012, title={Reversed Cyclic Flexural Behavior of Spiral DSAW and Single Seam ERW Steel Pipe Piles}, volume={138}, ISSN={["0733-9445"]}, DOI={10.1061/(asce)st.1943-541x.0000553}, abstractNote={This paper presents the findings of an investigation on the flexural performance of hollow steel pipe piles subjected to reversed cyclic loading. The testing evaluated both spirally double submerged arc welded (DSAW) and traditional longitudinal single seam electric resistance welded (ERW) pipe piles to determine the effects of the spiral welding manufacturing process on the structural performance of the pile. Some of the tests were conducted on previously driven piles to study the effects of driving stresses. The experimental results and observations indicated that the undesirable failure mode of spiral weld cracking did not control the ultimate limit state in any of the spirally welded specimens considered. Although weld fracture did occur in each spirally welded specimen, it did not develop until the specimen was subjected to large inelastic deformations and was ultimately the result of locally increased strains caused by local buckling. Each traditional single seam specimen failed in a similar manner with pile wall local buckling developing at inelastic deformation levels comparable to those of the spirally welded specimens.}, number={9}, journal={JOURNAL OF STRUCTURAL ENGINEERING-ASCE}, author={Fulmer, Steven J. and Kowalsky, Mervyn J. and Nau, James M. and Hassan, Tasnim}, year={2012}, month={Sep}, pages={1099–1109} }
 @article{hassan_colak_clayton_2011, title={Uniaxial Strain and Stress-Controlled Cyclic Responses of Ultrahigh Molecular Weight Polyethylene: Experiments and Model Simulations}, volume={133}, ISSN={["1528-8889"]}, DOI={10.1115/1.4003109}, abstractNote={Thermoplastics such as ultrahigh molecular weight polyethylene (UHMWPE) are used for a wide variety of applications, such as bearing material in total replacement of knee and hip components, seals, gears, and unlubricated bearing. Accurate prediction of stresses and deformations of UHMWPE components under service conditions is essential for the design and analysis of these components. This, in turn, requires a cyclic, viscoplastic constitutive model that can simulate cyclic responses of UHMWPE under a wide variety of uniaxial and multiaxial, strain, and stress-controlled cyclic loading. Such a constitutive model validated against a broad set of experimental responses is not available mainly because of the lack of experimental data of UHMWPE. Toward achieving such a model, this study conducted a systematic set of uniaxial experiments on UHMWPE thin-walled, tubular specimens by prescribing strain and stress-controlled cyclic loading. The tubular specimen was designed so that both uniaxial and biaxial experiments can be conducted using one type of specimen. The experimental responses developed are presented for demonstrating the cyclic and ratcheting responses of UHMWPE under uniaxial loading. The responses also are scrutinized for determining the applicability of the thin-walled, tubular specimen in conducting large strain cyclic experiments. A unified state variable theory, the viscoplasticity theory based on overstress for polymers (VBOP) is implemented to simulate the recorded uniaxial responses of UHMWPE. The state of the VBOP model simulation is discussed and model improvements needed are suggested.}, number={2}, journal={JOURNAL OF ENGINEERING MATERIALS AND TECHNOLOGY-TRANSACTIONS OF THE ASME}, author={Hassan, Tasnim and Colak, Ozgen U. and Clayton, Patricia M.}, year={2011}, month={Apr} }
 @article{abdi_peters_kowalsky_hassan_2011, title={Validation of a single-mode polymer optical fiber sensor and interrogator for large strain measurements}, volume={22}, ISSN={["1361-6501"]}, DOI={10.1088/0957-0233/22/7/075207}, abstractNote={A single-mode polymer optical fiber (POF) in a Mach–Zehnder interferometer configuration is validated for the measurement of tensile nominal elongation of the POF up to 10%. The single-mode POF sensors were unmounted and surface mounted on aluminum tensile coupons for strain measurements. The measured strains from the POF sensors were compared to extensometer measurements for validation. The phase response of the interferometer was measured with a 3 × 3 coupler interrogator. The coupler arrangement was configured to permit the extraction of potential intensity changes in the sensor arm. The phase-shift–strain response of the POF sensors was repeatable for the loading and unloading measurements. The nonlinearity of the phase-shift–strain response was greater than that measured during pure tensile loading of the POF, presumably due to the behavior of the adhesive between the optical fiber and the aluminum coupons.}, number={7}, journal={MEASUREMENT SCIENCE AND TECHNOLOGY}, author={Abdi, Omid and Peters, Kara and Kowalsky, Mervyn and Hassan, Tasnim}, year={2011}, month={Jul} }
 @inproceedings{krishna_hassan_2010, title={Constituitive modeling for simulating a broad set of uniaxial and multiaxial cyclic and ratcheting responses}, DOI={10.1115/pvp2009-77816}, abstractNote={A set of cyclic and ratcheting experimental responses obtained under proportional to various degrees of nonproportional loading cycles are simulated using the modified Chaboche model in its rate-independent and rate-dependent forms. Features of the modified Chaboche nonlinear-kinematic hardening model needed for simulating cyclic hardening-softening, cyclic relaxation and ratcheting responses under uniaxial and multiaxial loading are elaborated. Significance of “rate-dependent” and novel “back stress shift” modeling features in improving the hysteresis loop and ratcheting rate simulations are demonstrated. Influence of the isotropic and kinematic hardening parameters in improving the multiaxial ratcheting response simulation by the modified Chaboche model are illustrated.}, booktitle={Computer Applications/Technology and Bolted Joints}, author={Krishna, S. and Hassan, T.}, year={2010}, pages={289–293} }
 @inproceedings{fulmer_kowalsky_nau_hassan_2010, title={Ductility of Welded Steel Pile to Steel Cap Beam Connections}, ISBN={9780784411308}, url={http://dx.doi.org/10.1061/41130(369)21}, DOI={10.1061/41130(369)21}, abstractNote={This paper discusses the seismic behavior of a bridge bent system that consists of round HSS piles welded to a steel HP section cap beam. Past practice has typically utilized a simple fillet weld with no backer ring to complete the connection between the pile and cap beam. The results of the research indicate that the overall ductility capacity of this system is controlled by the configuration of the welded connection between the piles and cap beam. Due to the lack of prior knowledge concerning this type of connection, six full scale bridge bent tests have been conducted at North Carolina State University’s Constructed Facilities Laboratory to evaluate the performance of the system when subjected to incremental simulated seismic loading. The two main goals of the research were to first evaluate the behavior of the system with a fillet weld which mimics the current typical design practice, and secondly to improve performance by investigating alternative weld configurations and connection details. The results indicate that the use of a simple fillet weld led to connection failure at a low ductility level rendering the detail inadequate for even moderate seismic regions. Subsequent tests showed that the use of other weld configurations, such as full joint penetration welds, improved the capabilities of the system but were still inadequate for higher seismic regions. However, promising results were obtained from a connection in which the flexural hinge region was relocated away from the pile to cap beam connection weld. This connection system remained essentially elastic at the pile to cap beam interface, which allowed for a more ductile base metal failure away from the connection.}, booktitle={Structures Congress 2010}, publisher={American Society of Civil Engineers}, author={Fulmer, S. J. and Kowalsky, M. J. and Nau, J. M. and Hassan, T.}, year={2010}, month={May} }
 @inproceedings{cheng_hassan_2010, title={Residual stress and strain responses of welded piping joints under low-cycle fatigue loading}, DOI={10.1115/pvp2009-77813}, abstractNote={It is well known that residual stress of welded joints influence their fatigue lives. This influence of residual stress is manifested through strain ratcheting response at the weld toe. Among many other reasons, strain ratcheting at the weld toe is anticipated to be a reason of many premature fatigue failure of welded joints. Hence, accurate simulations of weld toe residual stress and strain responses are essential for fatigue life simulation of welded joints. This paper presents results form an ongoing study on fatigue failure of welded piping joints. A modeling scheme for simulating weld toe residual stress and strain response is developed. Uncoupled, thermo-mechanical, finite element analyses are employed for imitating the welding procedure, and thereby simulating the temperature history during welding and initial residual stresses. Simulated residual stresses are validated by comparing against the measured residual stresses. Finite element simulations indicate that both residual stress and resulting strain responses near the weld toe are the key factors in inducing fatigue cracks at the weld toe. Research needs in revealing the fatigue failure mechanisms at the weld toe are discussed.}, booktitle={ASME Pressure Vessels and Piping Conference 2009, vol 1: Codes and Standards}, author={Cheng, P. Y. and Hassan, T.}, year={2010}, pages={95–101} }
 @inproceedings{hassan_rahman_2010, title={Simulation of ratcheting responses of elbow piping components}, DOI={10.1115/pvp2009-77819}, abstractNote={Ratcheting damage accumulation in piping components may occur under repeated reversals of loading induced by earthquakes, mechanical and thermal operating conditions, and other extreme loading conditions. Ratcheting damage accumulation can cause failure of structures through fatigue cracks or plastic collapse. A major challenge in structural mechanics is the prediction of ratcheting responses of structures under various cyclic loading conditions. Accurate prediction of ratcheting-fatigue and ratcheting-collapse of elbow components is imperative in order to incorporate the ratcheting related failures into the ASME design Code in a rational manner. This would require predictions of both local (stress-strain) and global (load-deflection) responses simultaneously. Towards achieving this goal, a set of experimental responses of elbow piping components is developed. Advanced cyclic plasticity models, such as, modified Chaboche, Ohno-Wang, modified Ohno-Wang and Abdel Karim-Ohno, are implemented into ANSYS for simulating the experimental responses. Results from the experimental and simulation studies are presented in order to demonstrate the state of structural ratcheting response simulation by these models.}, booktitle={ASME Pressure Vessels and Piping Conference 2009, vol 1: Codes and Standards}, author={Hassan, T. and Rahman, S. M.}, year={2010}, pages={103–108} }
 @article{kiesel_peters_hassan_kowalsky_2009, title={Calibration of a single-mode polymer optical fiber large-strain sensor}, volume={20}, ISSN={["1361-6501"]}, DOI={10.1088/0957-0233/20/3/034016}, abstractNote={We calibrate the phase shift as a function of applied displacement in a polymethylmethacrylate (PMMA) single-mode optical fiber interferometer, operating at a wavelength of 632.8 nm. The phase sensitivity is measured up to 15.8% nominal strain in the fiber. The measured phase–displacement response is compared to a previous analytical formulation for the large deformation response of the polymer optical fiber strain sensor. The formulation includes both the finite deformation of the optical fiber and nonlinear strain-optic effects at large deformations. Using previously measured values for the linear and nonlinear mechanical response of the fiber, the nonlinear strain-optic effects are calibrated from the current experimental data. This calibration demonstrates that the nonlinearities in the strain-optic effect are of the same order of magnitude as those in the mechanical response of the PMMA optical fiber sensor.}, number={3}, journal={MEASUREMENT SCIENCE AND TECHNOLOGY}, author={Kiesel, Sharon and Peters, Kara and Hassan, Tasnim and Kowalsky, Mervyn}, year={2009}, month={Mar} }
 @article{abdi_wong_hassan_peters_kowalsky_2009, title={Cleaving of solid single mode polymer optical fiber for strain sensor applications}, volume={282}, ISSN={["1873-0310"]}, DOI={10.1016/j.optcom.2008.11.046}, abstractNote={Single mode polymer optical fibers (smPOFs) can be applied for measuring large strains in numerous applications, such as civil engineering infrastructure assessment and health monitoring. Because of the large light attenuation of solid smPOFs, small lengths of the fiber would need to be coupled to silica optical fibers (SOFs) for practical applications of the smPOF as a strain sensor. This coupling requires smooth cleaving of the smPOFs. In this work, several cleaving techniques previously demonstrated to provide smooth cross-sections of multimode POFs were applied to the smPOF. From these techniques, hot-knife cutting was determined to be a feasible method for cleaving when the blade was heated to 80 °C and the smPOF heated in the range of 30 °C to 40 °C. In addition, focused ion beam machining which produces high-precision cleaves of the solid smPOF cross-section, was performed to set a bench mark and thus evaluate the quality of cleaving from other methods used in this study.}, number={5}, journal={OPTICS COMMUNICATIONS}, author={Abdi, O. and Wong, K. C. and Hassan, T. and Peters, K. J. and Kowalsky, M. J.}, year={2009}, month={Mar}, pages={856–861} }
 @article{krishna_hassan_ben naceur_sai_cailletaud_2009, title={Macro versus micro-scale constitutive models in simulating proportional and nonproportional cyclic and ratcheting responses of stainless steel 304}, volume={25}, ISSN={["1879-2154"]}, DOI={10.1016/j.ijplas.2008.12.009}, abstractNote={A recent study by Hassan et al. [Hassan, T., Taleb, L., Krishna, S., 2008. Influences of nonproportional loading paths on ratcheting responses and simulations by two recent cyclic plasticity models. Int. J. Plasticity, 24, 1863–1889.] demonstrated that some of the nonproportional ratcheting responses under stress-controlled loading histories cannot be simulated reasonably by two recent cyclic plasticity models. Two major drawbacks of the models identified were: (i) the stainless steel 304 demonstrated cyclic hardening under strain-controlled loading whereas cyclic softening under stress-controlled loading, which depends on the strain-range and which the existing models cannot describe; (ii) the change in biaxial ratcheting responses due to the change in the degree of nonproportionality were not simulated well by the models. Motivated by these findings, two modified cyclic plasticity models are evaluated in predicting a broad set of cyclic and ratcheting response of stainless steel 304. The experimental responses used in evaluating the modified models included both proportional (uniaxial) and nonproportional (biaxial) loading responses from Hassan and Kyriakides [Hassan, T., Kyriakides, S., 1994a. Ratcheting of cyclically hardening and softening materials. Part I: uniaxial behavior. Int. J. Plasticity, 10, 149–184; Hassan, T., Kyriakides, S., 1994b. Ratcheting of cyclically hardening and softening materials. Part II: multiaxial behavior. Int. J. Plasticity, 10, 185–212.] and Hassan et al. [Hassan, T., Taleb, L., Krishna, S., 2008. Influences of nonproportional loading paths on ratcheting responses and simulations by two recent cyclic plasticity models. Int. J. Plasticity, 24, 1863–1889.] The first model studied is a macro-scale, phenomenological, constitutive model originally proposed by Chaboche et al. [Chaboche, J.L., Dang-Van, K., Cordier, G., 1979. Modelization of the strain memory effect on the cyclic hardening of 316 stainless steel. In: Proceedings of the Fifth International Conference on SMiRT, Div. L, Berlin, Germany, L11/3.]. This model was systematically modified for incorporating strain-range dependent cyclic hardening–softening, and proportional and nonproportional loading memory parameters. The second model evaluated is a polycrystalline model originally proposed by Cailletaud [Cailletaud, G., 1992. A micromechanical approach to inelastic behavior of metals. Int. J. Plasticity, 8, 55–73.] based on crystalline slip mechanisms. These two models are scrutinized against simulating hysteresis loop shape, cyclic hardening–softening, cross-effect, cyclic relaxation, subsequent cyclic softening and finally a broad set of ratcheting responses under uniaxial and biaxial loading histories. The modeling features which improved simulations for these responses are elaborated in the paper. In addition, a novel technique for simulating both the monotonic and cyclic responses with one set of model parameters is developed and validated.}, number={10}, journal={INTERNATIONAL JOURNAL OF PLASTICITY}, author={Krishna, Shree and Hassan, Tasnim and Ben Naceur, Ilyes and Sai, Kacem and Cailletaud, Georges}, year={2009}, month={Oct}, pages={1910–1949} }
 @article{montejo_kowalsky_hassan_2009, title={Seismic behavior of flexural dominated reinforced concrete bridge columns at low temperatures}, volume={23}, DOI={10.1061/(ASCE)0887-381X(2009)23:1(18)}, abstractNote={This paper presents the results from Phase II of an experimental study on the behavior of reinforced concrete bridge columns in cold seismicly active regions. Six half-scale circular reinforced concrete columns, designed to be flex- ural dominated, were tested under reversed cyclic loading while subjected to temperatures ranging from 36°C 33°F to 22°C 72°F. Four of the units tested were reinforced concrete filled steel tube RCFST columns and the other two were ordinary reinforced concrete columns. Results obtained reiterated the observations made in Phase I, which is that low temperatures cause an increase in the flexural strength and initial stiffness as well as a reduction in the spread of plasticity and displacement capacity of the column. Another important observa- tion made was that the plastic hinge length is drastically reduced in the RCFST units compromising the displacement capacity of this type of column even at room temperature conditions. Current predictive models were revised and modi- fied to account for the low-temperature effect. DOI: 10.1061/ASCE0887-381X200923:118 CE Database subject headings: Temperature effects; Reinforced concrete; Tubes; Seismic effects; Buckling; Bridges; Concrete columns.}, number={1}, journal={Journal of Cold Regions Engineering}, author={Montejo, L. A. and Kowalsky, M. J. and Hassan, T.}, year={2009}, pages={18–42} }
 @article{montejo_kowalsky_hassan_2009, title={Seismic behavior of shear-dominated reinforced concrete columns at low temperatures}, volume={106}, DOI={10.14359/56610}, abstractNote={Extreme cyclic load reversals and very low temperatures can cause brittle failure of structures in seismically active cold regions. This study seeks to identify the effect of low temperatures on the seismic behavior of shear-dominated columns. Two pairs of reinforced concrete squat columns were tested under cyclic load reversals while subjected to freezing (-36 °C [-33 °F]) and room temperatures (22 °C [72 °F]). Findings showed that cold specimens exhibited an increase in the shear strength and elastic stiffness. The experimental results were used to evaluate existing models for assessment and design of shear strength. Results suggest that current models are conservative for low temperature conditions even if the appropriate low temperature material properties are taken into account.}, number={4}, journal={ACI Structural Journal}, author={Montejo, L. A. and Kowalsky, M. J. and Hassan, T.}, year={2009}, pages={445–454} }
 @article{montejo_sloan_kowalsky_hassan_2008, title={Cyclic response of reinforced concrete members at low temperatures}, volume={22}, DOI={10.1061/(ASCE)0887-381X(2008)22:3(79)}, abstractNote={An experimental study was undertaken to investigate the influence of cold temperatures on the seismic behavior of reinforced concrete members. This paper summarizes the results of Phase I of a multiphase research project that consisted of the reversed cyclic testing of four identical large scale reinforced concrete circular columns subjected to temperatures ranging from −40°C (−40°F) to 20°C (68°F) . An extensive literary review is also presented. It was found that most of the past research focused on the material level, i.e., the independent behavior of plain concrete and reinforcing bars. Data collected from past works were complemented with the results obtained from the material tests performed in this research, and used to develop empirical equations for the estimation of the mechanical properties of concrete and steel reinforcement at low temperature. Past research shows an increase in strength without any loss in the deformation capacity of plain concrete and reinforcing steel bars tested at low tem...}, number={3}, journal={Journal of Cold Regions Engineering}, author={Montejo, L. A. and Sloan, J. E. and Kowalsky, M. J. and Hassan, T.}, year={2008}, pages={79–102} }
 @article{rahman_hassan_corona_2008, title={Evaluation of cyclic plasticity models in ratcheting simulation of straight pipes under cyclic bending and steady internal pressure}, volume={24}, ISSN={["1879-2154"]}, DOI={10.1016/j.ijplas.2008.02.010}, abstractNote={This paper evaluates seven cyclic plasticity models for structural ratcheting response simulations. The models evaluated are bilinear (Prager), multilinear (Besseling), Chaboche, Ohno–Wang, Abdel Karim–Ohno, modified Chaboche (Bari and Hassan) and modified Ohno–Wang (Chen and Jiao). The first three models are already available in the ANSYS finite element package, whereas the last four were implemented into ANSYS for this study. Experimental responses of straight steel pipes under cyclic bending with symmetric end rotation history and steady internal pressure were recorded for the model evaluation study. It is demonstrated that when the model parameters are determined from the material response data, none of the models evaluated perform satisfactorily in simulating the straight pipe diameter change and circumferential strain ratcheting responses. A detailed parameter sensitivity study with the modified Chaboche model was conducted to identify the parameters that influence the ratcheting simulations and to determine the ranges of the parameter values over which a genetic algorithm can search for refinement of these values. The refined parameter values improved the simulations of straight pipe ratcheting responses, but the simulations still are not acceptable. Further, improvement in cyclic plasticity modeling and incorporation of structural features, like residual stresses and anisotropy of materials in the analysis will be essential for advancement of low-cycle fatigue response simulations of structures.}, number={10}, journal={INTERNATIONAL JOURNAL OF PLASTICITY}, author={Rahman, Syed M. and Hassan, Tasnim and Corona, Edmundo}, year={2008}, month={Oct}, pages={1756–1791} }
 @article{hassan_taleb_krishna_2008, title={Influence of non-proportional loading on ratcheting responses and simulations by two recent cyclic plasticity models}, volume={24}, ISSN={["1879-2154"]}, DOI={10.1016/j.ijplas.2008.04.008}, abstractNote={Aubin and her coworkers conducted a unique set of experiments demonstrating the influence of loading non-proportionality on ratcheting responses of duplex stainless steel. In order to further explore their new observation, a set of experiments was conducted on stainless steel (SS) 304L under various biaxial stress-controlled non-proportional histories. This new set of data reiterated Aubin and her coworkers’ observation and illustrated many new responses critical to model development and validation. Two recent and different classes of cyclic plasticity models, the modified Chaboche model proposed by Bari and Hassan and the version of the multi-mechanism model proposed by Taleb and Cailletaud, are evaluated in terms of their simulations of the SS304L non-proportional ratcheting responses. A modeling scheme for non-proportional ratcheting responses using the kinematic hardening rule parameters in addition to the conventionally used isotropic hardening rule parameter (yield surface size change) in the modified Chaboche model is evaluated. Strengths and weaknesses of the models in simulating the non-proportional ratcheting responses are identified. Further improvements of these models needed for improving the non-proportional ratcheting simulations are suggested in the paper.}, number={10}, journal={INTERNATIONAL JOURNAL OF PLASTICITY}, author={Hassan, Tasnim and Taleb, Lakhdar and Krishna, Shree}, year={2008}, month={Oct}, pages={1863–1889} }
 @article{kiesel_peters_hassan_kowalsky_2008, title={Large deformation in-fiber polymer optical fiber sensor}, volume={20}, ISSN={["1941-0174"]}, DOI={10.1109/LPT.2008.916929}, abstractNote={We demonstrate the measurement of the phase shift in a polymethylmethacrylate single-mode optical fiber interferometer, operating at a wavelength of 632.8 nm, up to 15.8% nominal strain in the fiber. The phase-displacement sensitivity is measured to be 1.39 x10 radldrm-1 for this strain range. This strain range is well beyond the yield strain of the polymer fiber and that previously measured for polymer Bragg gratings and silica optical fiber sensors.}, number={5-8}, journal={IEEE PHOTONICS TECHNOLOGY LETTERS}, author={Kiesel, Sharon and Peters, Kara and Hassan, Tasnim and Kowalsky, Mervyn}, year={2008}, pages={416–418} }
 @article{kiesel_peters_hassan_kowalsky_2007, title={Behaviour of intrinsic polymer optical fibre sensor for large-strain applications}, volume={18}, ISSN={["1361-6501"]}, DOI={10.1088/0957-0233/18/10/S16}, abstractNote={This paper derives the phase response of a single-mode polymer optical fibre for large-strain applications. The role of the finite deformation of the optical fibre and nonlinear strain optic effects are derived using a second order strain assumption and shown to be important at strain magnitudes as small as 1%. In addition, the role of the core radius change on the propagation constant is derived, but it is shown to be negligible as compared to the previous effects. It is shown that four mechanical and six opto-mechanical parameters must be calibrated to apply the sensor under arbitrary axial and transverse loading. The mechanical nonlinearity of a typical single-mode polymer optical fibre is experimentally measured in axial tension and is shown to be more significant than that of their silica counterpart. The mechanical parameters of the single-mode polymer optical fibre are also measured for a variety of strain rates, from which it is demonstrated that the strain rate has a strong influence on yield stress and strain. The calibrated constants themselves are less affected by strain rate.}, number={10}, journal={MEASUREMENT SCIENCE AND TECHNOLOGY}, author={Kiesel, Sharon and Peters, Kara and Hassan, Tasnim and Kowalsky, Mervyn}, year={2007}, month={Oct}, pages={3144–3154} }
 @article{hassan_lucier_rizkalla_zia_2007, title={Modeling of L-shaped, precast, prestressed concrete spandrels}, volume={52}, ISSN={["0887-9672"]}, DOI={10.15554/pcij.03012007.78.92}, abstractNote={This paper presents results of nonlinear finite element (FE) analyses conducted to model the behavior of L-shaped, precast, prestressed concrete spandrels built with open web reinforcement. The FE model was calibrated using experimental results from recent tests of slender, L-shaped, precast, prestressed concrete spandrels. Detailed correlative studies between analytical and experimental results are provided, demonstrating the capability of the FE program to describe the observed experimental behavior. The feasibility of using open web reinforcement in compact, L-shaped, precast, prestressed concrete spandrels to achieve a more construction-friendly reinforcement scheme is also examined. Five different web reinforcement configurations for the compact spandrels were studied in order to evaluate the contribution of closed stirrups to the spandrels' shear-torsion behavior. The behavior, ultimate load-carrying capacity, and mode of failure of both the slender and compact L-shaped precast, prestressed concrete spandrels are presented. For loading values near the ultimate, the out-of-plane bending behavior of compact, L-shaped, precast, prestressed concrete spandrels is strongly influenced by the web-reinforcement configuration. Results from the analysis show that for long-span, compact spandrels, open web reinforcement can be used effectively to resist torsional forces throughout the member.}, number={2}, journal={PCI JOURNAL}, author={Hassan, Tarek and Lucier, Gregory and Rizkalla, Sarni and Zia, Paul}, year={2007}, pages={78–92} }
 @inproceedings{humphreys_hassan_hubbard_2006, title={Fatigue and residual stress responses of welded piping joints}, booktitle={Trends in Welding Research, Proceedings}, author={Humphreys, A. E. and Hassan, T. and Hubbard, C. R.}, year={2006}, pages={595–600} }
 @article{hassan_rizkalla_2004, title={Bond mechanism of near-surface-mounted fiber-reinforced polymer bars for flexural strengthening of concrete structures}, volume={101}, DOI={10.14359/13458}, abstractNote={This paper presents both experimental and analytical investigations undertaken to evaluate bond characteristics of near-sutfacemounted (NSM) carbon fiber-reinforced polymer (CFRP) bars. A total of eight concrete beams. strengthened with NSM CFRP bars, were tested under monotonic static loading. Different embedment lengths are studied to determine the development length of the fiber-reinforced polymer (FRP) reinforcement. The performance of two different adhesives used to bond the bars to the surrounding concrete is examined. A general methodology to evaluate the development length of NSM FRP bars of different configurations and types of fibers is presented. A quantitative criterion governing debonding failure is established. The proposed bond model assumes linear elastic behavior jar the concrete, adhesive, and the NSM FRP bars, following the same philosophy oj the ACI provisions for bond analysis and design. The proposed analytical model is validated by comparing the predicted values with test results as well as to nonlinear finite element modelling. The influence of key parameters, including the thickness of the adhesive cover, groove width, groove spacing, and inte171ai steel reinforcement configuration, are discussed.}, number={6}, journal={ACI Structural Journal}, author={Hassan, T. K. and Rizkalla, S. H.}, year={2004}, pages={830–839} }
 @article{hassan_rizkalla_2003, title={Investigation of bond in concrete structures strengthened with near surface mounted carbon fiber reinforced polymer strips}, volume={7}, DOI={10.1061/(ASCE)1090-0268(2003)7:3(248)}, abstractNote={Fiber reinforced polymer (FRP) materials are currently produced in different configurations and are widely used for the strengthening and retrofitting of concrete structures and bridges. Recently, considerable research has been directed to characterize the use of FRP bars and strips as near surface mounted reinforcement, primarily for strengthening applications. Nevertheless, in-depth understanding of the bond mechanism is still a challenging issue. This paper presents both experimental and analytical investigations undertaken to evaluate bond characteristics of near surface mounted carbon FRP (CFRP) strips. A total of nine concrete beams, strengthened with near surface mounted CFRP strips were constructed and tested under monotonic static loading. Different embedment lengths were used to evaluate the development length needed for effective use of near surface mounted CFRP strips. A closed-form analytical solution is proposed to predict the interfacial shear stresses. The model is validated by comparing t...}, number={3}, journal={Journal of Composites for Construction}, author={Hassan, T. and Rizkalla, S.}, year={2003}, pages={248–257} }
 @article{bari_hassan_2002, title={An advancement in cyclic plasticity modeling for multiaxial ratcheting simulation}, volume={18}, ISSN={["0749-6419"]}, DOI={10.1016/S0749-6419(01)00012-2}, abstractNote={In a search for a constitutive model for ratcheting simulations, the models by Chaboche, Ohno–Wang, McDowell, Jiang–Sehitoglu, Voyiadjis–Basuroychowdhury and AbdelKarim–Ohno are evaluated against a set of uniaxial and biaxial ratcheting responses. With the assumption of invariant shape of the yield surface during plastic loading, the ratcheting simulations for uniaxial loading are primarily a function of the plastic modulus calculation, whereas the simulations for multiaxial loading are sensitive to the kinematic hardening rule of a model. This characteristic of the above mentioned models is elaborated in this paper. It is demonstrated that if all parameters of the kinematic hardening rule are determined from uniaxial responses only, these parameters primarily enable a better plastic modulus calculation. However, in this case the role of the kinematic hardening rule in representing the ratcheting responses for multiaxial loading is under-appreciated. This realization motivated many researchers to incorporate multiaxial load dependent terms or parameters into the kinematic hardening rule. This paper evaluates some of these modified rules and finds that none is general enough to simulate the ratcheting responses consistently for the experiments considered. A modified kinematic hardening rule is proposed using the idea of Delobelle and his co-workers in the framework of the Chaboche model. This new rule introduces only one multiaxial load dependent parameter to the Chaboche model, but performs the best in simulating all the ratcheting responses considered.}, number={7}, journal={INTERNATIONAL JOURNAL OF PLASTICITY}, author={Bari, S and Hassan, T}, year={2002}, pages={873–894} }
 @article{hassan_rizkalla_2002, title={Flexural strengthening of prestressed bridge slabs with FRP systems}, volume={47}, ISSN={["0887-9672"]}, DOI={10.15554/pcij.01012002.76.93}, abstractNote={Fiber reinforced polymer (FRP) materials offer great potential for cost effective retrofitting of concrete structures. In response for the growing need for strengthening and rehabilitation of concrete structures and bridges, an experimental program was conducted to investigate the feasibility of using different strengthening techniques as well as different types of FRP for strengthening prestressed concrete members. Half scale models of a prestressed concrete bridge were constructed and tested to failure. The test specimens consisted of one simple span and two overhanging cantilevers. Each specimen was tested three times using a different load location in each case. The applicability of a nonlinear finite element analysis of post-tensioned bridge slabs strengthened with near surface mounted FRP reinforcement is enumerated.}, number={1}, journal={PCI JOURNAL}, author={Hassan, T and Rizkalla, S}, year={2002}, pages={76–93} }
 @article{bari_hassan_2001, title={Kinematic hardening rules in uncoupled modeling for multiaxial ratcheting simulation}, volume={17}, ISSN={["0749-6419"]}, DOI={10.1016/S0749-6419(00)00031-0}, abstractNote={An earlier paper by the authors evaluated the performance of several coupled models in simulating a series of uniaxial and biaxial ratcheting responses. This paper evaluates the performance of various kinematic hardening rules in an uncoupled model for the same set of ratcheting responses. A modified version of the Dafalias–Popov uncoupled model has been demonstrated to perform well for uniaxial ratcheting simulation. However, its performance in multiaxial ratcheting simulation is significantly influenced by the kinematic hardening rules employed in the model. Performances of eight different kinematic hardening rules, when engaged with the modified Dafalias–Popov model, are evaluated against a series of rate-independent multiaxial ratcheting responses of cyclically stabilized carbon steels. The kinematic hardening rules proposed by Armstrong–Frederick, Voyiadjis–Sivakumar, Phillips, Tseng–Lee, Kaneko, Xia–Ellyin, Chaboche and Ohno–Wang are examined. The Armstrong–Frederick rule performs reasonably for one type of the biaxial ratcheting response, but fails in others. The Voyiadjis–Sivakumar rule and its constituents, the Phillips and the Tseng–Lee rules, can not simulate the biaxial ratcheting responses. The Kaneko rule, composed of the Ziegler and the prestress directions, and the Xia–Ellyin rule, composed of the Ziegler and Mroz directions, also fail to simulate the biaxial ratcheting responses. The Chaboche rule, with three decomposed Armstrong–Frederick rules, performs the best for the whole set of ratcheting responses. The Ohno–Wang rule performs well for the data set, except for one biaxial response where it predicts shakedown with subsequent reversal of ratcheting.}, number={7}, journal={INTERNATIONAL JOURNAL OF PLASTICITY}, author={Bari, S and Hassan, T}, year={2001}, pages={885–905} }
 @article{hassan_liu_2001, title={On the difference of fatigue strengths from rotating bending, four-point bending, and cantilever bending tests}, volume={78}, ISSN={["0308-0161"]}, DOI={10.1016/S0308-0161(00)00080-6}, abstractNote={Comparisons of piping fatigue data demonstrate that the fatigue strength from rotating bending tests is lower than that from cantilever and four-point bending tests, especially in the low-cycle fatigue life range. The lower strength from the rotating bending test is generally believed to result from the fact that in this test all the points on the piping surface are subjected to the maximum stress range. Consequently, the weakest point in the specimen always initiates and causes failure. On the contrary, in cantilever and four-point bending tests, the maximum stress range occurs only at the top and bottom extreme fibers, which may not contain the weakest point in the specimen. Hence, the pipes in rotating bending tests usually fail earlier in comparison with the other two tests. Finite element analyses for the three tests revealed another and more compelling reason for the lower fatigue strength from the rotating bending test. The results demonstrated that, for the same prescribed bending moment range, the inelastic strain range in rotating bending is higher than the ranges in four-point and cantilever bending tests. Experimental data also demonstrate a similar trend. The new observation suggests that fatigue data from these three tests should be analyzed or compared in terms of strain range, instead of nominal stress range.}, number={1}, journal={INTERNATIONAL JOURNAL OF PRESSURE VESSELS AND PIPING}, author={Hassan, T and Liu, Z}, year={2001}, month={Jan}, pages={19–30} }
 @article{bari_hassan_2000, title={Anatomy of coupled constitutive models for ratcheting simulation}, volume={16}, ISSN={["0749-6419"]}, DOI={10.1016/S0749-6419(99)00059-5}, abstractNote={This paper critically evaluates the performance of five constitutive models in predicting ratcheting responses of carbon steel for a broad set of uniaxial and biaxial loading histories. The models proposed by Prager, Armstrong and Frederick, Chaboche, Ohno-Wang and Guionnet are examined. Reasons for success and failure in simulating ratcheting by these models are elaborated. The bilinear Prager and the nonlinear Armstrong-Frederick models are found to be inadequate in simulating ratcheting responses. The Chaboche and Ohno-Wang models perform quite well in predicting uniaxial ratcheting responses; however, they consistently overpredict the biaxial ratcheting responses. The Guionnet model simulates one set of biaxial ratcheting responses very well, but fails to simulate uniaxial and other biaxial ratcheting responses. Similar to many earlier studies, this study also indicates a strong influence of the kinematic hardening rule or backstress direction on multiaxial ratcheting simulation. Incorporation of parameters dependent on multiaxial ratcheting responses, while dormant for uniaxial responses, into Chaboche-type kinematic hardening rules may be conducive to improve their multiaxial ratcheting simulations. The uncoupling of the kinematic hardening rule from the plastic modulus calculation is another potentially viable alternative. The best option to achieve a robust model for ratcheting simulations seems to be the incorporation of yield surface shape change (formative hardening) in the cyclic plasticity model.}, number={3-4}, journal={INTERNATIONAL JOURNAL OF PLASTICITY}, author={Bari, S and Hassan, T}, year={2000}, pages={381–409} }
 @article{hassan_zhu_matzen_1998, title={Improved ratcheting analysis of piping components}, volume={75}, ISSN={["0308-0161"]}, DOI={10.1016/S0308-0161(98)00070-2}, abstractNote={It is well known that ratcheting (defined as the accumulation of deformation with cycles) can reduce fatigue life or cause failure of piping components or systems subjected to seismic or other cyclic loads. This phenomenon is sometime referred to as fatigue-ratcheting, which is yet to be understood clearly. Commercial finite element codes cannot accurately simulate the ratcheting responses recorded in tests on piping components or systems. One of the reasons for this deficiency has been traced to inadequate constitutive models in the existing analysis codes. To overcome this deficiency, an improved cyclic plasticity model, composed of the Armstrong–Frederick kinematic hardening rule and the Drucker–Palgen plastic modulus equation, is incorporated into an ANSYS material model subroutine. The modified ANSYS program is verified against three sets of experimental results. The simulations from this modified ANSYS show a significant improvement over the unmodified ANSYS and the ABAQUS codes.}, number={8}, journal={INTERNATIONAL JOURNAL OF PRESSURE VESSELS AND PIPING}, author={Hassan, T and Zhu, Y and Matzen, VC}, year={1998}, month={Jul}, pages={643–652} }