@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{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{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} }
 @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_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{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{islam_rana_ahsan_ghani_2014, title={An optimized design of network arch bridge using global optimization algorithm}, volume={17}, number={2}, journal={Advances in Structural Engineering}, author={Islam, N. and Rana, S. and Ahsan, R. and Ghani, S. N.}, year={2014}, pages={197–210} }