@article{idolor_berkowitz_guha_grace_2022, title={Nondestructive examination of polymer composites by analysis of polymer-water interactions and damage-dependent hysteresis}, volume={287}, ISSN={["1879-1085"]}, DOI={10.1016/j.compstruct.2022.115377}, abstractNote={Polymer composites are currently replacing metals in applications requiring design flexibility, high strength-to-weight ratio, and corrosion resistance. However, the damage modes in these materials are very different from metals and require specialized techniques to detect internal flaws which may exist even in the absence of visible surface damage. This study proposes a technique for damage detection in polymer composites which uses naturally absorbed moisture as an ‘imaging’ agent. The locally higher concentration of water in the ‘free’ state at damaged regions and the tendency of such water to quickly migrate to and from damage sites—exhibiting damage-dependent hysteresis—is leveraged for damage detection. To identify damaged regions, a machine learning approach is adopted using logistic regression to classify local regions as ‘undamaged’ or ‘damaged’. New possibilities resulting from higher sensitivity levels achievable by damage-dependent hysteresis are highlighted, providing a pathway to field deployment of the novel damage detection technique.}, journal={COMPOSITE STRUCTURES}, author={Idolor, Ogheneovo and Berkowitz, Katherine and Guha, Rishabh Debraj and Grace, Landon}, year={2022}, month={May} }
 @article{idolor_guha_berkowitz_grace_2021, title={An experimental study of the dynamic molecular state of transient moisture in damaged polymer composites}, volume={42}, ISSN={["1548-0569"]}, url={https://doi.org/10.1002/pc.26066}, DOI={10.1002/pc.26066}, abstractNote={AbstractPolymeric composites absorb measurable amounts of moisture from their environment in almost all operating conditions. This absorbed moisture exists either in the “free” state, without any interactions, or in the “bound” state—interacting with the polymer matrix via secondary bonding mechanisms. The ratio and distribution of these water states within a moisture‐contaminated polymer composite are sensitive to physical damage. However, the water state distribution is also affected by variations in total water content resulting from humidity or precipitation‐driven fluctuations in the ambient environment, which could confound the ability to detect damage within the polymer matrix using this technique. To understand the effect of moisture content variations on water state distribution, low levels of barely visible impact damage were induced in epoxy/glass fiber composites. Spatial variations in polymer–water interactions were identified by their characteristic dielectric properties, measured using a split post dielectric resonator operating at 5 GHz. Gravimetric moisture uptake and relative permittivity were monitored during the absorption and desorption processes. Results indicate moisture absorption/desorption history has a significant effect on the sensitivity of damage detection using water state variations. Damage‐dependent hysteresis was observed in relative permittivity, highlighting an avenue by which the confounding effects of moisture absorption/desorption history may be mitigated.}, number={7}, journal={POLYMER COMPOSITES}, publisher={Wiley}, author={Idolor, Ogheneovo and Guha, Rishabh Debraj and Berkowitz, Katherine and Grace, Landon}, year={2021}, month={Jul}, pages={3391–3403} }
 @article{guha_idolor_berkowitz_pasquinelli_grace_2021, title={Exploring secondary interactions and the role of temperature in moisture-contaminated polymer networks through molecular simulations}, volume={17}, ISSN={["1744-6848"]}, url={https://doi.org/10.1039/D0SM02009E}, DOI={10.1039/D0SM02009E}, abstractNote={We investigated the effect of temperature variation on the secondary bonding interactions between absorbed moisture and epoxies with different morphologies using molecular dynamics simulations.}, number={10}, journal={SOFT MATTER}, publisher={Royal Society of Chemistry (RSC)}, author={Guha, Rishabh D. and Idolor, Ogheneovo and Berkowitz, Katherine and Pasquinelli, Melissa and Grace, Landon R.}, year={2021}, month={Mar}, pages={2942–2956} }
 @article{guha_idolor_berkowitz_pasquinelli_grace_2021, title={Exploring secondary interactions and the role of temperature in moisture-contaminated polymer networks through molecular simulations (vol 17, pg 2942, 2021)}, volume={5}, ISSN={["1744-6848"]}, DOI={10.1039/d1sm90100a}, abstractNote={Correction for ‘Exploring secondary interactions and the role of temperature in moisture-contaminated polymer networks through molecular simulations’ by Rishabh D. Guha et al., Soft Matter, 2021, 17, 2942–2956, DOI: 10.1039/D0SM02009E.}, journal={SOFT MATTER}, author={Guha, Rishabh D. and Idolor, Ogheneovo and Berkowitz, Katherine and Pasquinelli, Melissa and Grace, Landon R.}, year={2021}, month={May} }
 @article{idolor_guha_berkowitz_geiger_davenport_grace_2021, title={Polymer-water interactions and damage detection in polymer matrix composites}, volume={211}, ISSN={["1879-1069"]}, url={https://doi.org/10.1016/j.compositesb.2021.108637}, DOI={10.1016/j.compositesb.2021.108637}, abstractNote={Polymer matrix composites have a tendency to absorb measurable moisture in nearly all operating environments. This absorbed moisture either becomes bound to the polymer network via secondary bonding interactions or exists as free water with negligible interactions. Damage creates new internal free volume where water molecules can exist in the latter state. This study introduces a novel basis for non-destructive examination in polymer matrix composites which leverages locally-higher concentration of free water in damaged areas. Experiments involved impact-induced sub-surface damage in a laminate prior to moisture exposure. Polymer-water interaction—determining the free or bound state of water—was characterized by near-infrared spectroscopy and microwave-range relative permittivity. Results show a direct correlation between the extent of local damage and higher relative levels of free water at damage sites.}, journal={COMPOSITES PART B-ENGINEERING}, publisher={Elsevier BV}, author={Idolor, Ogheneovo and Guha, Rishabh Debraj and Berkowitz, Katherine and Geiger, Carl and Davenport, Matthew and Grace, Landon}, year={2021}, month={Apr} }
 @article{guha_idolor_grace_2020, title={An atomistic simulation study investigating the effect of varying network structure and polarity in a moisture contaminated epoxy network}, volume={179}, ISSN={["1879-0801"]}, DOI={10.1016/j.commatsci.2020.109683}, abstractNote={Absorbed moisture is a frequent contributor to performance loss in polymer-based composite structures operating in nearly all environments. However, the fundamental mechanisms that govern water-polymer interaction remain poorly understood. In this molecular dynamics study, the polarity and internal structure of an epoxy-based composite matrix was varied through manipulation of crosslink density. A commonly used epoxy-hardener combination (DGEBA- DETA) was chosen and four different models were created with crosslinking density of 20%, 51%, 65% and 81% respectively. The results indicate that the increase in crosslinking leads to a greater availability of polar sites with a concomitant rise in available free volume. The rise in network polarity aids the hydrogen bonding interactions between the absorbed water and the composite matrix, but the greater availability of free volume also allows water molecules to cluster together through mutual hydrogen bonding activity. This results in a subsequent decrease in moisture interaction with polar sites at very high crosslink densities. It was also found that the diffusivity and average dipole moment of the absorbed moisture are correlated with the state of water molecules and that a greater percentage of network-bonded molecules tends to lower both of these quantities. These results are consistent with previously published experimental results which have contemplated the dual nature of water molecules in an epoxy network. The results also highlight the potential of leveraging this phenomenon for non-destructive inspection of the physical and chemical state of a polymer network.}, journal={COMPUTATIONAL MATERIALS SCIENCE}, author={Guha, Rishabh Debraj and Idolor, Ogheneovo and Grace, Landon}, year={2020}, month={Jun} }
 @inproceedings{characterization of damage in polymer matrix composites by analyzing polymer-moisture interactions_2020, url={https://asnt.digitellinc.com/asnt/sessions/377/view}, booktitle={American Society for Nondestructive Testing Annual Conference 2020}, year={2020}, month={Nov} }
 @article{idolor_guha_berkowitz_grace_2020, title={Damage Detection in Polymer Matrix Composites by Analysis of Polymer-Water Interactions Using Near-Infrared Spectroscopy}, DOI={10.12783/asc35/34874}, abstractNote={Detection of early stage damage in polymer matrix composites (PMCs) is a challenge for current non-destructive examination (NDE) methods. New techniques based on sub-micron scale effects may enable identification and characterization of early, non-visible damage. Moisture, ubiquitous in most service environments and readily absorbed by PMCs, may present an opportunity for identifying and characterizing such damage. Water molecules absorbed naturally within PMCs exist either as 'bound water' (interacting with the polymer matrix via secondary bonding mechanisms), or as free water (having negligible external bonding interactions). Early-stage physical damage within a PMC takes the form of nano to micron scale free volume within the polymer matrix. This new free volume represents an opportunity for water to occupy and form clusters within the host polymer without directly interacting with the polymer matrix chemically or physically. Therefore, relative to a pristine region, a damaged region would contain a much higher concentration of free water. This altered spatial distribution of free water creates an opportunity for damage identification and characterization. This study explores this possibility by investigating the moisture state in undamaged and 3-joule impactdamaged 16-ply epoxy/glass fiber composite laminates. The specimens are exposed to moisture via immersion in deionized water to simulate long-term moisture exposure in a humid environment. The state of absorbed water is characterized quantitatively via near-infrared spectroscopy. Results show a significantly higher ratio of free-to-bound water in the 3-Joule damaged specimen when compared to the undamaged specimen at the same moisture content. This indicates the existence of free water within voids associated with matrix micro-cracks and delaminations created due to impact. Results from this study suggests that analysis of the nature of polymer-water interaction could be leveraged in developing a technique for damage detection even at very low damage levels in PMCs.}, journal={American Society for Composites 2020}, publisher={DEStech Publications, Inc.}, author={IDOLOR, OGHENEOVO and GUHA, RISHABH and BERKOWITZ, KATHERINE and GRACE, LANDON}, year={2020}, month={Sep} }
 @article{idolor_guha_bilich_grace_2019, title={2-Dimensional Mapping of Damage in Moisture Contaminated Polymer Composites Using Dielectric Properties}, DOI={10.12783/asc34/31312}, abstractNote={Polymer composite materials are increasingly being adopted in civil infrastructure, oil & gas, marine, automotive and aerospace industries. Achieving ubiquitous adoption would require new advances in development of non-destructive examination methods which are cost effective relative to composite structures under test, simple to utilize and sensitive enough to ascertain the structural integrity of composite parts, especially in the absence of visible damage. This study proposes a relatively simple method for non-destructive testing of polymer composites that detects 2-dimensional micro to nano-scale damage by analysis of the effects of polymer-moisture interactions on microwave-frequency dielectric properties. In this study, we apply this method to mapping of internal damage in two widely used aerospace composite laminates; a 12-ply bismaleimide/quartz and a 16-ply epoxy/7781 fiber glass composite laminate. Specimens are dried and then immersed in deionized water, to simulate long term exposure to humid environments while measuring gravimetric moisture uptake. Localized damage is induced in the laminate specimens via low velocity impact damage of 5 and 9 Joules. A split post dielectric resonator coupled with a vector network analyzer is used to determine the spatial variation in relative permittivity across the composite laminate. Generally, results show significant increase in relative permittivity towards the center of impact damage compared to surrounding undamaged areas. This increase is indicative of internal damage as a result of micro-crack formation around the point of impact. This new free volume in the damaged area is primarily occupied by free water; driving a local increase in the relative permittivity in the damaged area due to the locally-higher ratio of free to bound water.}, journal={American Society for Composites 2019}, publisher={DEStech Publications, Inc.}, author={IDOLOR, OGHENEOVO and GUHA, RISHABH and BILICH, LOGAN and GRACE, LANDON}, year={2019}, month={Oct} }
 @article{guha_idolor_grace_2019, title={Molecular Dynamics (MD) Simulation of a Polymer Composite Matrix with Varying Degree of Moisture: Investigation of Secondary Bonding Interactions}, DOI={10.12783/asc34/31367}, abstractNote={Epoxy-based polymer composites are widely used in structural applications. Environmental degradation through moisture absorption considerably deteriorates the performance of these materials, particularly in the aviation and automotive industries. Absorbed moisture behaves differently in pristine and damaged composites. In a pristine structure with fewer diffusion pathways and voids, absorption is generally less and there is a higher degree of interaction between the resin and water molecules, which limits water mobility. Conversely, in a damaged structure, voids and microcracks allow accelerated diffusion and a greater proportion of “free” molecular water. The ratio of free to bound water states can be exploited to characterize damage via the dielectric properties. Structures with greater damage, and thus greater proportion of free water molecules, will exhibit a higher relative permittivity than a pristine structure, independent of water content. Without damage, water is more restricted from rotating under the influence of an applied electromagnetic field, leading to a lower relative permittivity. To simulate this phenomenon on a molecular scale, MD simulations are performed to characterize the moisture interaction in a polymer composite matrix system with the same epoxy (DGEBA) and hardener (DETA) constituents but with varying moisture concentrations. The system is constructed using the MAPS Amorphous Builder and the simulations were performed in LAMMPS using the AMBER-Cornell force field. Cross-linking is executed based on a reaction cut-off distance and the desired degree of cross-linking using the crosslinker module in MAPS. The simulation results are then analyzed and different properties like the diffusion coefficients and dipole moment fluctuations are evaluated to establish a primary understanding of how moisture interacts with the polymer matrix. To gain a deeper insight into the distinction between “free” and “bound” water, the hydrogen bonds between different polar species are counted at varying moisture concentrations to understand how secondary bonding mechanisms influences diffusion and dielectric properties.}, journal={American Society for Composites 2019}, publisher={DEStech Publications, Inc.}, author={GUHA, RISHABH DEBRAJ and IDOLOR, OGHENEOVO and GRACE, LANDON}, year={2019}, month={Oct} }
 @article{idolor_guha_grace_2018, title={A Dielectric Resonant Cavity Method for Monitoring of Damage Progression in Moisture-Contaminated Composites}, DOI={10.12783/asc33/25963}, abstractNote={A method for monitoring of damage progression due to combined mechanical and hygroscopic loading in polymer composite materials is presented. Polymer-based materials have a tendency to absorb moisture from their operating environment. Dielectric properties of these materials are significantly affected by the total amount of absorbed moisture and the degree of its interaction with the host polymer. Bound water molecules which are restricted in their ability to rotate with an applied electromagnetic field contribute less to the bulk relative permittivity. 'Free' water molecules rotate without impediment and are therefore associated with a higher relative permittivity. The bulk relative permittivity as a function of total water content of a contaminated composite is a unique function of the internal physical and chemical characteristics of the specimen. Holding chemical contributions constant, physical characteristics dominate. Thus, relative permittivity provides insight into the physical state of composite, including amount of free space from processing-induced voids or, critically, the presence of physical damage such as cracks and voids across multiple length scales. Here, we demonstrate a method for leveraging this phenomenon to provide insight into the initiation and accumulation of physical damage in moisturecontaminated composites. This is accomplished using a split-post dielectric resonant technique operating in the low GHz frequency range, where dipolar contributions to relative permittivity dominate. Further, continuous and non-contact monitoring of relative permittivity is achieved by integrating a resonant cavity with a fatigue loading frame. Preliminary experimental assessment of this test method is supportive of its potential in damage tracking. Water-contaminated 12-ply bismaleimide (BMI) / quartz laminate specimens were tested in impact and flexural fatigue, while a 4-ply glass/epoxy laminate was tested in tensile fatigue while changes in relative permittivity were recorded. The results show a distinct rise in relative permittivity consistent with the expected magnitude and progression of damage in all cases.}, journal={American Society for Composites 2018}, publisher={DEStech Publications, Inc.}, author={IDOLOR, OGHENEOVO and GUHA, RISHABH and GRACE, LANDON}, year={2018}, month={Nov} }