@article{rajendran_ashmawi_zikry_2006, title={The modeling of the shock response of powdered ceramic materials}, volume={38}, ISSN={["1432-0924"]}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-33646550239&partnerID=MN8TOARS}, DOI={10.1007/s00466-005-0712-3}, number={1}, journal={COMPUTATIONAL MECHANICS}, author={Rajendran, AM and Ashmawi, WM and Zikry, MA}, year={2006}, month={Jun}, pages={1–13} }
 @article{ashmawi_zikry_wang_reeber_2004, title={Modeling of residual stresses for thermally strained GaN/Al2O3 heterostructures}, volume={266}, ISSN={["1873-5002"]}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-2442603520&partnerID=MN8TOARS}, DOI={10.1016/j.jcrysgro.2004.02.105}, abstractNote={A finite element model and a specialized constitutive formulation were used to predict the evolving interfacial thermal mismatch stresses and strains in gallium nitride/alumina epitaxial layered systems. The constitutive formulation was based on having the coefficients of thermal expansion vary as a function of temperature for both material systems, which were assumed to be transversely isotropic. Different layer configurations were investigated, and it is shown that layer geometry is controlled by the evolution of induced thermal mismatch properties and residual stresses.}, number={4}, journal={JOURNAL OF CRYSTAL GROWTH}, author={Ashmawi, WM and Zikry, MA and Wang, K and Reeber, RR}, year={2004}, month={Jun}, pages={415–422} }
 @article{ashmawi_zikry_2003, title={Grain boundary effects and void porosity evolution}, volume={35}, ISSN={["1872-7743"]}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-0037339303&partnerID=MN8TOARS}, DOI={10.1016/S0167-6636(02)00269-7}, abstractNote={A multiple-slip dislocation-density based constitutive crystalline formulation that is coupled to a kinematic scheme that accounts for grain boundary (GB) interfacial interactions with dislocation densities, and an internal porosity formulation have been used to predict how void porosity is affected by GB interactions, such as dislocation-density pile-ups at GB interfaces, partial and total dislocation-density transmission from one grain to neighboring grains, and dislocation-density absorption within GBs. Void nucleation and growth is represented by a single scalar that is a function of total dislocation density, stress triaxiality, accumulated plastic strains, and temperature. The proposed methodology provides an understanding of how interactions at the GB interface scale affect overall macroscopic behavior due to the interrelated effects of GB orientations, the evolution of mobile and immobile dislocation densities, and porosity evolution, which occur at smaller physical scales. It is shown that the accumulation of pile-ups at GB interfaces and GB absorption at different GB regions are the triggering mechanisms that lead to porosity localization, and subsequently to void nucleation and growth.}, number={3-6}, journal={MECHANICS OF MATERIALS}, author={Ashmawi, WM and Zikry, MA}, year={2003}, pages={537–552} }
 @article{ashmawi_zikry_2003, title={Grain-boundary interfaces and void interactions in porous aggregates}, volume={83}, ISSN={["1478-6443"]}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-0347592119&partnerID=MN8TOARS}, DOI={10.1080/14786430310001599423}, abstractNote={A multiple-slip dislocation-density-based formulation and computational schemes that are coupled to grain-boundary (GB) interfacial schemes and an internal porosity formulation are used to analyse the behaviour and interaction of different arrangements and geometries of explicit pairs of voids in a polycrystalline fcc aggregate. The GB regions are treated as regions with properties and topologies that are distinct from that of the grain bulk. The GB kinematic scheme accounts for dislocation density interactions with GBs, such as dislocation density impedance, blockage and GB absorption. These evolving interfacial conditions are monitored throughout the deformation history. The analysis indicated that void-to-void interactions result in dislocation density evolution and saturation and porosity localization that are intricately related to both dislocation density pile-ups and blockages at GB interfaces, and GB absorption within different GB regions.}, number={31-34}, journal={PHILOSOPHICAL MAGAZINE}, author={Ashmawi, WM and Zikry, MA}, year={2003}, pages={3917–3944} }
 @article{ashmawi_zikry_2003, title={Single void morphological and grain-boundary effects on overall failure in FCC polycrystalline systems}, volume={343}, ISSN={["1873-4936"]}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-0037465014&partnerID=MN8TOARS}, DOI={10.1016/s0921-5093(02)00325-8}, abstractNote={An investigation of how dislocation-density interactions, such as impedance, transmission, and absorption, with grain-boundaries (GBs) affect and control void growth and porosity evolution in F.C.C. aggregates has been conducted. A multiple-slip rate-dependent crystalline constitutive formulation that is coupled to the evolution of mobile and immobile dislocation-densities, a new internal porosity formulation for microvoid nucleation and growth, and specialized computational schemes have been developed to understand and quantify the interrelated effects of GB orientation, mobile and immobile dislocation-density evolution, and dislocation-density transmission and blockage on microvoid nucleation and growth. The effects of GB structure and orientation on ductile failure have been accounted for by the development of GB interfacial kinematic conditions that account for dislocation-density interactions with GBs, such as full and partial transmission, impedance, blockage, and absorption. Pile-ups and transmission regions are identified and monitored as the deformation and failure evolves. It is shown that mobile dislocation-density saturation, void size and shape, and dislocation-density interactions within the grains and the GBs are the interrelated triggering mechanisms that lead to porosity nucleation, growth, and localization.}, number={1-2}, journal={MATERIALS SCIENCE AND ENGINEERING A-STRUCTURAL MATERIALS PROPERTIES MICROSTRUCTURE AND PROCESSING}, author={Ashmawi, WM and Zikry, MA}, year={2003}, month={Feb}, pages={126–142} }
 @article{ashmawi_zikry_2002, title={Prediction of grain-boundary interfacial mechanisms in polycrystalline materials}, volume={124}, ISSN={["0094-4289"]}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-0012448372&partnerID=MN8TOARS}, DOI={10.1115/1.1421611}, abstractNote={A multiple slip dislocation-density based crystalline formulation has been coupled to a kinematically based scheme that accounts for grain-boundary (GB) interfacial interactions with dislocation densities. Specialized finite-element formulations have been used to gain detailed understanding of the initiation and evolution of large inelastic deformation modes due to mechanisms that can result from dislocation-density pile-ups at GB interfaces, partial and total dislocation-density transmission from one grain to neighboring grains, and dislocation density absorption within GBs. These formulations provide a methodology that can be used to understand how interactions at the GB interface scale affect overall macroscopic behavior at different inelastic stages of deformation for polycrystalline aggregates due to the interrelated effects of GB orientations, the evolution of mobile and immobile dislocation-densities, slip system orientation, strain hardening, geometrical softening, geometric slip compatibility, and localized plastic strains. Criteria have been developed to identify and monitor the initiation and evolution of multiple regions where dislocation pile-ups at GBs, or partial and total dislocation density transmission through the GB, or absorption within the GB can occur. It is shown that the accurate prediction of these mechanisms is essential to understanding how interactions at GB interfaces affect and control overall material behavior.}, number={1}, journal={JOURNAL OF ENGINEERING MATERIALS AND TECHNOLOGY-TRANSACTIONS OF THE ASME}, author={Ashmawi, WM and Zikry, MA}, year={2002}, month={Jan}, pages={88–96} }
 @article{zheleva_nam_ashmawi_griffin_davis_2001, title={Lateral epitaxy and dislocation density reduction in selectively grown GaN structures}, volume={222}, ISSN={["1873-5002"]}, DOI={10.1016/S0022-0248(00)00832-0}, abstractNote={The results of a comparative study of the defect microstructures at different regions in epitaxial, monocrystalline GaN structures grown selectively within windows in and laterally over SiO2 masks deposited on GaN/AlN/6H–SiC heterostructures are presented. The defects in the GaN grown within the SiO2 windows were predominantly threading dislocations of mostly mixed character with Burgers vector b=1/3〈112̄3〉 and edge dislocations with b=1/3〈112̄0〉 with a density range of 109–1010 cm−2, as determined using transmission electron microscopy (TEM). The regions of lateral epitaxial overgrowth (LEO-GaN) contained short dislocation segments parallel to the interfacial planes, which were usually aligned parallel or nearly parallel to the 〈11̄00〉 or 〈112̄0〉 directions and with densities of ⩽106 cm−2. Specific morphologies exhibited by the LEO-GaN were determined to be associated with the mechanism of stress relaxation. Finite element analysis of these complex heterostructures showed that the accommodation of the mismatches in the coefficients of thermal expansion among the different phases in the heterostructures was manifest in the formation of the curved surfaces observed in cross-sectional TEM.}, number={4}, journal={JOURNAL OF CRYSTAL GROWTH}, author={Zheleva, TS and Nam, OH and Ashmawi, WM and Griffin, JD and Davis, RF}, year={2001}, month={Feb}, pages={706–718} }
 @article{ashmawi_zikry_2000, title={Effects of grain boundaries and dislocation density evolution on large strain deformation modes in fcc crystalline materials}, volume={7}, ISSN={["0928-1045"]}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-0033876739&partnerID=MN8TOARS}, DOI={10.1023/A:1008717428264}, number={1}, journal={JOURNAL OF COMPUTER-AIDED MATERIALS DESIGN}, author={Ashmawi, WM and Zikry, MA}, year={2000}, pages={55–62} }
 @article{zheleva_ashmawi_jones_1999, title={Pendeo-epitaxy versus lateral epitaxial overgrowth of GaN: A comparative study via finite element analysis}, volume={176}, number={1}, journal={Physica Status Solidi. A, Applications and Materials Science}, author={Zheleva, T. S. and Ashmawi, W. M. and Jones, K. A.}, year={1999}, pages={545–551} }
 @article{zheleva_smith_thomson_gehrke_linthicum_rajagopal_carlson_ashmawi_davis_1999, title={Pendeo-epitaxy: A new approach for lateral growth of gallium nitride structures}, volume={4S1}, number={G3.38}, journal={MRS Internet Journal of Nitride Semiconductor Research}, author={Zheleva, T. S. and Smith, S. A. and Thomson, D. B. and Gehrke, T. and Linthicum, K. J. and Rajagopal, P. and Carlson, E. and Ashmawi, W. M. and Davis, R. F.}, year={1999} }
 @article{zheleva_ashmawi_nam_davis_1999, title={Thermal mismatch stress relaxation via lateral epitaxy in selectively grown GaN structures}, volume={74}, ISSN={["0003-6951"]}, DOI={10.1063/1.123017}, abstractNote={A reduction in the dislocation density of 104–105 cm−2 has been achieved via lateral epitaxial overgrowth (LEO) of GaN films selectively grown from stripes etched in SiO2 masks deposited on GaN/AlN/6H–SiC(0001) heterostructures. The magnitudes and distribution of stresses generated in the LEO GaN layer and the SiO2, due primarily to differences in the coefficients of thermal expansion, were modeled using finite element (FE) analysis. These calculations showed that localized compressive stress fields of ≈3 GPa occurred at the edges of the LEO GaN in the vicinity of the GaN/SiO2 interface. Localized compression along the GaN substrate/SiO2 interface and tension along the 〈0001〉 direction were responsible for the change in shape of the SiO2 stripes from rectangular with flat sides to an airfoil shape with curved sides. The FE calculations also revealed that an increase in the width of the LEO GaN regions over the SiO2 or the reduction in the separation between the GaN stripes (all other dimensions being fixed) resulted in a slight reduction in the compressive stresses along the LEO GaN/SiO2 interface and an increase in the compressive stress along [0001]. An increase in the shear stress, at the corners of the LEO GaN near the LEO GaN/SiO2 interface, with an increase in the width of the LEO GaN region were also indicated.}, number={17}, journal={APPLIED PHYSICS LETTERS}, author={Zheleva, TS and Ashmawi, WM and Nam, OH and Davis, RF}, year={1999}, month={Apr}, pages={2492–2494} }