Mohammed Zikry - Academia.edu (original) (raw)

Papers by Mohammed Zikry

Research paper thumbnail of Effect of temperature on the grain boundary movements during creep in copper and a copper-zinc alloy

Acta Physica Hungarica, Mar 1, 1992

Previous work by the present authors [1] showed that the presence of second phase particles of zi... more Previous work by the present authors [1] showed that the presence of second phase particles of zinc at the boundaries of copper grains blocked the absorption of lattice dislocations by these boundaries during sliding at steady state creep. This was detected from measuring the sensitivity parameter of the steady creep rate to the applied stress at wide range, either for pure copper or for copper 28 wt% zinc solid solution. In this work we cansider the effect of temperature and accordingly the redistribution and diffusion of these zinc atoms at the copper grain boundaries on this blocking phenomenon. It was found that changing the temperature from 200°C to 400°C did not alter the suppression of grain boundary movements and, accordingly, their cooperation to creep deformation was also suppressed.

Research paper thumbnail of Health Monitoring and Reliability of Adaptive Heterogeneous Structures

Structural Health Monitoring-an International Journal, Jul 1, 2002

New adaptive structural systems comprised of optimal material combinations, innovative sensors an... more New adaptive structural systems comprised of optimal material combinations, innovative sensors and actuators, and control subsystems need to be developed and tailored for damage tolerant applications associated with severe loading conditions and harsh environments. In this position paper, fundamental considerations related to the modeling, design, and performance prediction of adaptive heterogeneous structures and benchmark laboratory experiments that would be needed for developing new and reliable sensor-integrated adaptive structures are discussed. Recent progress in these areas is discussed and new research directions that are needed and the barriers to be overcome to fulfill these objectives are proposed. One major challenge is to optimally integrate a variety of sensors and actuators in the context of health monitoring in order to enhance structural performance and reliability. The other major challenge is of scaling material and structural behavior from the micro- and meso-levels to the macro-level for full-scale adaptive structural system performance.

Research paper thumbnail of Fabrication and characterization of three-dimensional cellular-matrix composites reinforced with woven carbon fabric

Composites Science and Technology, Dec 1, 2001

A low-density three-dimensional cellular-matrix composite reinforced with woven carbon fabric (3D... more A low-density three-dimensional cellular-matrix composite reinforced with woven carbon fabric (3DCMC), was fabricated by means of a pressure-quenching molding technique with nitrogen gas as the blowing agent. Epoxy resins in the interstices of yarns in the 3DCMC samples were vacated during the foaming process and needle shaped voids were also generated between fibers in yarns. The average density of the

Research paper thumbnail of In-situ impact-induced damage assessment of woven composite laminates through a fibre Bragg grating sensor network

Journal of the Royal Aeronautical Society, Jun 1, 2009

Woven composite specimens with embedded fibre Bragg grating (FBG) sensor networks were impacted a... more Woven composite specimens with embedded fibre Bragg grating (FBG) sensor networks were impacted at low velocities, while global measurements of contact forces and dissipated energies were obtained from drop tower measurements, and local residual, post-impact strain values were obtained from the FBG sensors. Critical damage events were identified in the global data for these specimens and damage signatures in the residual strain data corresponding to these critical damage events were correlated. The results indicate that the full spectral scan information from the sensor network, although obtainable at a lower scan rate, provide more reliable residual lifetime information than average residual strains.

Research paper thumbnail of Microstructural Behavior and Fracture in Crystalline Materials: Overview

Springer eBooks, Sep 4, 2014

Research paper thumbnail of Dislocation density crystalline plasticity modeling of lath martensitic microstructures in steel alloys

Philosophical Magazine, Nov 21, 2009

Research paper thumbnail of An accurate and stable algorithm for high strain-rate finite strain plasticity

Computers & Structures, Feb 1, 1994

Research paper thumbnail of Demonstration of failure identification methodology incorporating sensor degradation

Proceedings of SPIE, Jul 29, 2004

... Pearson, Kara Peters, Mohammed Zikry. Abstract. The long-term goal of this project is the dev... more ... Pearson, Kara Peters, Mohammed Zikry. Abstract. The long-term goal of this project is the development of embedded, optimally distributed, multi-scale sensing methodologies that can be integrated into material systems for failure identification in structural systems. The coupling ...

Research paper thumbnail of The onset of spallation in nanocrystalline copper: An atomic scale study

Bulletin of the American Physical Society, Jun 30, 2009

in nanocrystalline metals can be understood based on the mechanisms of plastic deformation and fa... more in nanocrystalline metals can be understood based on the mechanisms of plastic deformation and failure at high strain rates. The bulk of current research on nanocrystalline metals focuses on deformation mechanisms; however, the research on the failure mechanisms (spallation) at high strain rates is still at a stage of the initial exploration. We examine the micro-mechanisms related to dynamic failure of nanocrystalline Cu at high strain rates through a series of large-scale MD simulations. Void nucleation and growth is studied in nanocrystalline copper for conditions of deformation that lead to the onset of spallation during shock loading. The high tensile triaxial stress states result in the nucleation of nanocscale voids at the grain boundaries that grow and coalesce to form the microscopic crack. The effect of shock pressure, strain rates, and grain size on the spall strength and microscopic failure mechanisms as obtained from MD simulations will be discussed.

Research paper thumbnail of Tunneling Effects and Electrical Conductivity of CNT Polymer Composites

MRS Proceedings, 2011

ABSTRACTA three-dimensional (3D) carbon nanotube (CNT) network computational model was developed ... more ABSTRACTA three-dimensional (3D) carbon nanotube (CNT) network computational model was developed to investigate the electrical conductivity and current flow in polymer composites with randomly dispersed CNTs. A search algorithm was developed to determine conductive paths for 3D CNT arrangements and to account for electron tunneling effects. Tunneled currents were obtained as a function of tunneling distance and matrix material. Several possible CNT conductive paths were obtained and finite-element representative volume elements (RVEs) were then used to predict current densities in different CNT arrangements. The predictions indicate that random CNT arrangements can be optimized for current transport.

Research paper thumbnail of Radiation and cladding modes as independent measurements of Bragg grating sensor integrity

Proceedings of SPIE, May 16, 2005

ABSTRACT

Research paper thumbnail of Characterization of fatigue damage in adhesively bonded lap joints through dynamic, full-spectral interrogation of fiber Bragg grating sensors

Proceedings of SPIE, Apr 11, 2013

In this study we evaluate the measurements of a fiber Bragg grating (FBG) sensor embedded at the ... more In this study we evaluate the measurements of a fiber Bragg grating (FBG) sensor embedded at the adhesive layer of a single composite lap joint subjected to harmonic excitation after fatigue loading. After a fully-reversed cyclic fatigue loading is applied to the composite lap joint, the full spectral response of the sensor is interrogated in reflection at 100 kHz during two states: with and without an added harmonic excitation. The dynamic response of the FBG sensor indicates strong nonlinearities as damage progresses. The short-time Fourier transform (STFT) is computed for the extracted peak wavelength information to reveal time-dependent frequencies and amplitudes of the dynamic FBG sensor response. Pulse-phase thermography indicates a progression in defect size at the adhesive layer that strongly suggests non-uniform loading of the FBG sensor.

Research paper thumbnail of Optimization of fiber Bragg sensor configurations for in situ failure identification in woven composites throughout impact

Proceedings of SPIE, Apr 6, 2007

In this study, the development and optimization of embedded fiber Bragg grating (FBG) sensor netw... more In this study, the development and optimization of embedded fiber Bragg grating (FBG) sensor networks within composite materials was investigated. Various densities of optical fibers were embedded within composite laminates, and low-velocity impact damage responses were evaluated to determine the effects on the mechanical behavior of the laminates. The woven composites were subjected to multiple strikes at 2 m/s until perforation occurred, and the impactor position and acceleration were monitored throughout each event. From these measurements, we obtained dissipated energies and contact forces for specimens with and without embedded optical fibers. Embedded fibers were interrogated with light to determine the degree to which light could pass through them for each density and arrangement. Cross sectional optical micrographs of the specimens were used to determine the local effects of the embedded fibers on neighboring fibers and the surrounding matrix material, both before and after impact events. Currently FBG sensors are being calibrated and prepared for embedment in specifically chosen configurations within the composite. They will be serially multiplexed together to create a single fiber sensing network capable of monitoring damage over a large area. Real time strain information will be gathered as future embedded laminates are subjected to impact events, and the resulting data will be used to better monitor and predict damage in the composite system.

Research paper thumbnail of High-speed full-spectrum fiber Bragg gratings interrogator system and testing

Proceedings of SPIE, May 15, 2011

ABSTRACT

Research paper thumbnail of High-speed full-spectrum interrogation of fiber Bragg gratings for composite impact sensing

Proceedings of SPIE, Mar 25, 2010

This paper presents a means for the high repetition rate interrogation of fiber Bragg gratings (F... more This paper presents a means for the high repetition rate interrogation of fiber Bragg gratings (FBG's). The new system highlights a method that allows a tradeoff between the full spectrum capture rate and the wavelength range and/or the spectral resolution of the technique. Rapid capture of the entire reflection spectrum at high interrogation rates shows important features that are missed

Research paper thumbnail of In situ failure identification in woven composites throughout impact using fiber Bragg grating sensors

Smart Structures and Materials 2006: Sensors and Smart Structures Technologies for Civil, Mechanical, and Aerospace Systems, Mar 16, 2006

In this study, measurements from low-impact velocity experiments and surface mounted optical fibe... more In this study, measurements from low-impact velocity experiments and surface mounted optical fiber Bragg grating (FBG) sensors were used to obtain detailed information pertaining to damage progression in two-dimensional laminate woven composites. The woven composites were subjected to multiple strikes at 2m/s until perforation occurred, and the impactor position and acceleration were monitored throughout each event. From these measurements, we obtained dissipated energies and contact forces. The FBG sensors were surface mounted at different critical locations near penetration-induced damaged regions. These FBG sensors were used to obtain initial residual strains and axial and transverse strains that correspond to matrix cracking and delamination. The transmission and the reflection spectra were continuously monitored throughout the loading cycles. They were used, in combination with the peak contact forces, to delineate repeatable sensor responses corresponding to material failure. From the FBG spectra, fiber and matrix damage were separated by an analysis based on the behavior of individual Bragg peaks as a function of evolving and repeated impact loads. This provided an independent feedback on the integrity of the Bragg gratings. Thus, potential sources of error such as sensor debonding were eliminated from the strain data throughout the measurements. A comparison by number of impact strikes and dissipated energies corresponding to material perforation indicates that these measurements can provide accurate failure strains.

Research paper thumbnail of Microwave Excitation of Crystalline Energetic Composites

IEEE Access, 2018

Hotspots produced by microwaves radiating into an energetic composite of RDX crystals and an esta... more Hotspots produced by microwaves radiating into an energetic composite of RDX crystals and an estane binder are examined using transient electromagnetic (EM) and coupled EM-thermal analyses. Hotspots, localized regions where energetic activity is likely to initiate, manifest as regions of peak electric field or high temperature. Stress caused by these high fields and temperatures may result in molecular breakdown, creating a chain reaction leading to the release of chemical energy via deflagration (burning and melting) in the absence of a mechanical shock wave. High peak electric fields up to three times higher than the incident field result from subwavelength scattering and occur near crystalline surfaces while peak temperatures occur in the binder, with both generally coinciding at the binder-RDX interface. Abstractions enable materials to be modeled with crystals having an average characteristic dimension of 100 µm. With an incident field of 1 MV/m the peak electric field in the composite was 2.9 MV/m and peak temperature increased by 75 K in the binder and 65 K in the RDX after 3.7 ms. The RDX fill factor of the composite was 37%, typical of an improvised explosive.

Research paper thumbnail of Reliable Local Strain Characterization on Si/SiGe Structures in Biaxial Tension

MRS Proceedings, 2006

The strain in the strained Si layer on a blanket strained Si/SiGe structure could not be determin... more The strain in the strained Si layer on a blanket strained Si/SiGe structure could not be determined with only convergent beam electron diffraction due to high order Laue zone (HOLZ) line splitting. Combined with CBED and finite element calculations, we quantified the deformation field by measuring HOLZ line splitting and demonstrated a procedure to determine the initial strain in the strained Si layer. Our results also gave us insights in strain relaxation in a TEM sample. The CBED technique combined with FE modeling has the potential for strain measurements on new generation strain-enhanced structures.

Research paper thumbnail of Computational Modeling and Design of Adaptive Thin-Film Composite Coatings

MRS Proceedings, 2006

The tailoring of thin film coatings comprised of high strength constituents, such as diamond like... more The tailoring of thin film coatings comprised of high strength constituents, such as diamond like carbon and partially stabilized zirconia and ductile constituents, such as gold and molybdenum is investigated by new microstructurally-based finite-element techniques for applications related to the wear, durability, and performance of these coatings over a broad range of temperatures and loading conditions. The effects of contact transfer films, grain-shape sizes and distributions, grain-boundary structure and sliding, texture, and strength are used to determine the optimal thin film coating compositions. Comparisons are made with experimental measurements and observations, and guidelines for optimal thin film composite coatings are proposed.

Research paper thumbnail of Inelastic microstructural failure mechanisms in crystalline materials with high angle grain boundaries

Journal of The Mechanics and Physics of Solids, Nov 1, 1996

Microstructurally-induced failure mechanisms in crystalline materials with coincident site-lattic... more Microstructurally-induced failure mechanisms in crystalline materials with coincident site-lattice (CSL) high angle grain boundaries (CBS) have been investigated. A multiple-slip rate-dependent crystalline constitutive formulation that is coupled to the evolution of mobile and immobile dislocation densities and specialized computational schemes have been developed to obtain a detailed understanding of the interrelated physical mechanisms that result in material failure. A transmission scalar has also been introduced to investigate slip-rate transmission, blockage and incompatibility at the GB. The combined effects of high angle GB misorientation, mobile and immobile dislocation densities, strain hardening, geometrical softening, localized plastic strains, and slip-rate transmission and blockage on failure evolution in face centered cubic (f.c.c.) crystalline materials have been studied. Results from the present study are consistent with experimental observations that single dislocation pileups result in a transgranular failure mode for the C9 CSL GB, and that symmetric double dislocation pileups result in an intergranular failure mode for the Zl7b CSL GB.

Research paper thumbnail of Effect of temperature on the grain boundary movements during creep in copper and a copper-zinc alloy

Acta Physica Hungarica, Mar 1, 1992

Previous work by the present authors [1] showed that the presence of second phase particles of zi... more Previous work by the present authors [1] showed that the presence of second phase particles of zinc at the boundaries of copper grains blocked the absorption of lattice dislocations by these boundaries during sliding at steady state creep. This was detected from measuring the sensitivity parameter of the steady creep rate to the applied stress at wide range, either for pure copper or for copper 28 wt% zinc solid solution. In this work we cansider the effect of temperature and accordingly the redistribution and diffusion of these zinc atoms at the copper grain boundaries on this blocking phenomenon. It was found that changing the temperature from 200°C to 400°C did not alter the suppression of grain boundary movements and, accordingly, their cooperation to creep deformation was also suppressed.

Research paper thumbnail of Health Monitoring and Reliability of Adaptive Heterogeneous Structures

Structural Health Monitoring-an International Journal, Jul 1, 2002

New adaptive structural systems comprised of optimal material combinations, innovative sensors an... more New adaptive structural systems comprised of optimal material combinations, innovative sensors and actuators, and control subsystems need to be developed and tailored for damage tolerant applications associated with severe loading conditions and harsh environments. In this position paper, fundamental considerations related to the modeling, design, and performance prediction of adaptive heterogeneous structures and benchmark laboratory experiments that would be needed for developing new and reliable sensor-integrated adaptive structures are discussed. Recent progress in these areas is discussed and new research directions that are needed and the barriers to be overcome to fulfill these objectives are proposed. One major challenge is to optimally integrate a variety of sensors and actuators in the context of health monitoring in order to enhance structural performance and reliability. The other major challenge is of scaling material and structural behavior from the micro- and meso-levels to the macro-level for full-scale adaptive structural system performance.

Research paper thumbnail of Fabrication and characterization of three-dimensional cellular-matrix composites reinforced with woven carbon fabric

Composites Science and Technology, Dec 1, 2001

A low-density three-dimensional cellular-matrix composite reinforced with woven carbon fabric (3D... more A low-density three-dimensional cellular-matrix composite reinforced with woven carbon fabric (3DCMC), was fabricated by means of a pressure-quenching molding technique with nitrogen gas as the blowing agent. Epoxy resins in the interstices of yarns in the 3DCMC samples were vacated during the foaming process and needle shaped voids were also generated between fibers in yarns. The average density of the

Research paper thumbnail of In-situ impact-induced damage assessment of woven composite laminates through a fibre Bragg grating sensor network

Journal of the Royal Aeronautical Society, Jun 1, 2009

Woven composite specimens with embedded fibre Bragg grating (FBG) sensor networks were impacted a... more Woven composite specimens with embedded fibre Bragg grating (FBG) sensor networks were impacted at low velocities, while global measurements of contact forces and dissipated energies were obtained from drop tower measurements, and local residual, post-impact strain values were obtained from the FBG sensors. Critical damage events were identified in the global data for these specimens and damage signatures in the residual strain data corresponding to these critical damage events were correlated. The results indicate that the full spectral scan information from the sensor network, although obtainable at a lower scan rate, provide more reliable residual lifetime information than average residual strains.

Research paper thumbnail of Microstructural Behavior and Fracture in Crystalline Materials: Overview

Springer eBooks, Sep 4, 2014

Research paper thumbnail of Dislocation density crystalline plasticity modeling of lath martensitic microstructures in steel alloys

Philosophical Magazine, Nov 21, 2009

Research paper thumbnail of An accurate and stable algorithm for high strain-rate finite strain plasticity

Computers & Structures, Feb 1, 1994

Research paper thumbnail of Demonstration of failure identification methodology incorporating sensor degradation

Proceedings of SPIE, Jul 29, 2004

... Pearson, Kara Peters, Mohammed Zikry. Abstract. The long-term goal of this project is the dev... more ... Pearson, Kara Peters, Mohammed Zikry. Abstract. The long-term goal of this project is the development of embedded, optimally distributed, multi-scale sensing methodologies that can be integrated into material systems for failure identification in structural systems. The coupling ...

Research paper thumbnail of The onset of spallation in nanocrystalline copper: An atomic scale study

Bulletin of the American Physical Society, Jun 30, 2009

in nanocrystalline metals can be understood based on the mechanisms of plastic deformation and fa... more in nanocrystalline metals can be understood based on the mechanisms of plastic deformation and failure at high strain rates. The bulk of current research on nanocrystalline metals focuses on deformation mechanisms; however, the research on the failure mechanisms (spallation) at high strain rates is still at a stage of the initial exploration. We examine the micro-mechanisms related to dynamic failure of nanocrystalline Cu at high strain rates through a series of large-scale MD simulations. Void nucleation and growth is studied in nanocrystalline copper for conditions of deformation that lead to the onset of spallation during shock loading. The high tensile triaxial stress states result in the nucleation of nanocscale voids at the grain boundaries that grow and coalesce to form the microscopic crack. The effect of shock pressure, strain rates, and grain size on the spall strength and microscopic failure mechanisms as obtained from MD simulations will be discussed.

Research paper thumbnail of Tunneling Effects and Electrical Conductivity of CNT Polymer Composites

MRS Proceedings, 2011

ABSTRACTA three-dimensional (3D) carbon nanotube (CNT) network computational model was developed ... more ABSTRACTA three-dimensional (3D) carbon nanotube (CNT) network computational model was developed to investigate the electrical conductivity and current flow in polymer composites with randomly dispersed CNTs. A search algorithm was developed to determine conductive paths for 3D CNT arrangements and to account for electron tunneling effects. Tunneled currents were obtained as a function of tunneling distance and matrix material. Several possible CNT conductive paths were obtained and finite-element representative volume elements (RVEs) were then used to predict current densities in different CNT arrangements. The predictions indicate that random CNT arrangements can be optimized for current transport.

Research paper thumbnail of Radiation and cladding modes as independent measurements of Bragg grating sensor integrity

Proceedings of SPIE, May 16, 2005

ABSTRACT

Research paper thumbnail of Characterization of fatigue damage in adhesively bonded lap joints through dynamic, full-spectral interrogation of fiber Bragg grating sensors

Proceedings of SPIE, Apr 11, 2013

In this study we evaluate the measurements of a fiber Bragg grating (FBG) sensor embedded at the ... more In this study we evaluate the measurements of a fiber Bragg grating (FBG) sensor embedded at the adhesive layer of a single composite lap joint subjected to harmonic excitation after fatigue loading. After a fully-reversed cyclic fatigue loading is applied to the composite lap joint, the full spectral response of the sensor is interrogated in reflection at 100 kHz during two states: with and without an added harmonic excitation. The dynamic response of the FBG sensor indicates strong nonlinearities as damage progresses. The short-time Fourier transform (STFT) is computed for the extracted peak wavelength information to reveal time-dependent frequencies and amplitudes of the dynamic FBG sensor response. Pulse-phase thermography indicates a progression in defect size at the adhesive layer that strongly suggests non-uniform loading of the FBG sensor.

Research paper thumbnail of Optimization of fiber Bragg sensor configurations for in situ failure identification in woven composites throughout impact

Proceedings of SPIE, Apr 6, 2007

In this study, the development and optimization of embedded fiber Bragg grating (FBG) sensor netw... more In this study, the development and optimization of embedded fiber Bragg grating (FBG) sensor networks within composite materials was investigated. Various densities of optical fibers were embedded within composite laminates, and low-velocity impact damage responses were evaluated to determine the effects on the mechanical behavior of the laminates. The woven composites were subjected to multiple strikes at 2 m/s until perforation occurred, and the impactor position and acceleration were monitored throughout each event. From these measurements, we obtained dissipated energies and contact forces for specimens with and without embedded optical fibers. Embedded fibers were interrogated with light to determine the degree to which light could pass through them for each density and arrangement. Cross sectional optical micrographs of the specimens were used to determine the local effects of the embedded fibers on neighboring fibers and the surrounding matrix material, both before and after impact events. Currently FBG sensors are being calibrated and prepared for embedment in specifically chosen configurations within the composite. They will be serially multiplexed together to create a single fiber sensing network capable of monitoring damage over a large area. Real time strain information will be gathered as future embedded laminates are subjected to impact events, and the resulting data will be used to better monitor and predict damage in the composite system.

Research paper thumbnail of High-speed full-spectrum fiber Bragg gratings interrogator system and testing

Proceedings of SPIE, May 15, 2011

ABSTRACT

Research paper thumbnail of High-speed full-spectrum interrogation of fiber Bragg gratings for composite impact sensing

Proceedings of SPIE, Mar 25, 2010

This paper presents a means for the high repetition rate interrogation of fiber Bragg gratings (F... more This paper presents a means for the high repetition rate interrogation of fiber Bragg gratings (FBG's). The new system highlights a method that allows a tradeoff between the full spectrum capture rate and the wavelength range and/or the spectral resolution of the technique. Rapid capture of the entire reflection spectrum at high interrogation rates shows important features that are missed

Research paper thumbnail of In situ failure identification in woven composites throughout impact using fiber Bragg grating sensors

Smart Structures and Materials 2006: Sensors and Smart Structures Technologies for Civil, Mechanical, and Aerospace Systems, Mar 16, 2006

In this study, measurements from low-impact velocity experiments and surface mounted optical fibe... more In this study, measurements from low-impact velocity experiments and surface mounted optical fiber Bragg grating (FBG) sensors were used to obtain detailed information pertaining to damage progression in two-dimensional laminate woven composites. The woven composites were subjected to multiple strikes at 2m/s until perforation occurred, and the impactor position and acceleration were monitored throughout each event. From these measurements, we obtained dissipated energies and contact forces. The FBG sensors were surface mounted at different critical locations near penetration-induced damaged regions. These FBG sensors were used to obtain initial residual strains and axial and transverse strains that correspond to matrix cracking and delamination. The transmission and the reflection spectra were continuously monitored throughout the loading cycles. They were used, in combination with the peak contact forces, to delineate repeatable sensor responses corresponding to material failure. From the FBG spectra, fiber and matrix damage were separated by an analysis based on the behavior of individual Bragg peaks as a function of evolving and repeated impact loads. This provided an independent feedback on the integrity of the Bragg gratings. Thus, potential sources of error such as sensor debonding were eliminated from the strain data throughout the measurements. A comparison by number of impact strikes and dissipated energies corresponding to material perforation indicates that these measurements can provide accurate failure strains.

Research paper thumbnail of Microwave Excitation of Crystalline Energetic Composites

IEEE Access, 2018

Hotspots produced by microwaves radiating into an energetic composite of RDX crystals and an esta... more Hotspots produced by microwaves radiating into an energetic composite of RDX crystals and an estane binder are examined using transient electromagnetic (EM) and coupled EM-thermal analyses. Hotspots, localized regions where energetic activity is likely to initiate, manifest as regions of peak electric field or high temperature. Stress caused by these high fields and temperatures may result in molecular breakdown, creating a chain reaction leading to the release of chemical energy via deflagration (burning and melting) in the absence of a mechanical shock wave. High peak electric fields up to three times higher than the incident field result from subwavelength scattering and occur near crystalline surfaces while peak temperatures occur in the binder, with both generally coinciding at the binder-RDX interface. Abstractions enable materials to be modeled with crystals having an average characteristic dimension of 100 µm. With an incident field of 1 MV/m the peak electric field in the composite was 2.9 MV/m and peak temperature increased by 75 K in the binder and 65 K in the RDX after 3.7 ms. The RDX fill factor of the composite was 37%, typical of an improvised explosive.

Research paper thumbnail of Reliable Local Strain Characterization on Si/SiGe Structures in Biaxial Tension

MRS Proceedings, 2006

The strain in the strained Si layer on a blanket strained Si/SiGe structure could not be determin... more The strain in the strained Si layer on a blanket strained Si/SiGe structure could not be determined with only convergent beam electron diffraction due to high order Laue zone (HOLZ) line splitting. Combined with CBED and finite element calculations, we quantified the deformation field by measuring HOLZ line splitting and demonstrated a procedure to determine the initial strain in the strained Si layer. Our results also gave us insights in strain relaxation in a TEM sample. The CBED technique combined with FE modeling has the potential for strain measurements on new generation strain-enhanced structures.

Research paper thumbnail of Computational Modeling and Design of Adaptive Thin-Film Composite Coatings

MRS Proceedings, 2006

The tailoring of thin film coatings comprised of high strength constituents, such as diamond like... more The tailoring of thin film coatings comprised of high strength constituents, such as diamond like carbon and partially stabilized zirconia and ductile constituents, such as gold and molybdenum is investigated by new microstructurally-based finite-element techniques for applications related to the wear, durability, and performance of these coatings over a broad range of temperatures and loading conditions. The effects of contact transfer films, grain-shape sizes and distributions, grain-boundary structure and sliding, texture, and strength are used to determine the optimal thin film coating compositions. Comparisons are made with experimental measurements and observations, and guidelines for optimal thin film composite coatings are proposed.

Research paper thumbnail of Inelastic microstructural failure mechanisms in crystalline materials with high angle grain boundaries

Journal of The Mechanics and Physics of Solids, Nov 1, 1996

Microstructurally-induced failure mechanisms in crystalline materials with coincident site-lattic... more Microstructurally-induced failure mechanisms in crystalline materials with coincident site-lattice (CSL) high angle grain boundaries (CBS) have been investigated. A multiple-slip rate-dependent crystalline constitutive formulation that is coupled to the evolution of mobile and immobile dislocation densities and specialized computational schemes have been developed to obtain a detailed understanding of the interrelated physical mechanisms that result in material failure. A transmission scalar has also been introduced to investigate slip-rate transmission, blockage and incompatibility at the GB. The combined effects of high angle GB misorientation, mobile and immobile dislocation densities, strain hardening, geometrical softening, localized plastic strains, and slip-rate transmission and blockage on failure evolution in face centered cubic (f.c.c.) crystalline materials have been studied. Results from the present study are consistent with experimental observations that single dislocation pileups result in a transgranular failure mode for the C9 CSL GB, and that symmetric double dislocation pileups result in an intergranular failure mode for the Zl7b CSL GB.