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Epoxy resin (DGEBA/TETA) is commonly used as an adhesive material. When this material is subjected to low temperatures for an extended period of time, it undergoes changes in inter/intramolecular v...

Even though there have been several studies in literature of 6H SiC, a proper physics based understanding of the molecular deformation mechanisms of the material under different loading conditions is still lacking. Experimentally, the brittle nature of the material leads to difficulties associated with in-situ determination of molecular deformation mechanisms of the material under an applied load; whereas, the complex material structure along with the bonding environment prevents proper computational identification of different types of inelasticity mechanisms within the material. Molecular dynamics study (on successful verification of the interatomic potential with experimental results) of pristine single crystals of 6H SiC have been used to probe the physics of molecular deformation mechanisms of the material along with its inherent orientational anisotropy. The study elucidates the experimentally observed mechanisms of defect nucleation and evolution through a detailed analysis of radial distribution functions, x-ray diffraction as well as phonon vibrational studies of the single crystal. Studies have been presented at room temperature, initial high temperature and different types of confinement effects of the material (including hydrostatic and different biaxial loading cases). The confinement resulted in an increase in stress and stiffness whereas increase in initial temperature resulted in a decrease compared to uniaxial stress loading conditions at room temperature.

Performance evaluation of cementitious mortar reinforced with natural fibers (jute) and synthetic (polypropylene) fibers is carried out in this article with respect to mechanical response a...

It is been widely experimentally reported that Si under static compression (typically in a Diamond Anvil Setup-DAC) undergoes different phase transitions. Even though numerous interatomic potentials are used for numerical studies of Si under different loading conditions, the efficacy of different available interatomic potentials in determining the phase transition behavior in a simulation environment similar to that of DAC has not been probed in literature which this manuscript addresses. Hydrostatic compression of Silicon using seven different interatomic potentials demonstrates that Tersoff(T0) performed better as compared to other potentials with regards to demonstration of phase transition. Using this Tersoff(T0) interatomic potential, molecular dynamics simulation of cubic diamond single crystal silicon has been carried out along different directions under uniaxial stress condition to determine anisotropy of the samples, if any. β -tin phase could be observed for the [0 0 1] direction loading whereas Imma along with β -tin phase could be observed for [0 1 1] and [1 1 1] direction loading. Amorphization is also observed for [0 1 1] direction. The results obtained in the study are based on rigorous X-ray diffraction analysis. No strain rate effects could be observed for the uniaxial loading conditions.

The surface morphology of fibers influence the global performance of fiber-reinforced cementitious composites. Commercially available polypropylene fibers may be coated with polyglycol este...

A molecular dynamic study of the structure-property relationship of tricalcium aluminate subjected to uniaxial tensile loading has been investigated in this study. The suitability of interface force potential, being used in this study, has been verified against experimental observations and first principle calculations. The study demonstrates that energy associated with the non-bonded terms (Van der Walls, Coulombic and long-range interactions) contributes to the major part of the total energy which increases steadily with strain. Nucleation of voids are also observed in the post-peak regime. Different bond lengths and angles of the puckered chain, which constitute the crystal structure of the ionic-covalent solid, are observed to increase (either uniformly or non-uniformly) with application of load in the prepeak regime. It can also be observed that even though coplanarity of the O and Al atoms in the puckered chain is separately maintained throughout the straining process; the angle between the two different planes in the puckered chain are observed to increase with strain. For the first time in literature, this study provides details of the deformation mechanism at a molecular level of this ionic-covalent solid when subjected to uniaxial tensile loading situations.

In a previous study (Asp et al., 1995) the experimentally observed low strains to transverse tensile failure of unidirectional (UD) polymer matrix composites were explained as an effect of triaxial (composite- like) stress state in the epoxy matrix. Assuming cavitation as an underlying mechanism for brittle cracking under triaxial stress states, a dilatation energy density based criterion was put forth (Asp et al., 1996) and was shown to predict well the transverse failure of epoxy based UD composites (Asp et al., 1996). The assumption of cavitation in the epoxy matrix has hitherto not been supported by a mechanism study. The current study attempts to provide a systematic clarification of the cavitation mechanism by molecular dynamic simulation. By imposing uniaxial, equi-biaxial and equi-triaxial tension on a simulation cell of a crosslinked epoxy, the degrees of cavitation at various stages of the stress- strain response are revealed. The results show that triaxiality of the stress states is a governing factor in cavitation of epoxies.

Twinning plays an important role on the microstructure evolution of HCP metals. In this work, we perform c-axis compression of single crystal Ti under uniaxial stress conditions using molecular dynamics simulations. The objective is to demonstrate activation and evolution of compression twins along with dislocations when Ti single crystals are loaded in uniaxial stress conditions using two commonly utilized interatomic potentials. We find that activation of only { 1 0 1 ¯ 1 } compression twins with the Kim potential is inconsistent with the Schmid criterion while activation of both { 1 0 1 ¯ 1 } and { 1 1 2 ¯ 2 } compression twins with the Hennig potential is consistent with the Schmid criterion. Twin variants activated do not contribute equally to the total twinned volume even with equal Schmid factor for both potentials and thereby demonstrating inability of the Schmid criterion to predict dominance and evolution of twin variants. The Kim potential shows large amount of dislocation density in comparison to the Hennig potential. The strain rate sensitivity on activation of compression twin systems is observed with the Hennig potential. The study can be utilized to further investigate the evolution of compression twins required to develop/improve twin volume fraction evolution laws for crystal plasticity finite element simulations.

We perform molecular dynamics simulations to study the evolution of tension twinning in single crystal Ti under uniaxial strain conditions. The objective is to understand the evolution trends of twin volume fraction as well as number of twins and express them in terms of the individual twin dynamics. The compressive strain applied along 〈 2 1 ¯ 1 ¯ 0 〉 and 〈 0 1 1 ¯ 0 〉 directions leads to the activation of { 1 0 1 ¯ 2 } twin variants. We find that the activation of twin variants follows the Schmid criterion. However, they do not activate at the same time even with equal Schmid factor indicating the stochastic nature of the twin variant activation. For the case where four variants activate (loading along 〈 2 1 ¯ 1 ¯ 0 〉 direction), high nucleation events occur compared to the case where only two variants activate (loading along 〈 0 1 1 ¯ 0 〉 direction). The activated variants in each case do not evolve with same rate even with equal Schmid factor and a clear dominance of twin variants is observed. For the case where only two variants activate, average twin growth rate and overall twin volume fraction are high compared to the case where four variants activate. A correlation between the number of activated variants and the overall twin volume fraction is observed. The size distribution of the nucleated twins follows the power law function. The expressions provided for overall number of twins and overall twin volume fraction can serve as a basis to develop the physics-based twin evolution laws to model twinning at higher length scales.

Identification of interfacial delamination in the glass fiber/polymer-foam-core sandwich composites is difficult if the delamination does not propagate to the side surface of the specimen. However, these damages may eventually lead to compromising the sandwich composite structural component. A cost-effective novel embedded fiber optic sensor is being proposed in this manuscript, which works on the principle of multimode interference, to perform distributed sensing of interfacial delamination within the sandwich composites while in service. Even though this easy to use methodology has been used to identify interfacial delamination, this methodology can also be used for different other types of interfacial/interlaminar distributed strain sensing of samples under mechanical as well as thermal loads.

We have carried out multimillion atom non-equilibrium molecular dynamics simulations for investigating the effect of crystallographic orientation over the evolution of deformation pathway of single crystal copper under shock compression. Based on symmetry, three different crystallographic directions, 〈 1 0 0 〉 , 〈 1 1 0 〉 and 〈 1 1 1 〉 are selected and taken as shock directions. Shock Hugoniot points has been calculated and compared among these different directions up to ∼ 450 GPa of shock pressure i.e. piston velocity of 3.0 km/s. Orientational anisotropy has been observed for the bulk Cu single crystals shock loaded along these three different directions. Even though this feature may not show up explicitly in experimental investigations which typically measures shock-velocity and density Hugoniot curve, it is apparent from large scale atomistic simulations which measures the temperature Hugoniot curve quite accurately. Differences are observed in the von-Mises strain and stress plot distributions for shock loading of different intensities along the three directions. Large directional dependency is also evident in the evolution mechanism of deformation. Temperature profiles at different piston velocities for the shock front and the shock equilibrated regions shows significantly different and interesting patterns along the three orientations. Maxwell-Boltzmann distribution is observed in the atomic velocities (thereby the temperature profiles also) for both the shock front region as well as the shock equilibrated region for shock loading along all the three directions.

Microstructural changes in concrete, mortar, and cementitious composite material were investigated to determine the efficacy of these materials subjected to shock loading. An experimental m...

Hydrated cement produces the strength and stiffness of concrete, a widely utilized infrastructural material. Estimation of elastic constants of hydrated cement, a heterogeneous material con...

We perform molecular dynamics simulations to simulate the c-axis compression of single crystal Ti at high strain rates under uniaxial strain conditions. Since it is well known that molecular dynamics simulations heavily rely upon the type of potential used, a comprehensive study is presented in which four different commonly utilized potentials for Ti (Ackland, Mishin, Kim and Hennig) are evaluated against their abilities to demonstrate different variants of compression twins, dislocation structures and structural phase transformation. We find that { 1 0 1 ¯ 1 } and { 1 1 2 ¯ 2 } twins activate for Ackland and Mishin potentials, while only { 1 0 1 ¯ 1 } twins activate for Kim potential. No compression twin systems activate for Hennig potential. The c-vector analysis of unknown structure generated with Ackland, Mishin and Kim potentials shows that the unknown structure has { 1 0 1 ¯ 1 } twin-like orientations and the structure factor analysis gives a signature of pressure-induced ω phase for the twin-like oriented unknown structure. No signature of twin-like oriented unknown structure and ω phase is observed for Hennig potential. The large amount of dislocation density is observed for Ackland potential followed by Mishin, Kim and Hennig potentials. The presence of compression twins and high dislocation density for Ackland, Mishin and Kim potentials suggest that the c-axis deformation is accommodated by twins and slip together, while only slip accommodates the c-axis deformation for Hennig potential. Based on these observations and as well as on the formulation of the above mentioned potentials, Kim potential is being recommended for use under c-axis uniaxial compressive strain loading situations.

An experimental investigation on sandwich composite materials composed of glass-fiber face sheet and polyvinyl-chloride foam core has been carried out. The research demonstrates improvement in mixed-mode delamination fracture toughness values of samples under mixed-mode bending condition. The improvement is recorded with addition of a certain percentage by weight of multiwalled carbon nanotubes in comparison to conventional samples. An easy and cost-effective methodology of multiwalled carbon nanotube insertion through sonication of epoxy resin followed by mixing with hardener and vacuum resin infusion technology for manufacturing of sandwich composites has been utilized in this study. The study also identifies the optimum weight percentage of multiwalled carbon nanotube addition in the resin system for maximum performance gain in mixed-mode fracture toughness. The results of observations in this study have been supported by field emission scanning electron microscope studies as well as high-resolution transmission electron microscope analysis.

We perform molecular dynamics simulations to investigate the microstructural evolution and role of twinning on ω -phase transformation in single crystal Ti for loading perpendicular to the c-axis under uniaxial strain conditions. We find that both tension twinning and ω -phase evolve simultaneously and compete with each other. The number of activated tension twin variants not only affects the overall twin volume fraction but also the ω -phase volume fraction. For the case where four twin variants activate, the overall twin volume fraction is lowest and ω -phase volume fraction is highest in comparison to the case where only two twin variants activate. Significant amount of unconsumed parent HCP structure occurs for the case where four twin variants activate in comparison to the case where only two twin variants activate. This suggests that the number of activated twin variants and the spatial distribution of twins belonging to these variants play an important role on the amount of unconsumed parent HCP structure. The presence of high dislocation density for the case where four twin variants activate in comparison to the case where only two variants activate indicates that the number of activated twin variants also affects the overall dislocation density. The foregoing observations can be useful to develop a dynamic material strength model which can account for the coupled evolution of plasticity and phase transformation.

DFT calculations (typically done at 0 K), Helmholtz free energy and Gibbs free energy calculations (for high temperature) within the quasi-harmonic approximation have been done in this manuscript to probe a metastable phase of Cu observed at high temperature and high pressure. The high electronic thermal conductivity observed for this new metastable phase of Cu may be beneficial for high temperature engineering applications in electronic and spacecraft device designs.

Face–core delamination is one of the major problems of using sandwich composite structures composed of a thin composite face sheets of Eglass/epoxy along with a low density PVC foam core. An innovative cost-effective methodology of inserting pre-manufactured shear keys in foam-core grooves has been proposed in this manuscript which increases the delamination resistance capacity of initially delaminated sandwich composite column subjected to in-plane compressive loading. Manufacturing of the sandwich composite panels with shear keys along with detailed experimental and numerical investigations have been performed in this manuscript to demonstrate the potential of the proposed simple methodology which can be utilized for face–core delamination sensitive design and analysis of sandwich composite structures and/or components. Numerical simulation considering cohesive damage at the face/core interface is shown to accurately simulate behavior as observed in experimental investigation. A numerical parametric study has also been carried out to determine the effect of change in stiffness of the face sheets and the shear keys on the in-plane and out-of-plane global response of sandwich composite panels.

A model for use in simulating the response of reinforced concrete interior beam-column joints is developed and evaluated using an extensive experimental data set. This model builds on previous work by Lowes and Altoontash in 2003, modifying the previously proposed model to improve prediction of response and extend the range of applicability. First, a new element formulation is proposed to improve simulation of joint response mechanisms. Second, a new method for simulating the shear stress-strain response of the joint core is developed. This method assumes joint shear is transferred through a confined concrete strut and simulates strength loss due to load history and joint damage following yielding of beam longitudinal steel. Third, modifications are made to enable better simulation of anchorage zone response. Comparison of simulated and observed response histories indicates that the new model represents well stiffness and strength response parameters for joints with a wide range of design parameters.

Even though there are numerous experiments and molecular dynamic simulations of Cu under shock loading, there appears to be no literature on the evolution of different types of dislocation mechanisms and their mutual interactions during the process of shock loading, which this article addresses through molecular dynamic simulations using the Mishin EAM potential for Cu. Three different directions , , and that have been considered in this article are subjected to shock compression with piston velocities ranging between 0.3–3 km s−1. The evolution of Hirth locks, Lomer–Cottrell locks, cross-slips, jogs, and dislocation-originated stacking-fault tetrahedra are demonstrated in this article for different direction shock loading of single-crystal Cu.

Molecular dynamics simulations were carried out using numerous force potentials to investigate the shock induced phenomenon of pure bulk liquid water. Partial phase transition was observed at single shock velocity of 4.0 km/s without requirement of any external nucleators. Change in thermodynamic variables along with radial distribution function plots and spectral analysis revealed for the first time in the literature, within the context of molecular dynamic simulations, the thermodynamic pathway leading to formation of ice VII from liquid water on shock loading. The study also revealed information for the first time in the literature about the statistical time-frame after passage of shock in which ice VII formation can be observed and variations in degree of crystallinity of the sample over the entire simulation time of 100 ns.