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This study compares the performance of four classical force fields (FFs)—OPLS, PCFF, TraPPE-UA, and ReaxFF—with density functional theory (DFT) for modeling polyethylene (PE) configurations under extreme conditions. We evaluate their accuracy in predicting energies, forces, and stresses. Additionally, the FFs are used to generate a pressure–temperature phase diagram to compare PE's melting behavior against experimental observations. The results indicate that PCFF and OPLS exhibit the closest agreement with DFT, while PCFF, TraPPE-UA, and ReaxFF perform equally when compared to experimental data. This research provides valuable insights for complex simulations, such as high-pressure shock compression studies on PE. [ABSTRACT FROM AUTHOR]

The dynamic mechanical behavior characterization of epoxy-cast stoichiometric mixtures of nano- or micron-scale aluminum and hematite (Fe[sub2]O[sub3]) powders is investigated in this work. Experiments conducted on rod-shaped samples, using instrumented reverse Taylor impact tests employing high-speed imaging and velocity interferometry, show that these composites exhibit viscoelastic deformation and brittle fracture behaviors. Upon impact, the samples display significant elastic and plastic deformation during both the loading and unloading stages, as determined from quantitative high-speed camera measurements of the transient deformation states. Approximately 50 pct elastic recovery of total axial strain was observed to occur rapidly (within tens of microseconds) after impact. A one-dimensional elastic-plastic wave propagation analysis was used for estimating the composite's dynamic average yield stress and total plastic strain. The results reveal that the nano-Al + Fe[sub2]O[sub3]-containing epoxy composite is most resilient, has the highest strength, and is more capable of absorbing impact energy. The analysis additionally provides detailed information about elastic and plastic wave interactions for discrete times, up to the final state of the material. Calculations and observations through the coupling of high-speed camera images and velocity interferometry (VISAR) measurements show that the elastic recovery coincides with peak axial strain and the interaction of elastic and plastic waves propagating within the rod-shaped specimen. Hence, such an instrumented Taylor test provides a detailed view of the general wave structure within the material upon impact and, at the same time, enables a complete description of the stress-strain response. [ABSTRACT FROM AUTHOR]

A kinetics model based on mass and heat transport has been developed for Ti3SiC2 formation via shock-activated reaction synthesis of powder precursors. The model allows prediction of heat treatment conditions under which an otherwise steady-state reaction is taken over by a "run-away" combustion-type reaction during post-shock reaction synthesis of Ti3SiC2. Shock compression of Ti, SiC, and graphite precursors generates a densely packed highly activated state of reactants, which lowers the activation energy and results in an increased rate of formation of Ti3SiC2 at a lower temperature and in shorter times. The predictive model correlated with experimental results of fraction reacted as a function of time at heat-treatment temperatures of 1400 and 1600 °C illustrates an increased rate of reaction due to lowering activation energy, which also results in the reaction at 1600 °C being taken over by a "run-away" combustion-type reaction, as the rate of heat release due to reaction exceeds the rate of heat dissipation through the compact. Correlation of the model with experimental results illustrates that the predictive model can be used to optimize reaction synthesis conditions in shock-densified compacts of Ti3SiC2-forming powder precursors, to better understand the processes leading to a steady-state reaction being taken over by the combustion mode. [ABSTRACT FROM AUTHOR]

The high-strain-rate mechanical properties, deformation mechanisms, and fracture characteristics of a bulk metallic glass (BMG)-matrix composite, consisting of an amorphous Zr57Nb5Cu15.4Ni12.6Al10 (LM106) matrix with crystalline tungsten reinforcement particles, were investigated using gas gun anvil-on-rod impact experiments instrumented with velocity interferometry (VISAR) and high-speed digital photography. The time-resolved elastic-plastic wave propagation response obtained through VISAR and the transient deformation states captured with the camera provided information about dynamic strength and deformation modes of the composite. Comparison of experimental measurements with AUTODYN-simulated transient deformation profiles and free surface velocity traces allowed for validation of the pressure-hardening Drucker-Prager model, which was used to describe the deformation response of the composite. The impacted specimens recovered for post-impact microstructural analysis provided further information about the mechanisms of dynamic deformation and fracture characteristics. The overall results from experiments and modeling revealed a strain to failure of ~45% along the length and ~7% in area, and the fracture initiation stress was found to decrease with increasing impact velocity because of the negative strain-rate sensitivity of the BMG. [ABSTRACT FROM AUTHOR]