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Spall failure is a phenomenon commonly observed under dynamic loading when shock waves are involved, leading to the nucleation, growth and coalescence of porosity. At elevated strain rates, such as those observed under laser-driven spall experiments, inertia at the microstructural scale becomes significant and can affect the porosity growth process and thereby affect the damage process. A semi-coupled Cocks-Ashby model accounting for microinertia is used to show microinertial contributions to spall experiments. The results show that the presence of microinertia alters the internal stress state as well as the apparent macroscopic spall strength and can lead to underestimations of both if ignored. The effect of microinertia is seen to grow with increasing strain rate. Complementary analysis shows that this effect is mitigated by thicker targets leading to lower strain rates and spall strengths. Attention is also brought to the importance of laser drive conditions, demonstrating the difference between square and triangular pulses. Square pulses lead to higher stresses and more diffuse damage compared to triangular pulses. These concepts can help to better understand spall at high strain rates and also expand the envelope for designing spall experiments that can access a wider range of stress and strain states. [ABSTRACT FROM AUTHOR]

We study the competition between twinning and slip in hexagonal close packed metals. Under a set of reasonable simplifying assumptions, we perform an explicit homogenization to derive an expression relating the macroscopic flow stress to the flow stresses of the different deformation mechanisms. The resulting expression is surprisingly simple and is parameterized by a single dimensionless variable related to the rate of deformation. Utilizing this expression, we pose a simple model for inelastic deformation in hexagonal close packed metals and apply it to examine stress-strain data in beryllium. [ABSTRACT FROM AUTHOR]

We present a multi-scale strength model that includes phase transformation. In each phase, strength depends on pressure, strain rate, temperature, and evolving dislocation density descriptors. A donor cell type of approach is used for the transfer of dislocation density between phases. While the shear modulus can be modeled as smooth through the BCC to rhombohedral transformation in vanadium, the multi-phase strength model predicts abrupt changes in the material strength due to changes in dislocation kinetics. In the rhombohedral phase, the dislocation density is decomposed into populations associated with short and long Burgers vectors. Strength model construction employs an information passing paradigm to span from the atomistic level to the continuum level. Simulation methods in the overall hierarchy include density functional theory, molecular statics, molecular dynamics, dislocation dynamics, and continuum based approaches. We demonstrate the behavior of the model through simulations of Rayleigh Taylor instability growth experiments of the type used to assess material strength at high pressure and strain rate. [ABSTRACT FROM AUTHOR]

We present here the first dynamic Rayleigh-Taylor (RT) strength measurement of a material undergoing solid-solid phase transition. Iron is quasi-isentropically driven across the pressure-induced bcc (α-Fe) → hcp (ε-Fe) phase transition and the dynamic strength of the α, ε and reverted α′ phases have been determined via proton radiography of the resulting Rayleigh-Taylor unstable interface between the iron target and high-explosive products. Simultaneous velocimetry measurements of the iron free surface yield the phase transition dynamics and, in conjunction with detailed hydrodynamic simulations, allow for determination of the strength of the distinct phases of iron. Forward analysis of the experiment via hydrodynamic simulations reveals significant strength enhancement of the dynamically-generated ε-Fe and reverted α′-Fe, compareable in magnitude to the strength of austenitic stainless steels. [ABSTRACT FROM AUTHOR]