Session F6: Inelastic Deformations, Fracture and Spall III: Critical Conditions

Critical conditions for failure; stress levels, length scales, time scales~

There is a range of thresholds for the response of condensed matter under loading in compression, from the yield~point to that at which the bond strength is overcome and warm dense matter is formed. Yield stress shows a correlation~between the length scale swept by the rise of the pulse and the defect distribution within the target for a range of~materials. Strain rate is also a useful term that reflects the~evolution of the stress state within a target but must also be defined for a volume element containing a particular defect~distribution to reflect continuum conditions acting within and thus applies to a defined length scale within a target. Examples are shown using shock pulses to spall metal targets. Different stacking shows differing behaviour yet in each case momentum is conserved. This~overview of behaviour suggests concepts borrowed from rate-independent plasticity may be viewed in a different~manner and accompanying behaviours such as brittle-ductile transition may be reviewed when explaining a range of~dynamic failure modes under load for materials and structures. ~   

Influence of Sample Geometry on Sweeping-Detonation-Wave Spallation in Tantalum

Widespread research over the past five decades has provided a wealth of experimental data and insight concerning shock hardening and the spallation response of materials subjected to square-topped shock-wave loading profiles. Less quantitative data have been gathered on the effect of direct, in-contact, sweeping-wave high explosive (HE)-driven Taylor wave loading profile shock loading on the shock hardening, damage evolution, or spallation response of materials. Sweeping-wave loading is a significantly different loading history than that achieved by a square-topped impulse or 1-D HE-driven plane-wave shock in terms of the evolving spherical and shear stresses applied to the specimen. The goal of this research is to quantify the combined influence of shockwave obliquity evolution plus sample geometry on the spallation response of Tantalum(Ta) by subjecting a curved Ta plate to HE-driven sweeping detonation-wave loading and quantify both the wave propagation and the post-mortem damage evolution. This talk will summarize our current understanding of the similarity and differences between the shock hardening and damage evolution during sweeping detonation-wave spallation loading observed in flat and curved Ta plate samples.   

Microstructural Effects on the Spall Properties of ECAE Magnesium and Magnesium Alloys

Magnesium and magnesium alloys are light weight materials and hence, are being increasingly employed as light armor in military applications. However, because of its limited slip systems (HCP) magnesium and magnesium alloys are relatively brittle as compared to FCC and BCC lattice structures. For this study, the effects of microstructure on the spall properties of magnesium and magnesium alloys processed using Equi-Channel Angular Extrusion (ECAE) were investigated using a 51 mm and 105 mm bore gas guns. Symmetric spall and recovery plate impact experiments were performed at impact velocities ranging from approximately 100 m/s and 400 m/s. Free surface velocity profiles of the shocked samples were obtained using Photonic Doppler Velocimetry (PDV). The spall strength and Hugoniot Elastic Limit (HEL) were extracted from the free surface velocity profiles. In addition, the microstructures of the pre-shocked and post-shocked magnesium and magnesium alloys were acquired using Scanning Electron Microscope (SEM) and Transmission Electron Microscope (TEM).