In situ observation of material flow in composite media under shock compression

Abstract

When a shock wave enters a heterogeneous material, local differences in density and compressibility drive pressure and velocity gradients which can be nearly as sharp as the incident shock. Subsequent momentum equilibration is comparatively slow, relying on shock reverberation, particle collisions, and shear flow. The latter phenomenon depends on constitutive behavior that is often poorly known and so the process can be problematic to simulate. This is compounded by the fact that conventional diagnostics, such as velocimetry, blend the response of both phases and provide only a homogenized comparison for modeling efforts. For these reasons, we have collected spatially resolved and phase-specific measurements on a model particulate composite using in situ synchrotron-based radiography. This revealed the post-shock internal motion, including nanosecond resolution measurement of the metal particles' trajectories and snapshots of the flow field within the polymer. Comparing this data to analytical and numerical predictions allowed the polymer's shear response to be inferred. The resulting constitutive model was then used in conjunction with direct numerical simulations to demonstrate the physical origins of the observed bulk composite response.