Full-Field Quantitative Visualization of Shock-Driven Pore Collapse and Failure Modes in PMMA

TL;DR

This study combines experimental and numerical methods to investigate pore collapse in PMMA under shock loading, revealing failure mode transitions and mechanisms of shear band formation and fracture at the pore scale.

Contribution

It introduces a novel internal digital image correlation technique for full-field deformation measurement and provides new insights into failure modes in porous materials under shock.

Findings

Identification of shear localization via adiabatic shear bands

Observation of dynamic fracture initiation at pore surfaces

Numerical models explaining failure mode transitions and shear band formation

Abstract

The dynamic collapse of pores under shock loading is thought to be directly related to hot spot generation and material failure, which is critical to the performance of porous energetic and structural materials. However, the shock compression response of porous materials at the local, individual pore scale is not well understood. This study examines, quantitatively, the collapse phenomenon of a single spherical void in PMMA at shock stresses ranging from 0.4-1.0 GPa. Using a newly developed internal digital image correlation technique in conjunction with plate impact experiments, full-field quantitative deformation measurements are conducted in the material surrounding the collapsing pore for the first time. The experimental results reveal two failure mode transitions as shock stress is increased: (i) the first in-situ evidence of shear localization via adiabatic shear banding and (ii) dynamic fracture initiation at the pore surface. Numerical simulations using thermo-viscoplastic dynamic finite element analysis provide insights into the formation of adiabatic shear bands (ASBs) and stresses at which failure mode transitions occur. Further numerical and theoretical modeling indicates the dynamic fracture to occur along the weakened material inside an adiabatic shear band. Finally, analysis of the evolution of pore asymmetry and models for ASB spacing elucidate the mechanisms for the shear band initiation sites, and elastostatic theory explains the experimentally observed ASB and fracture paths based on the directions of maximum shear.