A micromechanics-based framework to predict transitions between dimple and cup-cone fracture modes in shocked metallic glasses

In an earlier work (Tang et al., 2020), we derived evolution equations governing dynamic void growth in amorphous materials with a number of idealizing assumptions. Here, we extend and further generalize the constitutive theory to better account for general stress states, strain softening, stable and unstable void growth modes, as well as viscous and micro-inertial retarding effects on void growth rates. The enhanced theory is implemented into a commercial finite element software package via a user-defined material subroutine to understand transitions in fracture morphologies. In particular, metallic glasses exhibit a dimple type fracture mode at low impact velocities, which transitions to a cup-cone type fracture mode at higher impact velocities. Our theory reveals that two competing processes drive this transition: (i) strain-softening behavior that is inherent to many amorphous materials and (ii) a so-called stress plateau effect that arises due to bursts of stable and unstable void growth. Our simulation results also suggest that strain-softening during the dynamic compression phase, which precedes subsequent tensile failure, is essential to the formation of cup-cone fracture morphology.

Automated summary

This study extends previous work on dynamic void growth in amorphous materials by considering a wider range of stress states, strain softening, stable/unstable void growth modes, and viscous/micro-inertial effects on void growth rates. A new constitutive theory is proposed to understand transitions in fracture morphologies, with a focus on competing processes of strain-softening behavior and stress plateau effects in amorphous materials. The simulation results highlight the essential role of strain-softening in the formation of cup-cone fracture morphology during dynamic compression.