Strength Effects of Microfracture on Granular Microstructures Evaluated by FDEM Direct Numerical Simulation
Authors: Hagengruber T. , Boyce S. H. , Bennett K. C. , Rougier E. 
Published: September 2022

ABSTRACT:
We present results of an investigation into the mechanisms of damage in granular microstructures conducted through direct numerical simulation with the combined Finite-Discrete Element Method (FDEM). Scanning Electron Microscope (SEM) images of a pressed crystalline powder are directly meshed, resolving grain-grain interfaces. Semi-ductile microfracture is simulated by prescribing a combination of inter-granular brittle fracture and intra-granular grain plasticity. Pristine (undamaged) and damaged microstructures are simulated in uniaxial compression tests and compared to experimental uniaxial compression measurements from literature. The simulation results show that the observed microscale mechanisms of damage (microfracture predominantly around and sometimes through grains and crack associated pore-growth) can well explain degradation of strength observed in the laboratory measurements.
A method of tracing grain boundaries from SEM images is described and applied to meshing of a microstructure damaged through cyclic thermal loading. By calibrating the simulations to the damaged and undamaged experimental measurements, micro-mechanical/structural insight is gained into the mechanisms of damage for the material. The results show that the SEM-based microcharacterization of damage can explain the degradation in effective strength observed in the testing and can be accurately modeled using the presented methods.

1. INTRODUCTION
Granular, also called "particulate," materials typically exhibit some combination of microfracture and solid crystal plasticity under irrecoverable deformation (Lemaitre & Desmorat, 2005; Anderson, 2005; Arson, Jin, & He, 2021; Shen, Ding, Arson, Chester, & Chester, 2021), which suggests that multiscale damage models for these types of materials should account in some way for the interplay between these inelastic deformation mechanisms (Bennett & Borja, 2018; Bennett, 2020). An example is the plastically bonded explosive (PBX) 9502, which has been shown to exhibit microfracture damage accompanied by irreversible volume changes when subjected to thermal loading (Yaeger, Montanari, Woznick, Knepper, & Bennett, 2022), known as rachet growth (Naum & Jun, 1970; Kolb & Rizzo, 1979; Thompson, et al., 2009). Ratchet growth in PBX 9502 is generally attributed to the highly anisotropic thermoelastic properties of the "graphite-like" Triaminotrinitrobenzene (TATB) crystals (Kolb & Rizzo, 1979; Bennett, Zecevic, Luscher, & Lebensohn, 2020). Other composites of graphite-like polycrystals have shown similar behavior (Naum & Jun, 1970; Kolb & Rizzo, 1979). Ratchet growth is well-known to cause damage in the sense of degradation of strength (Thompson, et al., 2010) and increased sensitivity to detonation (Mulford & Swift, 2012). Microfracture associated with ratchet growth damage in PBX materials often arise in grain interfaces, signifying the importance of bond-strength in grain boundaries to predict microscale models (Rae, Palmer, Goldrein, Field, & Lewis, 2002; Yaeger, Ramos, Hooks, Majewski, & Singh, 2014). Recent multiscale experimental investigations into the driving mechanisms of ratchet growth (Yaeger, Montanari, Woznick, Knepper, & Bennett, 2022) have shown that ratchet growth volume changes are attributable to marked increase in microfractures and their associated crack openings predominantly around agglomerated crystal grains, suggesting multiscale models of PBX 9502 need to resolve grain-interfaces and intergranular microfracture.