Effects of Cyclic Loading and Time-Recovery on Microstructure and Mechanical Properties of Particle-Binder Composites

Abstract

We propose a systematic experimental procedure and quantitative analyses to investigate the effect of cyclic loading, and time-recovery, or aging, on the mechanical properties and microstructure of particle-binder composites. Specifically, we study three compositions that differ in aluminum content from the mock sugar formulation of plastic-bonded explosive PBXN-109. Cast cylindrical specimens are subjected to high-amplitude quasi-static cyclic compressive loading, before and after a 4-week time-recovery period, and their microstructures are analyzed using micro-computed tomography (CT). For quantitative analysis, we develop a procedure for identifying the spatial distribution of primary components of the formulation, including pore space, from micro-CT images. The study shows that the stress–strain response is highly nonlinear, without a distinct yield point, and exhibits hysteresis and cyclic stress softening, or Mullins effect, with cyclic stabilization. Specimens without aluminum exhibit considerable gain in stiffness and strength after the time-recovery or aging period, owing to the development of increased sucrose particle–particle interactions during the first cyclic loading. In contrast, specimens with aluminum micro-sized powder exhibit permanent loss of stiffness and strength, owing to large ductile plastic flow and irrecoverable damage. Further insight from micro-CT analysis is gained by observing that, for all compositions, the majority of microstructural changes occur near the specimen core. Specifically, affine radial deformation of the soft and debonded binder, as it is compressed by the non-affine longitudinal motion of stiffer sucrose crystals, is observed in the formulation without aluminum, whereas non-affine rearrangement of the binder toward the specimen core, and affine radial flow of sucrose particles away from the core due to ductile macroscopic deformation of the specimen, is observed in the formulations with aluminum content.