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

Abstract

<jats:title>Abstract</jats:title> <jats:p>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.</jats:p>

A finite-deformation constitutive model of particle-binder composites incorporating yield-surface-free plasticity

“…The range of behavior predicted by the proposed constitutive model is demonstrated by calibrating parameters for the 85-15 mock PBX formulation under cyclic compression (Agarwal & Gonzalez, 2020). Therefore, section 4.1 presents the incremental procedure for uniaxial cyclic loading under unconfined lateral conditions and incompressible material assumptions.…”

“…The resulting estimated stress-strain curve, compared with the experimentally obtained response for the 85-15-05 specimen (ref. Agarwal & Gonzalez (2020) for the specific naming convention of the tested mock PBX specimens) is presented in Figure 3a, along with the experimental and model-estimated values of the apparent stiffness (Figure 3b) and of the cumulative energy dissipation (Figure 3c). The apparent stiffness is calculated as the slope of the line connecting peak and valley stresses in each cycle, while the cyclic energy dissipation is calculated as the difference between the energy supplied to the material (i.e., the area under the loading path) and the energy recovered after a cycle (i.e., the area under the unloading path).…”

“…Uniaxial confined compression tests carried out on inert mock sugar-based specimens for PBX 9501 (Wiegand, 2000;Wiegand & Reddingius, 2005) under different confining pressures revealed increasing plastic deformation and strain hardening with increasing pressure. Recently, Agarwal & Gonzalez (2020) conducted unconfined compression tests at room temperature and low strain rate (10 −3 /s) on cylindrical specimens of three mock explosive formulations based on PBXN-109 (Lochert et al, 2002), with sugar as a substitute for the explosive HMX crystals. All three formulations contained 85% w/w of solids (sugar and aluminum), but differed from each other by the amount of aluminum content in the solids-composition, namely 85-00 (0% Al in solids, 0% in total), 85-15 (15% Al in solids, 12.75% in total) and 85-30 (30% additive in solids, 25.5% in total).…”

“…Lastly, Agarwal & Gonzalez (2020) also conducted large deformation, unconfined cyclic compression tests of cylindrical sugar-based mock PBX specimens at room temperature and low strain rate (10 −3 /s). Figure 1b shows the cyclic response of the 85-15 formulation.…”

“…Therefore, we develop a parameter identification method based on a nonlinear multivariate least-squares problem, which is expected to be non-convex and affected by multiple local optima. The range of behavior predicted by the proposed model is demonstrated by calibrating parameters for the 85-15 formulation under cyclic compression (Agarwal & Gonzalez, 2020). Finally, to gain a better insight into the dependence of material response on model parameters, the analysis is concluded by studying the sensitivity of the cyclic compressive response to variations in the estimated values of endochronic and isotropic damage parameters.…”