Peridynamic Investigation of Stress-Strain Response and Fracture Behavior Variations in Micro-CT Images of Mock Energetic Nanocomposites

Abstract

The stress-strain response and fracture behaviors of a mock energetic nanocomposite were investigated to reveal behaviors that contribute to variations throughout the sample. Approximately 100 XCT images that span the depth of a single sample were modeled in a peridynamics formulation to investigate changes in behaviors throughout the composite. The meshes, which included sugar, PDMS, and void phases, were modeled using tensile boundary conditions. The results from the y-direction elastic cases suggest that the overall sample had an effective stiffness of 4.396 +/- 1.153 GPa, though this varied significantly between images and image groupings. In addition, the effective stiffness of image groupings varied from 2.5 GPa to approximately 5 GPa as image number increased. These computationally determined effective stiffnesses were compared to the Mori-Tanaka approach for effective material property estimation, and the results were broadly consistent with the Mori-Tanaka solution. Damage-enabled cases were also conducted, and the results from the y-direction cases revealed unique characteristics associated with damage propagation for specific images. While the majority of the stress vs. strain curves displayed gradual damage evolution, some images exhibited rapid damage evolution caused by loading columns due to tightly packed sugar grains. In addition, variations were also caused by connection points, bridging the two halves of the mesh across voids. The variation of ultimate stress with respect to image number was also investigated, suggesting a slight increase in ultimate stress with image number. The combination of elastic and damage-enabled results displayed a general increase in stiffness for the material as image number increased. The isotropy investigation and averaged results indicated that the composite was transversely isotropic. The stress vs. strain curve for the damage-enabled cases were nearly identical for the y-direction and x-direction conditions, with a maximum percent difference of 16.4% for the duration of the simulation. The transverse isotropy was especially evident in the elastic comparison, where the average stress values had a maximum percent difference of 1.3% between strain values of 0.03% and 1%. The simulations also indicated an increase in effective stiffness and peak stress as a function of image number. Overall, the results demonstrate that the sample is statistically heterogeneous for both effective stiffness and fracture behavior. In addition, variations in effective material properties with respect to image number suggest the necessity to include this variability in sampling and modeling strategies. The fracture behaviors were also highly sensitive to the microstructure configurations, and would therefore need to be accounted for when characterizing this composite in advanced piezoresistive models.