Density, Microstructure, and Strain Rate-Dependent Behaviour of Polymeric Foams

Abstract

This thesis investigates the experimental mechanical behavior of polymeric foams, specifically studying the effects of microstructural features (pore size and wall thickness), foam density, and strain-rate-dependency on their stress-strain, lateral-axial strain, and failure. This thesis explores different polymeric foam materials that are used as liner materials in combat helmets, which are used in impact applications: 1. Polyurethane foams with varying densities from PORON, and 2. Advanced shear-thickening foam materials from D3O. Materials are primarily characterized before testing using scanning electron microscopy, and micro X-ray tomography to determine microstructural features (e.g., pore size). Experiments used to probe the mechanical response and failure behaviors include quasistatic testing, intermediate strain-rate experiments, and split-Hopkinson pressure bar dynamic testing. The deformation and failure behaviors are captured using ultra-high-speed imaging, which also allows for determination of 2D strain fields via multiple image-based techniques that are presented in the thesis (e.g., Digital Image Correlation). With these characterization and experimental measurements, contributions are made to phenomenological and scaling models, and mechanical response and damage evolution during rate dependent loading is demonstrated as a non-linear and non-monotonic behavior. The modelling provides a way to capture the combined effects of density, microstructure, and strain-rate. Long-term, we hope to bridge how new knowledge generated in the thesis on failure in foams can be leveraged to improve design and performance of polymer foams through partnership with end-users in the Canadian Department of National Defence and the US Army Research Laboratory.