Three-Dimensional Quantitative Visualization for Mechanics of Discontinuous Materials

Citation

Mac Donald, Kimberley Ann (2020) Three-Dimensional Quantitative Visualization for Mechanics of Discontinuous Materials. Dissertation (Ph.D.), California Institute of Technology. doi:10.7907/0M4F-FG13. https://resolver.caltech.edu/CaltechTHESIS:08092019-151803660

Abstract

The complexity and multiscale nature of material microstructures introduces significant intricacies to many mechanics problems for which we do not have a full theoretical understanding. Under loading, these microstructures can introduce significant nonlinearities that cannot be described sufficiently by current theories and models. This leads us to consider experiments we could perform to improve our understanding of such effects. This thesis describes the design of experiments exploring two aspects of material microstructure effects: (i) crack propagation and renucleation in soft brittle polymers and (ii) interparticle forces in granular materials.

First, experimental and analysis methods are developed to study fracture mechanics in soft brittle polymers with the goal of developing a more detailed understanding of the effects of microstructural heterogeneities on crack propagation and renucleation in three-dimensions. To better understand these processes, experiments on crack propagation in thin soft polymers using confocal microscopy images are conducted. Traditional metrics associated with crack propagation including stress intensity factor (SIF, K) and energy release rate (ERR, G) are calculated by direct measurement of the crack tip opening displacement (CTOD, δ_t) on the sub-millimeter scale. Errors in these calculations are comparable to those reported in the literature for more traditional fracture experiment geometries. Fluorescent speckle images are captured using confocal microscopy imaging, a fast and low cost 3D optical imaging technique, to study crack geometry during propagation. Images of renucleation events are also captured allowing investigation of factors contributing to slow crack roughening observed by earlier researchers. The goal of this study is to provide an experimental method to enhance understanding of crack interactions with microstructural heterogeneities and of renucleation events, which can significantly improve our ability to design material toughness.

To begin to understand the effects of engineered microstructural heterogeneities such as inclusions in materials, we must be able to produce such engineered systems and understand the interparticle interactions. To this end, a method to manufacture volumetrically speckled spheres in-house with controlled diameters was developed. Additionally, an experimental method combining confocal microscopy with digital volume correlation (DVC) was also used to study interparticle force transmission in 3D. Analysis of an in-plane 2D projection of volumetric surface data shows that three-dimensional effects play a significant role in the deformation of granular assemblies. Study of a single grain in 3D demonstrates progress in experimental capabilities and highlights the need for more studies to validate existing numerical models and theories for granular matter. Analysis of particle scale deformations and strains with the Granular Element Method (GEM) allows us to determine interparticle forces and understand the development and evolution of force chains in a granular assembly under a wide variety of loading conditions. These experiments can also lead to development of new understanding of the effects of inclusions on material properties, processes, and damage evolution.

Thesis Files