Dissertation/Thesis Abstract

Fracture and Contact Mechanics of Soft Materials: Theory and Experiment
by Qi, Yuan, Ph.D., University of Colorado at Boulder, 2020, 185; 27959130
Abstract (Summary)

Soft elastomers and gels are widely applied in many technologies such as pressure sensitive adhesives, soft robotics and stretchable electronics. Understanding their mechanical behaviors is critical for evaluating the mechanical reliability and performance. However, mechanics of soft materials is a challenging task due to its highly nonlinear nature. This is especially true in fracture problems where the present of cracks introduces a severely concentrated strain field at the crack tip.

Starting from the fracture toughness, which quantifies the work required to advance the crack by a unit area, we present a theoretical framework for calculating the energy dissipation associated with crack propagation. Specifically, we focus on a model material system with rate-independent hysteresis: a neo-Hookean solid with Mullins effect. We determine analytical relations between fracture toughness and the parameters governing bulk hysteresis, and quantitatively predict the reduction in fracture toughness due to prestretch.

Experimentally, we present a new approach to map the large deformation field by optically tracking the displacements of randomly distributed tracer particles whose trajectories are further converted to continuous displacement and strain fields through an interpolation scheme. Using a soft silicone elastomer as model system, we verified the accuracy of the method and experimentally observed, for the first time, the asymptotic solution of crack tip deformation field for nonlinear elastic solids with strain stiffening. This method also enables local evaluation of energy release rate through the J-integral and investigation on the crack growth resistance behavior in respect to true crack extension.

Next, we explore crack propagation in a highly stretchable viscoelastic material. We examine the steady state assumption adopted in most theories, where the strain and stress fields in the solid translate with the crack tip in a constant speed. We also study the relation between the viscoelastic dissipation and crack propagation velocity.

Lastly, we extended the particle tracking method in three dimensions to study the local contact mechanics between PDMS pillars and a polyacrylamide hydrogel. Using confocal microscopy, we track the fluorescent beads embedded in the hydrogel and analyze the influence of the indentation depth and the shape of the pillar on the global shear force.

Indexing (document details)
Advisor: Long, Rong
Commitee: Long, Rong, Vernerey, Franck, Rentschler, Mark, Xiao, Jianliang, Jimenez, Francisco Lopez
School: University of Colorado at Boulder
Department: Mechanical Engineering
School Location: United States -- Colorado
Source: DAI 81/11(E), Dissertation Abstracts International
Subjects: Mechanics
Keywords: Dissipation, Fracture mechanics, Large deformation, Particle tracking, Soft materials
Publication Number: 27959130
ISBN: 9798645450717
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