Localized deformation in soft solids around spherical cavities

Biological tissue, like internal organs, brain, or skin, are soft solids that may be exposed to injury inducing mechanisms at a range of rates, resulting in damage. Sport injuries, car crashes, traumatic brain injury, and ballistic impacts can produce strain rates from 10−10^5 1/s. The parameters governing failure in these soft solids (moduli < 1 MPa) are challenging to quantify, requiring modifications or defying the use of traditional characterization techniques. Driving bubble/droplet growth or contraction provides a way to characterize these materials while avoiding some limitations of traditional techniques. Large deformations in these spherical geometries induce localized deformations at the surface of the cavity. In this thesis, I leverage these localized deformations, cracks during expansion and creases during contractions, to investigate fracture energy and strain stiffening, respectively. A Small-scale Ballistic Cavitation device uses a high-pressure air reservoir to generate spherical deformation at high rates within soft solids. Air accelerates through a needle, reaching the speed of sound at the tip before delivery to the sample. The energy density of the air pulse matches that of bullets, producing small, ballistic-like cavitations. Independent control of pressure, needle diameter, and valve cycle time provides flexibility in experimental control variables not available in other ultrasoft solid cavitation techniques. Using these needle-mediated, high rate spherical expansions, I investigate the parameters governing fracture initiation in soft solids by adapting a theory of fracture traditionally used in dynamic failure in hard materials. I present results from cavity expansions in silicone and gelatin samples. Increasing the rate of expansion increases the number of cracks initiated in the cavity surface, leading to multi-lobed cracks, as opposed to the penny-shaped cracks present in quasi-static cavitations. The elastic wave speed-dependent fracture correlation model I adapt suggests that counting the number of cracks provides a measure of the soft solid’s fracture energy. Additionally, I include the implications of this model for analytical calculations in very tough, nonlinear materials. Finally, I demonstrate the multiple fracture phenomenon in ballistic impacts and present a method for analyzing damage that draws upon the understanding gained in from the bench-scale cavity expansions. In the last chapter, I report on crease morphology and evolution at the surface of contracting cavities embedded within elastomeric solids of varying degrees of crosslinking. Cavity contraction is achieved through evaporation of an embedded water droplet. In validation of theoretical predictions, strain-stiffening is found to govern both crease onset and crease density. Neo-Hookean solids are found to prefer initiating creasing with many short creases that join to form a collapsed state with only a few creases, whereas creasing in Gent solids initiates with a few creases that propagate across the cavity surface.

Name thesis:degree_name

Ph.D.

Level thesis:degree_level

Dissertation

Discipline thesis:degree_discipline

Mechanical Engineering

Grantor

University of Illinois at Urbana-Champaign

Year dc:date

2020

Author dc:creator

• Milner, Matthew P.

Contributors dc:contributor

• Hutchens, Shelby B.
• Lambros, John
• Lopez-Pamies, Oscar
• Hilgenfeldt, Sascha

• soft solid
• fracture
• high rate
• ballistic
• spherical

Statement dc:rights

• Copyright 2020 Matthew P. Milner

Language dc:language

en

Handle dc:identifier

http://hdl.handle.net/2142/108454

OAI identifier oai:identifier

oai:www.ideals.illinois.edu:2142/108454