GEORGIA INSTITUTE OF TECHNOLOGY

Under the provisions of the regulations for the degree

DOCTOR OF PHILOSOPHY

on Tuesday, April 3rd, 2018

4:00 PM
in MRDC 3515

will be held the

DISSERTATION DEFENSE

for

Wade Lanning

"The Steady-State Work Density Gradient: a New Parameter and Strategies for Characterizing Crack Propagation in Thin Ductile Sheets"

Committee Members:

Prof. Christopher Muhlstein, Advisor, MSE

Prof. Hamid Garmestani, MSE

Prof. Arun Gokhale, MSE

Prof. David McDowell, ME

Prof. Richard Neu, ME

Abstract:

The fracture toughnesses of ultra-thin FCC metal sheets reported in the literature are orders of magnitude lower than the bulk, which presents a serious obstacle to scaling the high strength of ultra-small specimens up to larger length scales. This work demonstrates that fracture toughnesses of thin and ultra-thin FCC metal are not low as suggested by the literature, and that fracture was by stable ductile crack propagation. The difficulty with such thin sheets is in finding an appropriate analysis framework to describe fracture in a system where large-scale plasticity, out-of-plane elastic buckling, and through-thickness necking impede the application of conventional mechanics toughness analyses.

While thin sheets pose problems for conventional modeling and scaling-effect based fracture toughness measurements, they also present an opportunity for in-situ measurements of crack growth and deformation within the sheet. This work lays out a foundation for a new fracture toughness parameter, the work density gradient, which describes steady-state crack propagation in ductile thin sheets. The work density gradient can be combined with boundary displacement measurements to compute a measurement of energy expenditure during crack propagation which can be compared to other fracture toughness parameters. It may also be combined with digital image correlation (DIC) strain measurements to reveal the spatial distribution of energy absorption by material around the fracture process zone. Unlike conventional fracture mechanics, the approaches used in this work are based solely on experimental measurements and require neither a model nor a constitutive relationship. The work density gradient framework is especially useful in exploring thin ductile sheet systems with deformation properties which are either difficult to characterize or unknown. In this presentation, we will explore the experimental development of the work density gradient and demonstrate how it may be applied to describe thin aluminum sheets or the effect of surface engineering on ultra-thin gold sheets.