Enhancing Fracture Toughness through Elastic Heterogeneity

Kristen John, California Institute of Technology

Fracture toughness is a material property describing the ability of a material containing a crack to resist fracture. My research focuses on determining whether it’s possible to increase a material’s fracture toughness through elastic heterogeneity. Fracture toughness involves the presence of an initial crack/flaw in the material. Once the energy release rate (G) equals the critical energy release rate (GC), the crack starts to grow. GC is a material property related to fracture toughness and tells us how difficult it is to break a material.

Fracture toughness is related to GC and solid mechanicians have shown that heterogeneous materials have enhanced failure resistance. Therefore, what is the role of elastic heterogeneity in enhancing fracture toughness? Think of the simplest heterogeneous material: laminates. A laminate consisting of two materials will have two GC’s. Instead, think of a laminate with the same GC throughout, but different elastic modulus (E). Surprisingly, at a macroscopic scale, the effective GC of the laminate is not the same as the GC’s of the materials. In order to induce elastic heterogeneity in a homogeneous material while keeping the same GC, I will create a Homalite specimen and cut a series of parallel “grooves”/slots such that the specimen has two different thicknesses of the same material.

This specimen now has uniform GC, but different E. To fracture the specimen, I will introduce a pre-crack, then load the specimen until it fractures (Mode-I) by performing a tensile test (high-strain, high-stress). I will measure loads, displacements, and crack propagation. From the results, I will compare the effective GC to a homogeneous material to characterize the enhancement and compare experimental results versus theory/simulations. Additionally, I will determine what happens to GC at a macroscopic scale, and determine if fracture toughness (via GC) can be controlled/increased by varying elastic moduli (via thickness).

Abstract Author(s): Kristen John