Experimental and theoretical aspects of dynamic crack growth along bimaterial interfaces

Citation

Fey, Kate Elizabeth (1996) Experimental and theoretical aspects of dynamic crack growth along bimaterial interfaces. Dissertation (Ph.D.), California Institute of Technology. doi:10.7907/ehea-x787. https://resolver.caltech.edu/CaltechETD:etd-12132007-082556

Abstract

This work presents findings of an experimental and theoretical study of dynamic bimaterial crack growth. Bimaterial systems composed of constituents with large material mismatch were investigated under dynamic loading conditions. The materials used in this study consisted of Poly-Methylmethacrylate (PMMA) and AISI 4340 Steel, bonded together using a Methylmethacrylate monomer. One point bend loading was achieved using a drop weight tower. Dynamic crack growth, with velocities up to eighty percent of the Rayleigh wave speed of PMMA, was observed using the lateral shearing interferometric technique of Coherent Gradient Sensing (CGS) in conjunction with high speed photography. The results of these experiments are first discussed within the realm of the validity of the linear, elastodynamic asymptotic stress fields. The complex interdependency of stress intensity and mode mixity with crack tip speed is also discussed. The interpretation of |K[superscript d]| and [phi superscript d] in a dynamic bimaterial crack is clarified through the experimental observation of crack growth. Complications in analysis arising from this interdependency between the dynamic K[superscript d]-field and velocity are examined for experimentally obtained CGS fringe patterns. Improvements of existing analyzing procedures are made, resulting in increased confidence in data obtained utilizing the method of CGS in dynamic bimaterial fracture. Special attention is given to the interaction of loading and velocity in the behavior of these crack tip fields. Previous methods of investigation have used an elastodynamic, asymptotic K[superscript d]-field to describe the deformations near a bimaterial crack tip. Attempts to develop a fracture criterion based on these results have suffered from the lack of natural length scale as the major criticism. Motivated by experimental observations, a cohesive zone model is presented in this thesis that allows an investigation of dynamic crack growth. The length of the cohesive zone is given by a combination of stress intensity and mixity, bimaterial behavior, and velocity, and emerges as a natural, time evolving length scale with which to examine the bimaterial crack problem. A fracture criterion based on critical cohesive displacements at the trailing edge of the cohesive zone is presented. This cohesive zone model is subsequently used to examine data obtained from experiment. The model enhances our ability to extrapolate our experimental measurements to the near tip region, and to thus study the neighborhood close to the propagating crack tip. Within experimental error, predictions of the proposed fracture criterion are shown to correspond to the experimentally observed dependence of |K[superscript d]| and [phi superscript d] on the instantaneous crack tip velocity. The fracture criterion based on the cohesive model presented in this paper provides the natural next step in understanding dynamic bimaterial crack growth. It provides a criterion based on physically motivated parameters, introduces a natural length scale into the problem, and increases our understanding of dynamic bimaterial fracture mechanics.

Thesis Files