Some Aspects of the Mechanics and Mechanisms of Dynamic Brittle Fracture
Kamath, Sundar Mangalore
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https://hdl.handle.net/2142/71694
Description
Title
Some Aspects of the Mechanics and Mechanisms of Dynamic Brittle Fracture
Author(s)
Kamath, Sundar Mangalore
Issue Date
1987
Department of Study
Theoretical and Applied Mechanics
Discipline
Theoretical and Applied Mechanics
Degree Granting Institution
University of Illinois at Urbana-Champaign
Degree Name
Ph.D.
Degree Level
Dissertation
Keyword(s)
Applied Mechanics
Abstract
Previous research in dynamic fracture mechanics has focused largely on steady-state growth in the absence of reflected stress waves. This is because of the analytical and experimental complexities of studying crack propagation in finite geometries. For example, inadequate spatial and temporal resolution is one potential limitation of high-speed photography, as used in the method of caustics, and in dynamic photoelasticity.
This study represents a major departure from current research. The problem chosen is that of brittle fracture in a finite-length beam under quasi-static flexure. The gradient stress field and boundary wave reflections present in this configuration lead to highly transient propagation which includes several intermittent arrests. Using newly developed optical methods, such as the Stress Intensity Factor Tracer (SIFT), dynamic stress intensity factor (K) and crack-tip velocity (v) have been measured continuously during fracture without using high-speed photography. The velocity-dependent fracture toughness relation, or K-v curve, has been obtained from a single specimen containing multiple run-arrests. Tests were conducted on PMMA (Plexiglass), an acrylic thermoset, and Homalite-100, a polyester resin, in their glassy states. The results for PMMA highlight the problems of characterizing history-dependent crack growth in rate-sensitive materials. The strong inertia effects observed during fracture were caused by boundary reflections of the unloading extensional waves and the Rayleigh surface waves emitted at fracture. Macroscopic measurements were supported by extensive fractography to gain insight into the mechanisms.
Several related aspects are also examined. A theoretical and experimental analysis of coherent light caustics for a Mode-I crack is presented in chapter 2. Errors in K estimation due to incorrect definitions of caustic diameter location caused by diffraction effects are discussed. Chapter 3 describes a sensitivity study of the SIFT method under quasi-static loading, to experimental variables like specimen geometry, notch root radius/near-tip effects, and focal plane aperture design. Also included is an extension of the SIFT focal plane mapping theory to characterize the Hutchinson-Tice-Rosengren singularity field for a stationary crack in power-law hardening materials. Chapter 4 provides an overview of critical issues concerning near-tip processes in dynamic fracture versus far-field characterization.
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