Date of Award

1998

Degree Type

Thesis

Degree Name

Master of Science in Mechanical Engineering and Applied Mechanics

Department

Mechanical Engineering and Applied Mechanics

First Advisor

Arun Shukla

Abstract

A search of the literature has shown that a large number of empirical models exist for predicting the fracture toughness of particle-matrix composites based on their component material properties, but there is a poor understanding of the correlation of these models to the true physical basis for the models. The extent to which variations in fracture toughness can be attributed to factors such as crack deflection, crack-tip bridging, crack-front bowing, and interfacial adhesion, is an important area for new research. The objective of the present work has been to characterize the role of some of these factors by making dynamic observations of a propagating crack in a particle filled brittle matrix composite. High speed photography has been used to obtain photoelastic images of the state of stress at the leading edge of dynamic cracks as they intersect and pass disperse spherical particles in a birefringent polyester matrix. In order to further characterize the nature of the crack tip in the area of embedded spherical particles, cracks were also induced to arrest in close proximity to spherical particles in composite materials of the same type as used in the dynamic experiments.

Experiments were performed using brittle polyester matrix materials with embedded disperse spherical particles. The particles were of three types including steel, glass, and rubber. Three series of experiments were performed. In the first series of experiments the physical characteristics of a polyester matrix material were evaluated including tensile strength, elastic modulus and birefringent properties. The second series of experiments utilized a CranzSchardin camera, and a circular polarizer to study the state of stress at the tip of dynamic cracks during progression of the cracks past particles in Modified Compact Tensile (MCT) test specimens. The third series of experiments was performed using the same component materials as the MCT experiments, in the form of single edge notch (SEN) specimens. In these experiments cracks were induced to arrest in close proximity to the spherical particles. Macro graphic analysis of the fracture surfaces of the dynamic MCT samples, and observation of the arrested crack fronts in the SEN samples were then used to determine the path and shape of the leading edge of the dynamic crack. These crack front profiles were then directly correlated to the stress intensity and velocity profiles obtained in the dynamic photoelastic studies.

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