Date of Award

5-2015

Degree Name

Doctor of Philosophy

Department

Mechanical and Aerospace Engineering

First Advisor

Dr. Daniel Kujawski

Second Advisor

Dr. Judah Ari-Gur

Third Advisor

Dr. Muralidhar Ghantasala

Fourth Advisor

Dr. Sam Ramrattan

Keywords

Fatigue, environmentally assisted, modeling, slow strain rate test, two parameter, chemical notch

Abstract

The formation of fatigue cracks and their propagation due to cyclic loading in metals have been a concern for more than hundred years. Since fatigue failure were first reported by the railroad industry in 1840s, tremendous progress has been achieved in understanding fatigue behavior of metals. But fatigue damage is still a concern due to its complex dependency on various environmental variable like humidity, temperature, time and corrosive environment. Although numerous theories and models have been proposed in the past, the effects of environment on fatigue crack growth (FCG) is not completely understood. This dissertation aims to shed light on the effect of environment on the FCG behavior in metals through experimental techniques (Part-I) and modeling (Part- II). Using the available concepts in literature and the generated experimental data, a new fatigue model is proposed which accounts for thickness, stress ratio and environmental effects on FCG behavior.

The slow-strain-rate tests revealed that the effect of liquid-air interface on tensile behavior of Al 7075-T651 can be explained by a concept of chemical notch at the liquid-air interface. The FCG behavior of Al 7075-T651 alloy in air and in vacuum were investigated by using an earlier developed custom matrix method based on △K and Kmax. The FCG data generated revealed the influence of stress ratio,(R), thickness and environment (air and vacuum) on FCG behavior. A new FCG model presented in Part-II includes the environmental effect through change in crack tip angle. In this model the mechanical fatigue damage is based on the two parameter approach, △K and Kmax. The model predictions of R-ratio, thickness and environment effects on FCG behavior in metals is compared with available data published in the literature and fairly good agreement was found.

Access Setting

Dissertation-Open Access

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