Candidate: Stoyan Stoychev

Degree of: Doctor of Philosphy

Department: Mechanical and Aeronautical Engineering

Title: Load Interaction Effects on Fatigue Crack Growth

Committee: Dr. Daniel Kujawski, Chair
Dr. Judah Ari-Gur
Dr. Philip Guichlelaar
Dr. Giora Kimmel

Date: Friday, May 13, 2005 10:30 a.m.-12:30 p.m.
Parkview Campus F-115

Abstract: It is a well known fact that the majority of mechanical failures in practice are caused by fatigue – a phenomenon where the stresses in the component may never exceed the yield strength of the material and still, a crack will nucleate and propagate, causing failure. It is of critical importance then, to answer these two questions: 1) Will the component fail? 2) How long before it fails? The quest for a solution of these questions started in 1829 (Albert - Effects of Repeated Loads). Almost 200 years later, it is still difficult to predict the fatigue life. The existing methods are largely empirical and often result in unacceptable errors. This dissertation is intended to shed light on the load history effects on the crack propagation. It will be shown that the ‘crack closure' concept, as defined by Elber more than 30 years ago, is not sufficient to explain these effects. There are a number of emerging theories that have the potential to replace the crack closure concept (mean stress effect, residual stresses, etc.). These new theories are evaluated and compared, based on experiments with overloads and block loading. Also a new methodology for fatigue crack growth prediction is (over) developed and evaluated. The new theoretical model shows good agreement with the experimental data. This work will aid in the development of fatigue life prediction methods under variable amplitude loading.

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