Author

Reddy

Date of Award

12-2008

Degree Name

Master of Science in Engineering

Department

Mechanical and Aerospace Engineering

Department

Mechanical and Aeronautical Engineering

First Advisor

Dr. Daniel Kujawski

Second Advisor

Dr. Muralidhar Ghantasala

Third Advisor

Dr. Dennis VanderBrink

Access Setting

Masters Thesis-Open Access

Abstract

A custom method to generate fatigue crack growth (FCG) data requires testing of multiple specimens at different load ratios. In addition, it includes the application of a load shedding procedure from pre-cracking level to threshold.

In this thesis, a novel method of testing has been investigated that utilizes a single specimen and a testing matrix in terms of Kmaxand ∆K values corresponding to predetermined R-ratios for which FCG data, da/dN, are recorded. Automatic K-controlled tests on 2324-T39 Al alloy were conducted using the ∆K increasing and ∆K decreasing procedures while Kmaxwas kept constant. Results show that the increasing ∆K procedure is more conservative than the ∆K decreasing procedure. Fatigue crack growth curves in terms of da/dN versus ∆K obtained from ∆K increasing are below the curves obtained from the ∆K decreasing procedure.

These differences are explained by means of load interaction between cyclic plastic zones and their effect on fatigue damage. The ∆K increasing procedure demonstrated minimal interaction effects and hence it is recommended for efficient FCG generation. The proposed procedure reduces testing time, the overall scatter associated with multiple samples and eliminates possible uncertainty linked to the load shedding procedure and its effect on threshold.

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