Ali Khawagi

Date of Award

8-17-2015

Document Type

Thesis

Degree Name

Master of Science in Materials Science

Department

Department of Aeronautics and Astronautics

First Advisor

Shankar Mall, PhD.

Abstract

This research investigates fatigue crack propagation behavior in both air and saltwater (3.5% NaCl) environments from pre-cracked notched circular hole in a 7075-T6 cruciform specimen. With stress ratio of 0.5, biaxility stress ratio of unity, and frequency of applied load of 10 Hz, the crack growth behavior was investigated under in-plane biaxial tension-tension fatigue with 45⁰, 90⁰ and 180⁰ phase difference conditions and then compared to previous fatigue tests with no phase difference to study the effect of changing the phase differences between the applied loads on the crack growth rate. Finite Element Analysis (FEA) was used to calculate cyclic variation of stress intensity factors (ΔK) at the crack tips. The crack growth rate was observed using optical microscopy. This study shows that in the biaxial fatigue tests with phase difference of 45⁰, 90⁰ and 180⁰, two fatigue cracks were shaped, symmetrical in case of 90° and 180°, while it’s unsymmetrical for 45⁰ phase difference case. For each phase difference and at a given average crack growth rate, the strain energy release rate of the non-split crack is equivalent to the sum of strain energy release rates of the two split cracks. In the saltwater environment, the corrosion accelerates the crack growth rate.

AFIT Designator

AFIT-ENY-MS-15-S-065

DTIC Accession Number

ADA621805

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