Date of Award

9-15-2016

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 the behavior of fatigue crack growth rate in both laboratory air and saltwater (3.5% NaCl) environments for notched cruciform specimen made from 7075-T6 aluminum alloy sheet at different stress ratios. With biaxility ratio of 1, and frequency of applied load of 10 Hz, the crack growth behavior was investigated under in-plane biaxial tension-tension fatigue with 0.1, 0.5, 0.7 stress ratios and then compared them to study the effect of stress ratio on the crack growth rate. Finite Element Analysis (FEA) was used to calculate cyclic variation of stress intensity factors (delta K) at the crack tips. The crack growth rate was observed using optical microscopy. In addition, the test generates more accurate definition of the Walker equation parameters and leads to more accurate prediction of fatigue crack growth rate at different stress ratio. This study shows that there is no effect of the stress ratio on crack path direction. Increase in stress ratio leads to increase in the fatigue crack growth rate under the biaxial loading test. The effective fatigue crack growth rate predicted by Walker equation is very close to the effective fatigue crack growth rate generated by the test in laboratory air and saltwater environments. In the saltwater environment, the corrosion accelerates the crack growth rate.

AFIT Designator

AFIT-ENY-MS-16-S-052

DTIC Accession Number

AD1018250

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