Eric M. Hunt

Date of Award

6-13-2013

Document Type

Thesis

Degree Name

Master of Science

Department

Department of Aeronautics and Astronautics

First Advisor

Shankar Mall, PhD.

Abstract

Research on fatigue crack formation from two types corrosion pits tangent to a circular hole in a 7075-T6 aluminum alloy subjected to uni-axial loads (R = 0.5, lambda = 0) in both an air and saltwater environment provides a method for exploring crack initiation and initial growth rates. This work focuses on a fracture mechanics approach to explore the transition from corrosion pit to crack growth. Specimens with a cylinder shaped through-pit tangent to a circular hole have a closed form solution to predict this delta K that closely resembles the finite element solutions. Specimens with a semi-circular corner-pit tangent to hole lack a closed form solution and finite element modeling was used to determine delta K of these specimens. Optical and electron microscopy provided an accurate way to measure and observe the crack growth rate (da/dN) and the cycles until initiation of the fatigue cracks. This research shows that corner-pit specimens initially have a slower crack growth rate than through-pit specimens due to the propagation of a quarter-circular crack front through the thickness of the sample. After initial crack growth, both corner-pit and through-pit samples have the same growth rate as their machine notched counterparts exposed air and saltwater environments.

AFIT Designator

AFIT-ENY-13-J-01

DTIC Accession Number

ADA581422

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