Date of Award

9-18-2014

Document Type

Thesis

Degree Name

Master of Science

Department

Department of Aeronautics and Astronautics

First Advisor

Shankar Mall, PhD.

Abstract

In this research, fatigue crack formation from two types of corrosion pits at a circular hole was investigated under uniaxial fatigue. Through pits and corner pits were created on the edge of a circular hole in test specimens using an electrochemical process. Specimens of 2024-T3 aluminum alloy were subjected to cyclic uniaxial loads with stress ratio of R = 0.5 in both air and saltwater environments. A fracture mechanics approach was used to investigate the crack initiation and crack growth from corrosion pits. Specimens with a through pit at the edge of a circular hole had a closed form solution to predict stress intensity factor range, ΔK, which was in agreement with finite element analysis. In addition, specimens with a corner pit do not have a closed form solution and finite element modeling was used to determine stress intensity range. Optical and electron microscopy provided an accurate method to measure the size of corrosion pits. Exposure to saltwater reduced the number of cycles for crack initiation in both types of corrosion pits. This reduction is up to 90% for through pits and up to 75% for corner pits. The required number of cycles for crack initiation for corner pit specimens is less than for through pit specimens. Here, the number of cycles decreases up to 94% in air and up to 88% in saltwater environment. There was a good agreement between crack growth rates in machined notch specimens and the specimen with through pit.

AFIT Designator

AFIT-ENY-T-14-S-05

DTIC Accession Number

ADA608841

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