Zafer Dolu

Date of Award

6-19-2014

Document Type

Thesis

Degree Name

Master of Science

Department

Department of Aeronautics and Astronautics

First Advisor

Shankar Mall, PhD.

Abstract

This thesis was investigated the effects of pre-existing corrosion on the fatigue crack initiation and fatigue crack growth behavior of the typical aircraft aluminum alloy 7075-T6, by using a fracture mechanics approach. Initiation and crack growth behavior was investigated under in-plane biaxial tension tension fatigue with stress ratio of 0.1 and biaxiality stress ratio, =1. Cruciform specimens with a center hole, having a corrosion pit at 45 to the specimen s arms, were tested in a biaxial fatigue test machine in laboratory air and saltwater (3.5% NaCl) environments. Crack initiated and propagated coplanar with the pit in L T orientation. For the sake of comparison, uniaxial fatigue tests with stress ratio of 0.1 in laboratory air and saltwater (3.5% NaCl) environments were also conducted. The number of cycles until initiation of the fatigue crack and the crack growth rate were observed and measured by optical and scanning electron microscopy. This research shows that corrosive environment of saltwater reduces the required number of cycles for crack initiation under uniaxial and biaxial loading conditions when compared to those in ambient laboratory air. In all tests corrosive environment shortens the fatigue life of the specimens under uniaxial and biaxial loading conditions. In ambient air, numbers of cycles for crack initiation under biaxial and uniaxial loading conditions are almost the same, on the other hand in saltwater environment crack occurs in lesser number of cycles in biaxial loading condition relative to uniaxial loading condition. The crack growth rate is faster in the saltwater environment than the corresponding value in ambient air environment at a given K in both loading conditions. In ambient air environment biaxial and uniaxial crack growth rates were almost similar. But in saltwater environment biaxial fatigue have faster crack growth rate than uniaxial fatigue.

AFIT Designator

AFIT-ENY-T-14-J-33

DTIC Accession Number

ADA602933

Download

Share

COinS