SEDL / STP / STP1332-EB / STP14962S

Stress-Level-Dependent Stress Ratio Effect on Fatigue Crack Growth

Sunder, R
Materials and Manufacturing Directoratetechnical directorresearch engineer and senior research engineer, Air Force Research Laboratory, AFRL/MLLNBiSS ResearchUniversity of Dayton Research Institute, Wright-Patterson AFB BangaloreDayton, OHOH

Porter, WJ
Materials and Manufacturing Directoratetechnical directorresearch engineer and senior research engineer, Air Force Research Laboratory, AFRL/MLLNBiSS ResearchUniversity of Dayton Research Institute, Wright-Patterson AFB BangaloreDayton, OHOH

Ashbaugh, NE
Materials and Manufacturing Directoratetechnical directorresearch engineer and senior research engineer, Air Force Research Laboratory, AFRL/MLLNBiSS ResearchUniversity of Dayton Research Institute, Wright-Patterson AFB BangaloreDayton, OHOH

Pages: 12    Published: Jan 1999

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Abstract

Fractographic measurements of fatigue crack growth rate for small cracks reveal stress-ratio effects even when fatigue crack closure is absent. These effects are restricted to low fatigue crack growth rates and become significant with increase in net stress levels. To characterize the effect, experiments and analyses were conducted on notched coupons of an Al alloy at stress levels producing inelastic conditions on initial loading. As a reference, fatigue crack growth rates were obtained for a long crack tested under fully elastic loading well below yield stress. The results indicate that for fatigue growth associated with low applied stress intensity range, minor changes in stress ratio can cause substantial variation in crack growth rate. A model is proposed for the small crack fatigue growth rate as a function of applied stress intensity and stress ratio. The model is based on crack growth rates obtained under several stress levels with crack size as small as 0.03 mm.