A Model for the Long-Life Fatigue Behavior of Small Notches
by
Chih-Hsien Ting
F. V., Lawrence
Department of Materials Science and Engineering
Abstract
The effect of notches and intrinsic defects on the long-life regime fatigue properties was studied. A crack-closure-at-a-notch (termed CCN) model for predicting the fatigue life and fatigue threshold stress was developed based on the assumption that the da/dN vs. ΔKeff relation is a unique material property independent of crack length. The variation of crack closure level with crack length for a crack emanating from a notch was found to depend on the material properties, the notch size, the notch acuity, and the applied load conditions.
The stress-life (S-N) properties of a cast aluminum alloy 319 were studied at room temperature under the R = -1 condition for both as-cast and polished cylindrical bar specimens. The test results showed that the fatigue strength of cast aluminum alloy 319 was not significantly affected by the surface conditions but strongly influenced by the size of defects which initiate fatigue cracks. Crack growth rate tests (da/dN vs. ΔK) for R = -1 were also carried out on single-edge-notch plate (SENP) specimens. The fatigue threshold stresses and fatigue lives for both cylindrical bar specimens and SENP specimens were successfully predicted by the CCN model. The validity of this model was further confirmed through comparison of predictions with the experimental results reported in the literature.
The CCN model considered intrinsic defects to be small notches. The limits of the CCN model applicability were studied. The maximum non-damaging notch depth for a material was quantitatively determined. A general description of the effect of a notch on the fatigue threshold stress was proposed.
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