An Evaluation of Damage Development During Multiaxial Fatigue of Smooth and Notched Specimens
by
ames Walter Fash
Department of Mechanical and Industrial Engineering
Abstract
Several theories have been proposed for multiaxial fatigue analysis, but a lack of consensus exists on the most appropriate for use in design. Five multiaxial fatigue theories are developed in the form of a strain parameter verses life relationship. Uniaxial, smooth specimen fatigue properties are employed to predict the results of two multiaxial fatigue test programs. Fatigue damage has been observed throughout both series of tests to relate the damage parameter for life analysis to the physical processes of fatigue.
Thin-wall tube specimens are tested in strain controlled, tension-torsion loading. A large volume of material is subjected to a uniform multiaxial strain state; hence, this geometry can be considered analogous to the smooth specimen for uniaxial fatigue. All five theoretical models result in good correlation of the thin-wall tube tests. A notched shaft designed to simulate a typical engineering component was tested under bending-torsion loading. Crack initiation occurs in a small volume of material in the vicinity of the notch, and subsequent growth is into a decreasing stress-strain field. Theoretical predictions and experimental results for the notch shaft program show considerably less correlation than that obtained for the thin-wall tube tests. Concepts of the local stress-strain fatigue analysis method suggest that if the local damage parameters for the smooth and notched specimen are equivalent, the fatigue lives will be equal. This
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