AN EVALUATION OF DAMAGE DEVELOPMENT DURING MULTIAXIAL FATIGUE OF SMOOTH AND NOTCHED SPECIMENS

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.;Crack behavior is observed using surface replicating techniques. Crack initiation in the thin-wall tube occurs on planes that experience the maximum range of shear strain. After a period of growth on this plane the crack changes direction and grows in a plane perpendicular to the maximum principal stress. For the 1045 steel considered, the crack size when this transition occurs is dependent on strain state and strain amplitude. Cracks in the notched shaft initiate in the notch plane rather than on planes of maximum shear for all tests except torsion. Growth to failure occurs on planes perpendicular to the maximum principal stress for low amplitude tests and in the notch plane for high amplitude tests. This behavior is reflected in the correlation of experimental results but is not accounted for in the theoretical models. (Abstract shortened with permission of author.);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.