FATIGUE BEHAVIOR UNDER MULTIAXIAL LOADING IN THE PRESENCE OF A NOTCH: METHODOLOGIES FOR THE PREDICTION OF LIFE TO CRACK INITIATION AND LIFE SPENT IN CRACK PROPAGATION (SHOULDER FILLET, PLASTIC STRAIN, PREDICTION).

摘要

The fatigue behavior of a specimen subjected to multiaxial loading and containing a stress concentration (notch) is investigated by considering two separate divisions of the total number of loading cycles to fracture. First, the number of cycles necessary to initiate a crack of a specified size at the notch is considered. Next, the subsequent life spent in crack growth to fracture is considered.;The implementation of several conventional and new crack initiation life prediction methods are described in detail and compared to data being generated by the Fatigue Design and Evaluation Committee (FD&E) of the Society of Automotive Engineers (SAE), for a round bar with a shoulder fillet subjected to combined bending and torsion, as well as other available data in literature. Improvements to the methods are suggested.;A procedure for estimating strains at a notch using a simplified elastic-plastic analysis is presented in detail for the SAE specimen. Extension of the approach to other notch geometries could circumvent the need for expensive elastic-plastic finite element analyses.;Methods for predicting propagation lives of cracks originating at the notch of specimens subjected to bending are proposed and predictions compared to available data. The effect of notch elastic stress distribution is taken into account.;A review is made of available initiation and propagation fatigue data trends and correlation methods for smooth and notched specimens under uniaxial and multiaxial loading.;An investigation is also made of the circumferential variation of notch stress concentration since it will influence both crack growth and notch strain predictions.;The data generated by the SAE-FD&E committee to data are entirely for constant amplitude, fully reversed loading. The bulk of the analysis in this thesis pertains to this type of loading. Recommendations for future work include the extension of the life prediction methodologies to variable amplitude loading, and to situations where material anisotropy is important.