FATIGUE LIFE, CRACK INITIATION, AND PROPAGATION OF 304 STAINLESS STEEL WITH NON-PROPORTIONAL AXIAL-TORSIONAL STRAIN-PATHS

In this thesis, the concepts of energy-probability, damage accumulation and path-dependence are used to formulate a criterion for multiaxial fracture under the condition of low-cycle fatigue.;In the investigation of fatigue life of metals under multiaxial loading condition, it has been recognized in recent years that the effect of path-dependence must play an important role. A set of strain-controlled experiments has been performed with strain-paths especially designed to highlight the characteristics of path-dependence in the multiaxial fatigue tests. The endochronic constitutive theory is first used to study the stress-strain responses of the designed strain-paths, and the result is directly applied to the new energy-probability fatigue criterion to account for the effect of path-dependence. It has been found that the theoretical results based on this new energy-probability fatigue criterion agree reasonably well with the test data.;Two other multiaxial fatigue fracture criteria, i.e., the effective mean strain range of Taira et al and the plastic work criterion of Garud are also compared with the experiment.;Crack initiation and propagation have also been studied based on the same set of multiaxial tests. Path-dependence does show significant influences on these effects. Scanning electron microscope (SEM) is heavily used to identify crack initiation and to observe fracture modes. A fracture mechanics model for the extension of a straight crack under nonsymmetric plane loading condition has also been applied to describe the observed direction of fatigue crack propagation under biaxial straining. Both mechanics analyses and SEM observations show that crack initiation and propagation are dominated by tensile mode. This is true even for the case of pure torsional fatigue with non-zero mean stress (strain).;A time-like parameter termed fatigue intrinsic time is introduced to measure damage accumulation in fatigue. This concept is an extension of the endochronic theory of viscoplasticity, which is based on the postulate that the history of deformation of a microelement is described with respect to its own intrinsic time scale, defined as the length of the plastic strain-path. A fatigue function is also introduced to regulate the rate of damage due to cycling. Fatigue fracture occurs when the fatigue intrinsic time is equal to a critical value.