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Because niobium-clad 304L stainless steel sheets are considered for use as bipolar plates in polymer electrolyte membrane (PEM) fuel cells, their mechanical behavior and failure mechanism are important to be examined. As-rolled and annealed specimens were tested in tension, bending and flattening. The effects of annealing temperature and time on the mechanical behavior and failure mechanism were investigated. Micrographic analyses of bent and flattened specimens showed that the as-rolled specimens have limited ductility and that the annealed specimens can develop an intermetallic layer of thickness of a few microns. The annealed specimens failed due to the breakage of intermetallic layer causing localized necking and the subsequent failure of Nb layer. The springback angles of the as-rolled and annealed specimens were also obtained from guided-bend tests.

The theoretical framework and closed-form stress intensity factor solutions in terms of the structural stresses for spot welds under various types of loading conditions are presented based on elasticity theories and fracture mechanics. A mechanics description of loading conditions for a finite plate with a rigid inclusion is first presented. The loading conditions of interest are the resultant loads on the inclusion in a plate and the surface tractions on the lateral surface of a plate. The surface tractions on the lateral surface of the plate can be decomposed into a load-balanced part and a self-balanced part. The resultant loads on the inclusion and the self-balanced resultant loads on the lateral surface are then decomposed into various types of symmetric and anti-symmetric parts. Based on the elasticity theories, closed-form moment, force and stress solutions are derived for a plate with a rigid inclusion subjected to various types of loading conditions.

Closed-form stress intensity factor solutions at the critical locations of spot welds in four types of commonly used specimens are obtained based on elasticity theories and fracture mechanics. The loading conditions for spot welds in the central parts of four types of specimens are first examined. The resultant loads on the weld nugget and the self-balanced resultant loads on the lateral surface of the central parts of the specimens are then decomposed into various types of symmetric and anti-symmetric parts. Closed-form structural stress and stress intensity factor solutions for spot welds under various types of loading conditions are then adopted from a recent work of Lin and Pan to derive new closed-form stress intensity factor solutions at the critical locations of spot welds in the four types of specimens.

In this investigation, spot friction welds in aluminum 6111-T4 lap-shear specimens were tested under both quasi-static and dynamic loading conditions. Micrographs of the spot friction welds after testing were examined to understand the failure modes of spot friction welds in lap-shear specimens under different loading conditions. The micrographs indicate that the spot friction welds produced by this particular set of welding parameters failed in interfacial failure mode under both quasi-static and dynamic loading conditions. The load and displacement histories for lap-shear specimens were obtained under quasi-static and dynamic loading conditions at three different impact velocities. The failure loads of spot friction welds in lap-shear specimens under dynamic loading conditions are about 7% larger than those under quasi-static loading conditions.

A fatigue crack growth model is adopted in this paper to investigate the fatigue lives of resistance spot welds in square-cup and lap-shear specimens of dual phase, low carbon and high strength steels under cyclic loading conditions. The fatigue crack growth model is based on the global stress intensity factor solutions for main cracks, the local stress intensity factor solutions for kinked cracks as functions of the kink length, the experimentally determined kink angles, and the Paris law for kinked crack propagation. The predicted fatigue lives based on the fatigue crack growth model are then compared with the experimental data. The results indicate that the fatigue life predictions based on the fatigue crack growth model are in agreement with or lower than the experimental results.

Fatigue behavior of laser welds in lap-shear specimens of high strength low alloy (HSLA) steels is investigated based on a fatigue crack growth model. Fatigue experiments of laser welded lap-shear specimens were conducted. Analytical global stress intensity factor solutions are developed and compared with finite element computational results. A fatigue crack growth model based on the analytical local stress intensity factor solutions of kinked cracks and the Paris law for crack growth is then adopted to estimate the fatigue lives of the laser welds under cyclic loading conditions. The estimated fatigue lives are compared with the experimental results. The results indicate that the fatigue life predictions based on the fatigue crack growth model are slightly longer than the experimental results.

In this paper, a generalized anisotropic hardening rule based on the Mroz multi-yield-surface model is derived. The evolution equation for the active yield surface is obtained by considering the continuous expansion of the active yield surface during the unloading/reloading process. The incremental constitutive relation based on the associated flow rule is then derived for a general yield function. Detailed incremental constitutive relations for materials based on the Mises yield function, the Hill quadratic anisotropic yield function and the Drucker-Prager yield function are derived as the special cases. The closed-form solutions for one-dimensional stress-plastic strain curves are plotted for materials under cyclic loading conditions based on the three yield functions.

Fatigue behavior of dissimilar Al/Fe spot friction welds between aluminum 6000 series alloy and coated steel sheets in lap-shear and cross-tension specimens is investigated based on experiments and three-dimensional finite element analyses. Micrographs of the welds after failure under quasi-static and cyclic loading conditions show that the Al/Fe welds in lap-shear and cross-tension specimens failed along the interfacial surface. Three-dimensional finite element analyses based on the micrographs of the welds before testing were conducted to obtain the J-integral solutions at the critical locations of the welds under lap-shear and cross-tension loading conditions. The numerical results suggest that the J-integral solutions at the critical locations of the welds can be used as a fracture mechanics parameter to correlate the experimental fatigue data of the Al/Fe spot friction welds in lap-shear and cross-tension specimens.