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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.

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.

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 failures of spot friction welds in lap-shear specimens of aluminum 6111-T4 sheets under cyclic loading conditions are investigated in this paper. The paths of fatigue cracks near the spot friction welds are first discussed. A fatigue crack growth model based on the Paris law for crack propagation and the global and local stress intensity factors for kinked cracks is then adopted to predict the fatigue lives of these spot friction welds. The global stress intensity factors and the local stress intensity factors based on the recent published works for resistance spot welds in lap-shear specimens are used to estimate the local stress intensity factors for kinked cracks with experimentally determined kink angles. The results indicate that the fatigue life predictions based on the Paris law and the local stress intensity factors as functions of the kink length agree well with the experimental results.

In this paper, the stress intensity factor solutions at the critical locations of spot welds in lap-shear specimens are investigated by finite element analyses. Three-dimensional finite element models are developed for lap-shear specimens to obtain accurate stress intensity factor solutions. Various ratios of the half specimen width to the nugget radius and the overlap length of the upper and lower sheets to the nugget radius are considered in this investigation. The computational results provide geometric functions in terms of the normalized specimen width and the normalized overlap length to the stress intensity factor solutions of Zhang [1,2] for lap-shear specimens. The computational results also indicate that when the spacing between spot welds decreases, the mode I stress intensity factor solution at the critical locations increases and the mode mixture of the stress intensity factors changes consequently.

In this paper, the local stress intensity factor solutions for kinked cracks near spot welds in lap-shear specimens are investigated by finite element analyses. Based on the experimental observations of kinked crack growth mechanisms in lap-shear specimens under cyclic loading conditions, three-dimensional finite element models are established to investigate the local stress intensity factor solutions for kinked cracks emanating from the main crack. The three-dimensional finite element computational results show that the critical local mode I stress intensity factor solution increases and then decreases as the kink depth increases. When the kink depth approaches to 0, the critical local mode I stress intensity factor solution appears to approach to that for vanishing kink depth based on the global stress intensity factor solutions and the analytical kinked crack solutions for vanishing kink depth.

In this paper, fracture and fatigue mechanisms of spot friction welds in aluminum 6111-T4 lap-shear specimens are investigated based on experimental observations. Optical and scanning electron micrographs of these spot friction welds before and after failure under quasi-static and cyclic loading conditions are examined. The micrographs show the fracture and fatigue mechanisms of spot friction welds under quasi-static and cyclic loading conditions. The experimental observations indicate that the fracture mechanisms depend on the microstructure and geometry of welds under quasi-static loading conditions. Under cyclic loading conditions, the fatigue mechanisms depend not only on the microstructure and geometry of welds but also on the load amplitudes.

The fatigue lives of spot friction welds in lap-shear specimens of aluminum 6111-T4 sheets are investigated here. The paths of fatigue cracks near spot friction welds are first discussed. A fatigue crack growth model based on the Paris law for crack propagation and the local stress intensity factors for kinked cracks is then adopted to predict the fatigue lives of spot friction welds. The global and local stress intensity factors based on a recent work of Wang and Pan for resistance spot welds in lap-shear specimens are used to estimate the local stress intensity factors of kinked cracks with experimentally determined kink angles. The results indicate that the fatigue life predictions based on the Paris law and the local stress intensity factors as functions of the kink length agree well with the experimental results.

In this paper, the effects of roller geometry on contact pressure and residual stress in crankshaft fillet rolling are investigated by a two-dimensional finite element analysis. The fillet rolling process is first introduced to review some characteristics of the rolling tools. A two-dimensional plane strain finite element analysis is then employed to qualitatively investigate the influence of the roller geometry. Computations have been conducted for eight different contact geometries between the primary roller and the secondary roller to investigate the geometry effect on the contact pressure distribution on the edge of the primary roller. Fatigue parameters of the primary rollers are also estimated based on the Findley fatigue theory. Then, computations have been conducted for three different contact geometries between the primary roller and the crankshaft fillet to investigate the geometry effect on the residual stress distribution near the crankshaft fillet.

In this paper, the local stress intensity factors for kinked cracks in spot weld cup specimens are investigated by finite element analyses. Based on the experimental observations of kinked crack growth mechanisms in square-cup specimens under cyclic loading conditions, axisymmetric finite element models are established to investigate the local stress intensity factor solutions for kinked cracks emanating from the main crack. Both circular and rectangular shaped notch tip and a sharp crack tip are considered for the main crack. Various kink lengths are considered. The local stress intensity factor solutions for kinked cracks are obtained. The results show that the local stress intensity factors for kinked cracks with finite kink lengths are much higher than those based on the closed-form solutions for kinked cracks with vanishing kink length. Finally, the implications of the local stress intensity factor solutions for kinked cracks on fatigue life prediction are discussed.

The failure mechanism of resistance spot welds in dual-phase steel lap-shear specimens is first investigated based on experimental observations. Optical micrographs of the cross sections of spot welds in lap-shear specimens of a dual-phase steel before and after failure are examined to understand the failure mechanism. The experimental results suggest that under lap-shear loading conditions, the necking failure is initiated in the sheet near the middle part of the nugget circumference under tension and then the failure propagates in the sheet along the nugget circumference to final fracture. Based on a two-dimensional elasticity theory, an analytic solution for an infinite plate containing a rigid circular inclusion subjected to a resultant shear force is used to investigate the stress and strain distributions near the nugget in lap-shear specimens.

Microstructures and failure mechanisms of spot friction welds in aluminum 6111-T4 lap-shear specimens are investigated based on experimental observations. Two types of tools, a Type I tool with a flat tool shoulder and a Type II tool with a concave tool shoulder, were used to join the aluminum sheets with different processing parameters. Optical micrographs of the cross sections of spot friction welds made by the two types of tools in lap-shear specimens before and after failure are examined. These spot friction welds show the failure mode of nugget pullout under lap-shear loading conditions. However, the micrographs show different microstructures and failure mechanisms for spot friction welds made by the two types of tools with different processing parameters.

Resonant frequencies of a resonant bending system with notched crankshaft sections are obtained experimentally and numerically in order to investigate the effect of notch depth on the drop of the resonant frequency of the system. Notches with the depths ranging from 1 to 5 mm, machined by an EDM (Electrical-Discharging Machining) system, were introduced in crankshaft sections at the fillet between the main crank pin and crank cheek. The resonant frequencies of the resonant bending system with the crankshaft sections with various notch depths were first obtained from the experiments. Three-dimensional finite element models of the resonant bending system with the crankshafts sections with various notch depths are then generated. The resonant frequencies based on the finite element computations are in good agreement with those based on the experimental results.

In this paper, the fatigue failure of the primary roller used in a crankshaft fillet rolling process is investigated by a failure analysis and a two-dimensional finite element analysis. The fillet rolling process is first discussed to introduce the important parameters that influence the fatigue life of the primary roller. The cross sections of failed primary rollers are then examined by an optical microscope and a Scanning Electron Microscope (SEM) to understand the microscopic characteristics of the fatigue failure process. A two-dimensional plane strain finite element analysis is employed to qualitatively investigate the influences of the contact geometry on the contact pressure distribution and the Mises stress distribution near the contact area. Fatigue parameters of the primary rollers are then estimated based on the Findley fatigue theory.

In this investigation, the stress and strain distributions at the first thread of a joint under cyclic uniaxial loads are examined by finite element analysis. The stress and strain histories of material elements along the thread surface are used in conjunction with multiaxial fatigue theories and the critical plane approach to determine the location, life, and direction of fatigue crack initiation for low cycle fatigue loading conditions. It is shown that fatigue cracks are initiated along the surface of the thread root at approximately 18° from the centerline of the thread shifted towards the loaded flank of the thread, and the inclination of the fatigue cracks are oriented at approximately 44° to the free surface of the bolt thread.

An effective fatigue driving stress is proposed to predict the failure of spot welds under cyclic combined loading conditions. The effective fatigue driving stress is obtained based on the Mises yield criterion in terms of the resultant forces and moments in a plastic collapse analysis of spot welds under complex combined loading conditions as discussed in Lin et al. [1]. The effective fatigue driving stress can be used to correlate the fatigue data of spot welds with consideration of the effects of the sheet thickness, nugget diameter and loading conditions. Experimental results for coach-peel and lap-shear specimens under cyclic loading conditions are used to evaluate the applicability of the effective fatigue driving stress. The experimental results for spot welds in both coach-peel and lap-shear specimens are correlated very well based on the effective fatigue driving stress.

Failure behavior of spot welds is investigated under impact loading conditions. Three different impact speeds were selected to test both HSLA steel and mild steel specimens under combined opening and shear loading conditions. A test fixture was designed and used to obtain the failure loads of spot weld specimens of different thicknesses under a range of combined opening and shear loads with different impact speeds. Accelerometers were installed on the fixtures and the specimens for investigation of the inertia effects. Optical micrographs of the cross sections of failed spot welds were obtained to understand the failure processes in both HSLA steel and mild steel specimens under different combined impact loads. The experimental results indicate that the failure mechanisms of spot welds are very similar for both HSLA steel and mild steel specimens with the same sheet thickness. These micrographs show that the sheet thickness can affect the failure mechanisms.

Failure loads of spot welds are investigated under static and impact loading conditions. A test fixture was designed and used to obtain maximum loads of spot welds under a range of combined opening and shear loads with different loading rates. Optical micrographs of the cross sections of spot welds before and after failure were obtained to understand the failure processes under various loading rates and different combinations of loads. The experimental results indicate that under nearly pure opening loads, the failure occurs along the nugget circumferential boundary. Under combined opening and shear loading conditions, the failure starts from the tensile side of the base metal near the nugget in a necking/shear failure mode. The effects of sheet thickness and combined load on the load carrying behavior of spot welds are investigated under static and impact loading conditions based on the experimental results.

In this paper, the formability of AA5754 aluminum laser-welded blanks produced by Nd:YAG laser welding is investigated under biaxial straining conditions. The mechanical behavior of the laser-welded blanks is first examined by uniaxial tensile tests conducted with the weld line perpendicular to the tensile axis. Shear failure in the weld metal is observed in the experiments. Finite element simulations under generalized plane strain conditions are then conducted in order to further understand the effects of weld geometry and strength on the shear failure and formability of these welded blanks. The strain histories of the material elements in the weld metal obtained from finite element computations are finally used in a theoretical failure analysis based on the material imperfection approach to predict the failure strains for the laser-welded blanks under biaxial straining conditions.