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Structural stress methods are now widely used in fatigue life assessment of welded structures and structures with stress concentrations. The structural stress concept is based on the assumption of a global stress distribution at critical locations such as weld toes or weld throats, and there are several variants of structural stress approaches available. In this paper, the linear traction stress approach, a nodal force based structural stress approach, is reviewed first. The linear traction stress approach offers a robust procedure for extracting linear traction stress components by post-processing the finite element analysis results at any given hypothetical crack location of interest. Pertinent concepts such as mesh-insensitivity, master S-N curve, fatigue crack initiation and growth mechanisms are also discussed.

This paper presents a numerical analysis of the effect of weld induced residual stress on the fatigue behavior of a T-joint. The thick-section T-joint contained 18 individual weld passes and was subjected to fully-reversed, zero-maximum, and zero-minimum fatigue cycling. The effect of the residual stress was demonstrated by comparing the result with and without residual stress. It was concluded that the local fatigue parameters (mean stress, alternating stress, and stress ratio) at the suspected crack initiation site were changed significantly by the residual stresses when the applied stress were other than fully reversed. In addition, the effect of the stress concentration at the weld was more significant that the effect of the residual stress for the applied fatigue loads levels that were considered. The analysis method presented can be used to assess weldment design and process variables.

In this study, an incrementally coupled finite element analysis procedure is used to analyze the electrical, thermal, and mechanical interaction during resistance spot welding processes. The results of the finite element analysis are validated by experimental measurements of the weld nugget sizes and dynamic resistance. The temperature results from the thermo-electric analysis are used as the input for the prediction of the microstructure evolution in the resistance spot welds of high strength steels. Consequently such welding parameters as welding current, electrode force, electrode designs, cooling water temperature and flow rate, and electrode holding time can be linked with the weld nugget size, microstructure and mechanical properties in spot welds, and eventually the residual stresses and performance of spot welded structures.

This paper summarizes the results of a recent study on a fatigue predictive technique for spot-welded automotive structures. The technique makes use of an equivalent stress intensity factor (Keq) as fatigue parameter for life predictions. A series of fatigue tests were conducted by using different types of fatigue specimens and weld arrangements. Using the raw test data collected, fatigue properties were processed in the form of ΔKeq versus fatigue life by a fracture mechanics based stress intensity factor technique. It is demonstrated that the fatigue properties are consistent among all the specimens tested and relatively geometry-independent. With the stress intensity factor based fatigue properties, the predictive technique was applied to more complex specimens with non-symmetric weld configurations and non-uniform loading conditions (resulting in mixed-mode loading on each weld). The results indicate good correlation between life predictions and test data.

This paper summarizes some recent results on fatigue evaluation of welded joints. A mesh-insensitive structural stress procedure was discussed and employed to characterize geometric stress raiser effects at welded joints. Existing weld fatigue data published in the open literature were analyzed using the structural stress parameter.

In the new design, the electrode employs composite electrode face construction with dissimilar materials. A cylindrical insert located in the electrode face center is made of low thermal and electrical conductivity material, such as stainless steel, and an annular outer sleeve is made of stainless steel and located at periphery of the electrode. Base material of the electrode is still made of copper alloys. With this electrode design, the electrical-thermal-mechanical conditions can be improved by confining the current flow path to reduce current level required for the weld nugget formation, and optimizing electrode pressure distribution, and minimizing electrode face heating and plastic deformation.

In this paper, an advanced computational framework is presented for integrated simulation of hydroforming effects and welding assembly operations. The finite element procedures take advantages of existing commercial finite element codes such as ABAQUS by employing a series of user-developed interface modules and a unified material constitutive model formulated with internal state variables that are used to track stress/strain histories induced during forming and welding operations. Its applications in design and welding assembly of hydrofomed components are demonstrated with a series of selected case studies. Based on the detailed finite element simulations described in the above, the following important observations can be made: Weld placements are extremely important in order to mitigate the significant cold work effects in hydroforming.

In this paper, a generalized spot weld model is presented for analyzing various performance attributes of spotwelded automotive structures. The spot weld model employs conventional definitions of beam- or nonlinear spring type elements. The relevant global mechanical properties are presented in the form of six pairs of generalized load-displacement relationships with respect to six degrees of freedom. The required generalized load-displacement relationships can be readily derived with assistance of local finite element welding process model along with limited single-weld coupon testing. As result, the effects of actual weld properties, welding-induced residual stress states, etc. can be incorporated for applications in finite element analysis of complex autobody structures. Its applications in conventional stress analysis for durability prediction, and limit load prediction, and crashworthiness simulation are also discussed with a few selected examples.

This paper addresses the modeling issues of resistance spot welds. The state of the art modeling techniques on weld process simulation, weld property prediction and weld engineering performance evaluation are presented. First, weld process simulation is performed using the incrementally coupled thermal-electrical-mechanical analyses. The resulted weld nugget size, weld residual stress and weld material property distributions are then used in determining the static performance of a single weld coupon. Comparisons with experimental measurements are presented as validations. Results generated from this single weld coupon is then used in the simulation of dynamic crush mechanism of a spot welded single hat section.

Tailor-welded blanks (TWB) have been increasingly used in the automotive industry as an effective way to reduce weight and costs. Although some of the joining processes for TWB are relatively well known, little independent information exists regarding welding procedure effects on weld/HAZ properties, particularly their effects on form-ability and structural performance under various conditions. In this paper, advanced computational modeling techniques were used to investigate the effects of welding procedures on weld property evolution and its impact on the formability issues. Two case studies were presented. One is on TIG welding of 6000 series aluminum tailored blanks, where thermomechanical effects on weldability was analyzed. Its implication on weld performance during forming will be discussed. The other case is on laser-beam welding of high strength steel to mild steel with a non-linear weld. The detailed thermal history and residual stress development will be presented.

A special spot weld element (SWE) is presented for simplified representation of spot joints in complex structures for structural durability evaluation using the mesh-insensitive structural stress method. The SWE is formulated using rigorous linear four-node Mindlin shell elements with consideration of weld region kinematic constraints and force/moments equilibrium conditions. The SWEs are capable of capturing all major deformation modes around weld region such that rather coarse finite element mesh can be used in durability modeling of complex vehicle structures without losing any accuracy. With the SWEs, all relevant traction structural stress components around a spot weld nugget can be fully captured in a mesh-insensitive manner for evaluation of multiaxial fatigue failure.