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MIG weld failures are commonly seen in chassis and frame structures in automobile industry. Until now, testing and CAE analysis based on local stresses in the vicinity of MIG weld were driving the design of these welds. With the advent of advanced methods and tools, it is possible to estimate fatigue life of MIG welds and support the design in the early stages of the vehicle program. Recently, fatigue damage models are developed for assessing the durability of MIG welds in aluminum auto structures. These damage models are based on advanced technologies like mesh-insensitive structural stress method, virtual node method, estimation of notch stress intensities and life predictions based on two-stage crack growth law. This paper outlines the theoretical aspects involved in deriving the master S-N curve.

Recent rapid advances in mesh-insensitive structural stress procedures are summarized in this report. In addition to their effectiveness in reliably calculating structural stresses for both simple weld details and complex structures, the mesh-insensitive structural stress procedure provides an effective mean for drastically simplifying stress intensity factor solutions for welded joints, by extending the structural stress definition for characterizing notch effects at welds. The results achieved so far and significant implications are summarized below: (1) The mesh-insensitive structural stresses can be consistently related to fatigue behavior in welded joints and the calculation methods can be readily implemented as processing procedures (2) As a stress based transformation process, the mesh-insensitive structural stress methods map complex 3D geometry and loading mode to a simple strip geometry subjected to a simple structural stress state.

Welding remains to be one of major manufacturing processes in the automotive industry. In addition to resistance welding, arc welding processes have recently received an increasing attention as some of the advanced designs of automotive structures are being introduced, such as hydro-formed frame structures. As such, it has become increasingly important that math-based design and manufacturing tools be developed to achieve concurrent design and welding process development in today's competitive environment. In this paper, some of the state of the art computational modeling techniques are first summarized. These modeling techniques can be used to establish welding procedure effects on weld quality, residual stresses, distortions, and more importantly, on structural integrity of the welded structures. The applications of some of the advanced analysis techniques are demonstrated by using a number of detailed case studies.

The effects of electrode wear on nugget development during resistance spot welding are major concerns in auto-body assembly and manufacturing. By considering detailed electrode-sheet interactions using advanced finite element modeling procedure, this paper presents a framework for detecting the electrode wear conditions and associated nugget development characteristics. Two important in-process parameters are studied in detail. They are the electrode movement and the dynamic resistance. It is found that the second-order derivative of the electrode movement and the first-order derivative of the dynamic resistance can be correlated in a fundamental form to identify the detailed nugget development process under various electrode wear conditions.

An important emerging technical area is computer-based modeling of the various manufacturing processes that are used in many diverse industries. These models are used to optimize manufacturing techniques to reduce fabrication costs and improve the service performance. One manufacturing process important in steel fabrication is welding. It can be a useful tool to aid in reducing fabrication costs and service durability by optimizing the weld process and is the subject of this paper.