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The highly competitive auto industry is looking for ways to reduce product development cycle time while meeting the corporate and government stringent vehicle performance requirements. Quasi-static or dynamic analytical fatigue life assessment of automotive structures consumes more time because of the use of long proving ground time histories and large finite element models with more than a million elements. A representative static load case that highlights all the durability concern locations is needed for making fast design iterations. This paper describes a simple technique to extract a static load case that correlates to minimum fatigue life at various locations of the body structure and the method of using this load case for fast iterations before validating the final design with fatigue analysis using full proving ground loads. The usefulness of this static load case in solving sheet metal and spot weld fatigue issues is demonstrated with an example.

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.

Finite element based stress analysis and fatigue predictions are practiced routinely in automotive body structural design and development. The accuracy of these simulation results is not fully understood or at least not well documented. Automotive body structures have many kinds of notches, metal thinning due to stamping and cold working etc. Modern fatigue assessment tools do take into account many of these complexities by Neuber corrections, mean-stress correction, critical plane selection, etc. Other challenges exist in the sensitivity to element quality, including warpage, size, element type, interpretation of results, etc. This case study is based on static loading and accelerated fatigue test conducted on a front-end body buck. The stress and fatigue correlations are designed to build confidence in the model and load inputs. The fatigue results are intended to reproduce durability issues that developed during a proving ground test and were then used to verify potential fixes.

MIG (Metal Inert Gas or Gas Metal Arc welding) and spot welding are the most common way of joining steel components in automotive body, and frame structures. The main design benefits of MIG welding are however the ability to join the parts with single side access and the reduction or elimination of flanges. Different finite element based methodologies exist for predicting the durability of welds. These methodologies are being used in the automotive industry to resolve potential and current durability issues in spot and MIG welded steel components and also to reduce expensive testing practices. However, the analysis results highly depend on the finite element modeling and the accuracy of weld data. This paper briefly describes some of the lessons learned while applying the weld life prediction technology for MIG and spot welds in automotive steel structural components.