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Because of increasing fuel costs and environmental concerns, the automotive industry is under enormous pressure to reduce vehicle weight. One strategy, downgaging, substitutes a reduced gage (thickness) steel in place of a thicker one, and is usually accompanied by a material grade change to a higher strength steel. Thus, Advanced High Strength Steels (AHSS) are increasingly used for lightweight automotive body structures. The critical durability concern with steels is the spot welds used to join them, since fatigue cracks in body structures preferentially initiate at spot welds. Hence, the Auto/Steel Partnership (A/SP) Sheet Steel Fatigue Taskforce undertook an investigation both to study the fatigue performance of AHSS spot welds, and to generate data for OEM durability analysis. The study included seven AHSS grades and, for comparison, mild steels and a conventional High Strength Low Alloy grade, HSLA340.

Self Pierced Rivets (SPRs) are being used in the automotive industry with aluminum structures due to their superior functional qualities and assembly processes. Fatigue behavior of SPR connections needs to be investigated experimentally and numerically to predict SPR fatigue lives. Testing of aluminum lap-shear and coach-peel coupons is performed to obtain the fatigue lives of SPR connections under different conditions. A damage model of the SPR fatigue life is developed using a global-local approach. The damage model is then validated using T-box structures with different loads and gauge combinations. The fatigue lives of aluminum car bodies are predicted using a Ford-developed tool based on the damage model.

A fatigue life reliability and robustness study on an aluminum U-box structure representing a typical car body joint with self-piercing rivets is described that combines CAE analysis using a finite element model with analytical reliability and robustness (AR & R) numerical techniques. The application of the AR & R process involved is first explained. Then the aspects of U-box modeling and a computer design of experiments are discussed. Also, the analytical results obtained from computer analysis are given that include the main effects and sensitivities of control factors that affect the design and the main interaction effects. Based on the main effects plots and sensitivities, the optimal U-box design is described to maximize the fatigue life of the structure for various fatigue damage parameter levels.

Since the environment of vehicle operation is dynamic in nature, dynamic methods should be used in vehicle durability analysis. Due to the constraints in current computer resources, simulation of vehicle durability tests and structural fatigue life assessment need special approaches and efficient CAE tools. The purpose of this paper is to present an efficient methodology and a feasible vehicle dynamic durability analysis process. Two examples of structural durability analysis using transient dynamics are given. The examples show that vehicle stress analysis and fatigue life prediction using dynamic method is now feasible by employing the presented method and process.

There is a trend in the automotive industry to reduce the number of physical prototypes and to rely more on Computer Aided Engineering (CAE) for sizing and final design of vehicle structures. The traditional deterministic approach does not necessarily clarify the degree of variability and conservatism. With small variability in influence parameters and a design factor for final design, the approach may be over conservative resulting in weight and cost penalty. On the other hand, with large variability and the same design factor, the deterministic approach may not satisfy durability requirements. It is important to identify the variability of all factors including road loads and sensitivities of the control parameters, and to minimize their effects on durability so that fatigue life distribution meets the durability requirements.

Automotive product development requires higher degree of quality upfront engineering, faster CAE turn-around, and integration with other functional requirements. Prediction of potential durability concerns using analytical methods for sheet metal structures subjected to road loads and other customer uses has become very important. A process has been developed to provide design direction based upon peak loads, simultaneous peak loads, and vehicle program analytical or measured loads. It identifies critical loads at each input location and load sets for multiple input locations, filters load time histories, selects critical areas and analyzes for fatigue life. Several case studies have been completed. The results show that the variations are consistent with the accuracies in finite element analysis, road load data acquisition, and fatigue calculation methods.