SAE 2008-01-0698 : 2008

Spot welding is the primary method of joining sheet metal for body and structural applications in the ground vehicle industry. A typical automobile may contain over 5000 spot welds. The fatigue failure of spot welded joints results in the degradation of both structural and Noise, Vibration, and Harshness (NVH) performance. Therefore, designers need reliable information about the total fatigue life of spot welded joints early on in the design phase. Currently, automotive structures are employing ever increasing amounts of Advanced High Strength Steels (AHSS) including dual-phase steels. As a result, automotive designers require fatigue strength information on AHSS spot welds. The Auto/Steel Partnership (A/SP) has conducted fatigue tests with lap shear and coach peel specimens made of AHSS, HSLA and low carbon steels. Overall, the test data showed good correlation with the applied load range for all of the materials, regardless of base material strength. However, some scatter was observed in fatigue test data, especially in the data obtained from coach peel testing. This scatter may have resulted from several sources. In order to understand the sources of scatter, the effect of geometric variations, material microstructure, and mechanical properties on fatigue life is investigated. Specifically, the variability in nominal values of spot weld diameter, specimen geometry including specimen thickness, width, etc., and hardness values obtained in the HAZ and fusion zone are examined. The results of this study shed light on the importance of geometric parameters, such as specimen thickness and weld nugget diameter, and their large impact on the fatigue strength variability.

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