Application of Weld Fatigue Evaluation Procedure for considering Multi-Axial Stress States in Weld End Locations for Automotive Structures Using the Battelle Structural Stress Method

Paper #:
  • 2017-01-0338

  • 2017-03-28
Even under uniaxial loading, seemingly simple welded joint types can develop multi-axial stress states, which must be considered when evaluating both the fatigue strength and failure location. Two well established examples of this are a hollow tube through a flat plate and a flat plate with an angled attachment plate. The stress distribution at these weld failure locations show significant in-plane shear stress in addition to the usual normal stress. Previously the author noted that when only the normal structural stress is considered for these joints the predictions of both the fatigue failure location and the fatigue life using the master S-N curve approach are inaccurate because the in-plane shear stress plays a significant role in the development of the crack. Based on the investigation of fatigue behavior for the multi-axial stress state, a procedure for fatigue behavior of welded joints with multi-axial stress states was proposed using an effective equivalent structural stress range parameter that is formulated as a von Mises form of the combined normal and in-plane shear equivalent structural stress ranges. In automotive structures, fatigue failure is often observed at weld end, which often show a complex stress state. Unfortunately, when analysts are creating finite element models, the mesh at the weld end locations are usually modeled as right-angled shape, which generates a sharp weld corner which is not representative of the real rounded weld end. Due to this type of weld end modeling, the fatigue failure prediction at the weld end tends to be overly conservative due to the excessive stress concentration at the right angle weld termination. In order to overcome this modeling limitation, an extended weld end correction procedure is proposed. This procedure considers the stress behavior of the element adjacent to the weld end and reduces the mesh sensitivity in this region. When the extended weld end correction procedure and the effective equivalent structural stress range parameter was applied to the failure at the weld end, the fatigue prediction are improved. In this article, the extended weld end correction procedure is described and validated. Additionally, the fatigue evaluation procedure using the effective equivalent structural stress range parameter is applied to various multiaxial stress state examples, including failures observed in the weld line and at the weld end. It is noted that the extended weld end correction procedure should be applied before applying the effective equivalent structural stress parameter for weld end. When the effective equivalent structural stress range parameter is employed, both the fatigue failure location and the fatigue life can be predicted correctly. Based on the multi-axial fatigue behavior observation, the cycles-to-failure data from the subject joint types are comparable with the master S-N curve for Mode I loading dominant behavior. Therefore, the master S-N curve developed for Mode I failures can be equally applicable for fatigue life prediction for multi-axial stress states by replacing the equivalent structural stress range with effective equivalent structural stress range.
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