Fatigue Life Prediction of Spot-Weld for Auto Body Based on Multiple Load Cases

Abstract

Objectives: A typical modern car body (BIW) has about 4,000−6,000 spot welds. It is important for the automotive design engineers to understand the mechanical behavior of different joints and furthermore, to incorporate static, impact, and fatigue strength of these joints using CAE methods. The main failure mode of spot welds for most of applications in a car body is fatigue failure. So, the way to layout the spot welds and predict the fatigue strength distribution in the early design stage of car body development has been one of the most interesting and significant topics in recent years. The main objective of this paper is to suggest multiple load cases considering customers usage for fatigue life prediction of spot-weld for the passenger car in order to improve simulation accuracy and CAE capability in vehicle performance development process. Methodology and Results: Loading, geometry and material fatigue properties (SN or EN curve) are three key elements for structural fatigue simulation. During the fatigue life simulation of spot welds in car body, the selection of load cases, the accuracy of welding fatigue SN curves, as well as weld nugget diameter of the selected spot weld and finite element modeling accuracy substantially affect spot weld fatigue damage and life prediction. This paper presents a method for fatigue analysis of spot-weld joints and the evaluation of the group of spot-welds having insufficient fatigue life. The authors focus on finding out more realistic load input condition and spot-welded joint modeling methods in order to conduct structural optimization design by using FEA. A multiple load case analysis process for spot weld fatigue life prediction is developed. Under the multiple load case combining torsion, bump, cornering, braking & accelerating, and by using nCode software, the spot weld fatigue analysis is done. As a result, the prediction of weak joints by simulation matches experimental results quite well, applications for several car body design programs conduct good results. Conclusion and Future Potential Researches: The advantages and the limitations of the multiple load case analysis method for spot weld fatigue life prediction in auto body design process are discussed. In the literature very few papers are focused on the loading condition improvement. The applications also indicate that current simulation technology is suitable for spot weld fatigue strength distribution prediction, weak spots identification, and A to B comparison in the early design process. However, the correlation analysis between fatigue test results and simulation results should be done. The accuracy of spot weld joint fatigue property (S–N curve) needs to be improved in the future because of the S–N curve significantly affecting the fatigue life and damage calculation.

F2012-E03-049