The Prediction of Fatigue Crack Initiation Life in Spot Welds

In this paper, strain-based fatigue life prediction method has been used to estimate the fatigue crack initiation life of spot-welded joints of Mild Steel JSC270D and Ultra-High Strength Steel JSC980Y. To do so, the joints were simulated using three-dimensional finite-element (FE) models, and then nonlinear FE analysis was performed to obtain the local stress and strain ranges and finally, the Morrow equation was applied to estimate the crack initiation lives. The results have been compared with those obtained from experimental crack growth morphology. In addition, the difference between fatigue limits for smooth specimens and spot-welded joints for mentioned materials has been briefly discussed. It has been shown that mean stress values in the Ultra-High Strength Steel can significantly decrease the fatigue limit of spot-welded joint because even at very low load level the stresses exceed the yield point at the root of nugget of spot-welded joint, while the amount of mean stress in the Mild Steel for the same load level is much less than that of Ultra-High Strength Steel. The comparison between numerical results of fatigue crack initiation lives and experimental data provided good agreement between numerical predictions and crack growth morphology observations. The results also shows that in some cases, depending on the joint type, the life spent in the nucleation phase can be an important part of the final failure lifetime.