| The fatigue properties of spot welded joints have gained significant attention in the past several decades. Typically, because the initiation site of the fatigue crack that leads to final failure of this joint is near the edge of the nugget between the two welded sheets, the fatigue crack observation and determination have been problems of estimating and verifying the fatigue life of the spot welded joints.; The objective of this dissertation was to explore and develop the new ways to observe and determine fatigue performance and crack propagation of spot welded joints. The investigation identifies the fatigue failure process, determines the fatigue crack geometry, and estimates the fatigue life of spot welding.; In this study, the x-ray technique has been verified to be an effective method to observe and measure the inside crack during the fatigue process. The experiment and finite element analysis demonstrated that the dynamic variables (natural frequency, vibrating mode, etc.) of structure would change at different stages of fatigue process or crack propagation. A theoretical model has been developed to evaluate the crack according to the changes of dynamic variables and strain energy during the fatigue crack propagation. The importance of this model is that, with some proper analysis, not only the crack geometry but also the crack location can be predicted. In addition, this model provides a relative complete base that can be used for further research of fatigue life analysis or multiple spot welded joints. The last part of this research used the strain-life relation method and fracture mechanics theory to estimate the fatigue life of this joint. |
