A statistical analysis of the fatigue behaviour of single and multi-spot welded joints.

Several models exist that can provide predictions for the fatigue properties of a spot welded joint based on geometric data. However, the accuracy of these models tends to diminish as the number of spot welds and complexity of the joint increases. This thesis reports on the findings of an investigative study to assess the applicability of a statistical approach for predicting the fatigue behaviour of a multi-spot welded joint, based on data that can be gathered on single spot welded joints that are geometrically equivalent to the individual spot welds present in the multi-spot welded joint. A series of Staircase and Probit fatigue testing techniques were used to gather extensive data on the fatigue behaviour of a given single spot welded joint geometry. This data was then used, along with the proposed model, to predict the fatigue behaviour of a series of simple multi-spot welded joints consisting of two, four, eight and sixteen nuggets, where all the individual welds present were all ostensibly geometrically identical to the single nugget joints. Actual values for the various multi-spot welded joints were then determined using a series of Staircase fatigue testing techniques and compared to those predicted by the model. The results showed an excellent correlation between predicted and experimental values for the standard deviation of the fatigue strength distribution, and a marginal increase in the experimentally determined mean fatigue strength, when compared to the values predicted by the model. This difference was attributed to a small increase in the width per spot weld in the multi-spot welded joints. Such increases in width have previously been shown to affect the fatigue properties of spot welded joints in this manner. Once this affect was taken into account, the results lend support to the proposed statistical approach for predicting the fatigue behaviour of multi-spot welded joints, based on known data regarding geometrically representative single spot welded joints.