One of the problems commonly associated with fatigue life prediction of various spot welded geometries has been the need to test each geometry. Various high strength low alloy and low carbon spot welded specimen geometries have been tested. Even for the same nugget diameter, specimen width and sheet thickness, the fatigue resistance in terms of the either the maximum or range of remote load differed from one specimen type to another. This could be attributed to the fact that the weld nugget region is associated with multiaxial stress fields, which are neglected in the remote load analyses.
An initial attempt to incorporate multiaxiality involved converting remote loads into nominal stresses in the vicinity of the spot weld using geometrical factors such as nugget eccentricity. Findley's parameter, a shear based multiaxial critical plane approach, was employed to evaluate the fatigue performance of the various geometries. A marked improvement in the life predictions resulted, indicating the importance of considering the multiaxiality when evaluating spot welds. A range of plate thickness and width, nugget size, and load ratios for single spot welded specimens were accounted for with the Findley parameter.
Subsequently a local stress index parameter, Ki, was developed as a refinement to the multiaxial nominal stress approach. In many ways the Ki parameter is conceptually analogous to Findley's parameter. However, the Ki parameter is based on crack tip fields rather than “nominal” stresses. The Ki parameter prvides an excellent representation of the fatigue data for different spot welded geometries, for which a single best fit equation has been obtained. Findley's parameter or Ki can be easily calculated once the load transferred by a weld nugget, which is available from most numerical analyses of spot welded structures, is determined.
However, the Ki analysis is less sensitive to some geometric factors. Implementation of the Ki parameter allows the designer to estimate the effects of different design variables on the fatigue performance of the spot welded structures without having to resort to further testing.