Fatigue life evaluation for multi-spot welded structures

A procedure using a conventional stress value (ERS, equivalent radial stress) to evaluate fatigue behaviour is applied to spot welded joints. The ERS is based on a closed-form solution of a theoretical bi-dimensional model of the spot weld area, under various types of loading conditions; ERS can be determined for every spot weld of the jointed structure. The closed-form solution is also digitalized in order to define a spot element which can be used in FE simulation of multi-spot structures, providing profit both in accuracy and in computing time saving. Through the use of ERS-N curves, fatigue data collected on different joint geometries can be successfully mixed together. One of the main aspects evidenced is that progressive damage deeply influences fatigue behaviour: a simple numerical solution neglecting accumulated damage is unable to foresee the whole fatigue life. Indeed, the final value of ERS is given accounting the progressive damage effects of spot welds using a non-linear damage-step procedure. In the present paper the method has been applied to many experimental results: it is shown that a single criterion is used to deal with several different structures and materials.