The results we have obtained so far present an apparent contradiction. On the one hand, the dissimilar aluminum alloy spot welds
with the addition of adhesive have larger nuggets, and one would expect longer fatigue life in these spot welds according to earlier research
stating that nugget size controls the fatigue life [1–2]. On the other hand, the two aluminum alloy dissimilar welds have the
same tensile-shear load capacity indicating that one would expect similar fatigue properties. Since the spot welds with the addition of
adhesive are indeed lower in nugget strength, one can even expect that their fatigue properties should also be reduced.
To help understand this contradiction, we can rationalize the fatigue results of these dissimilar aluminum alloy resistance spot
welds using the structural stress concept [15–18]. This concept (Fig. 8) developed by Rupp et al. [13], is based on a structural
stress range Δσ(SS) defined as