Investigation of Assessment for Fatigue Strength of Multi-Spot Welded Joints

Abstract

This investigation is carried out in order to establish an estimation procedure of fatigue strength under tensile shearing load of multi-spot welded joints arranged in perpendicular to the load direction, by using maximum load vs. number of repeated cycles curve (abbreviated to L-N curve, hereafter) of single-spot welded joint which has the same nugget diameter with those of multi-spot welded joints. In general, it has been known that static tensile shearing strength of multi-spot welded joints becomes nearly equal to the product of the spot number of the joint by the tensile shearing strength of single-spot welded joint which has the same specimen width with the spot space of multi-spot welded joints (abbreviated such a single-spot welded joint to the equivalent single-spot welded specimen), no matter how the mounting of multi-spot wleded specimen on the testing machine is made without sufficient cares. On the other hand, it is usual that fatigue strength of multi-spot welded joints becomes considerably lower than the product of the spot number of the joint by the fatigue strength of equivalent single-spot welded specimen, even if the mounting of multi-spot welded specimen on the fatigue testing machine is made with sufficient cares. (Here, we define that the fatigue life of multi-spot welded joints under a constant maximum load is the number of cycles when a first fatigue crack with the same length with the nugget diameter becomes visible around a spot weld of multi-spot welds.) It is made clear by the previous paper that this is mainly because of unequality of load bearing ratio of each spot weld in multi-spot welds. Moreover, it is recognized in our laboratory that when a special device is used for load bearing ratio of each spot weld in multi-spot welds to become equal, fatigue strength of multi-spot welded joints becomes nearly equal to the product of the spot number of the joint by the fatigue strength of equivalent single-spot welded specimen.
In practices of spot welded structures, load bearing ratio of each spot weld in multi-spot welds is scarcely equal. Moreover, depended on the unintentional variations of nugget diameter and spot space, unequality of load bearing ratio of each spot weld in multi-spot welds will become larger.
The purpose of this investigation is to find out a relationship between fatigue strengths of spot welded structures in practices as mentioned above and that of equivalent single-spot welded specimen. For this purpose, a hypothesis which is able to connect fatigue strength of multi-spot welded joints and that of equivalent single-spot welded specimen was set up, and experiments for the verification of the hypothesis were carried out. Materials of specimens used in these experiments are mild steel, high strength steel and commercially pure aluminum, respectively, of 0.8 mm thickness.