Fatigue Strength of Spot Welded Joint

To ascertain the effects of the pitch of spots on the fatigue strength, two kinds of spot welded joints with pitches of 10-60 mm were tested under pulsating tensile loads (stress ratio 0). The first was made by lapping and joining two stainless steel plates and was named SUS+SUS. The second was made of a stainless steel plate and a carbon steel plate lapped and joined together, SUS+SPC. In order to explain the relation between the pitch and the fatigue strength of joints, also studied were the effect of the pitch on the welding consequences (nugget size, structure of heat affected zone and welding residual stress) and the relation between the pitch and stress concentration at the nugget edge where the joints failed due to fatigue.In the results of fatigue test, the fatigue strength expressed by the load per one.spot, at any life, was not necessarily constant but had a tendency to lower as the pitch became smaller. In SUS+SPC joints, fatigue ratio (fatigue strength/tensile strength) was approximately constant regardless of the pitch. In SUS+SUS joints the ratios for lives up to 106 cycles were also constant, for longer lives over 106 cycles, however, the ratios decreased with an increase of the pitch.Effects of the pitch on the consequences of welding were obtained as follows. Concerning the size of nugget and the microstructure of heat affected zone around the nugget, no considerable differences by the pitch were observed. However, concerning residual stress caused by welding which was measured by X-ray diffraction (sin2 Ψ method), the differences due to the pitch were great. For instance, the residual stress at the nugget edge was estimated about 39 kg/mm2 with a pitch of 60 mm, while it was about 24 kg/mm2 with one of 20 mm (for SUS+SUS joints).By stress measurement with electric wire strain gauges and stress analysis, relations between the stress concentration at nugget edge and the pitch were obtained. In this result for SUS+SUS joints, the stress concentration with a pitch of 60 mm was about 2.4 times as that for 20 mm pitch.Taking the residual stress and the stress concentration at nugget edge into consideration on the fatigue strength at 107 cycles for SUS+SUS joints, for any pitches, the maximum stress of stress cycle at nugget edge was approximately identical to the proof stress of the base material, and so it was pointed out that the effect of the residual stress on fatigue strength was very important.