Influence of steel strength and loading mode on fatigue properties of resistance spot welded H beam components

Abstract

The drive to downgauge steel components by substituting mild steel with thinner high strength steel means that design engineers are now working closer to the design limits with respect to fatigue performance of spot welded structures. This study was carried out on an interstitial free mild steel and two high strength interstitial free steels, in sheet thicknesses 0 . 8 and 1 . 2 mm, representative of sheet thicknesses and grades in typical body-in-white automotive structures. The results obtained indicate that steel strength had little effect on fatigue performance for the range of steels studied. In contrast, both the loading mode and the sheet thickness had a significant influence. The results for shear loading were up to ten times greater when compared to peel loading conditions. Under shear loading conditions full plug failures were widely observed, whereas with peel loaded variants partial plug failures predominated. With respect to enhancing the fatigue properties the use of larger weld sizes was shown to be beneficial. MST/5870

Critical review of automotive steels spot welding: process, structure and properties

“…The IF mode can be observed during fatigue loading of spot welds with small nuggets at high loads, especially under peeling loads. 225 However, the tendency to fail in IF mode in fatigue loading is lower than quasi-loading condition and the PF mode is often observed for the fatigue tests. 225 Fatigue PF is divided into three stages: [226][227][228] (i) stage I: fatigue crack initiation and early growth (ii) stage II: crack growth through the sheet thickness (iii) stage III: crack growth through the sheet width.…”

“…225 However, the tendency to fail in IF mode in fatigue loading is lower than quasi-loading condition and the PF mode is often observed for the fatigue tests. 225 Fatigue PF is divided into three stages: [226][227][228] (i) stage I: fatigue crack initiation and early growth (ii) stage II: crack growth through the sheet thickness (iii) stage III: crack growth through the sheet width. In the following section, the factors affecting fatigue life of a spot weld are discussed.…”

“…Fatigue life of spot welds decreased when the shear component of loading conditions is increased; however, the tendency to fail in IF mode is increased. 225…”

“…Since a good part of crack growth life is spent on its propagation through sheet thickness, the thicker the sheet, the longer the fatigue life. 225 It is proven experimentally that spot welds fatigue strength is proportional to the square root of the sheet thickness. 229…”

Relationship between failure behaviour and weld fusion zone attributes of austenitic stainless steel resistance spot welds

“…Spot welds with d,d Cr tend to fail via interfacial failure and welds with d.d Cr tend to fail via nugget pull-out failure mode. Therefore, to obtain critical nugget diameter d Cr , equations (2) and (3) Figure 7 shows the cross-section of a sample failed through the pull-out failure mode during the tensile shear test. As can be seen, the location of the failure initiation of the austenitic stainless steel spot welds in the pull-out mode is at HAZ, beside the fusion zone.…”