PSI - Issue 37

Liting Shi et al. / Procedia Structural Integrity 37 (2022) 351–358 L Shi et al/ Structural Integrity Procedia 00 (2022) 000 – 000

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Figure 4 shows fatigue test results of positive and negative AA5754-HSLA RSWs in addition to that of the AA5754-AA5754 RSWs. Results in Fig. 4 demonstrate that both the positive and negative polarity AA5754-HSLA RSWs exhibited higher fatigue life compared to AA5754-AA5754 RSWs. However, large scatter was observed in tensile shear tests of the AA5754-HSLA RSWs, especially at low fatigue load. This behavior will be explained later with the notch root angle effect. In examination of the fractured tensile shear specimens, it was evident that pull-out fracture occurred at high load levels (2598N and above) while interfacial fracture occurred at lower load levels. For coach peel, pull-out or eye-brow fracture was observed for all specimens tested at all load levels. Following Rupp’s method in Rupp , et al. (1995), the structural stress range calculation for tensile shear fatigue tests is Δ σ ( SS ) = (1 − R ) [1.872 × 0.6 t 1/2 × M x /( dt 2 )− F y /( πdt )] (1) and for coach peel fatigue tests is Δ σ ( SS ) = 0.6 × (1 − R ) × t 1/2 [1.872 M z /( dt 2 )− 1.744 F z / t 2 ] (2) where R is 0.1 in the present study. 4.4. Structural stress analysis

Fig. 5. Relationship between structural stress range of fatigue life for resistance spot welds of AA5754 to itself and AA5754 to HSLA steel with aluminum connecting to either positive or negative electric polarity.

When plotting the structural stress ranges versus fatigue life results for all the RSWs studied (Fig. 5), it is evident that all fatigue life data points fall onto a master curve with a correlation coefficient of 0.833 indicating that the weld diameter is the dominate parameter controlling fatigue life. However, large scatter was still observed at the low structural stress range level which is a strong indication of the effect of notch root angle, as discussed in Shi et al. (2020b). 4.5. Effect of notch root angle via the maximum principal strain approach It was found in Shi, et al., 2020b that, in AA6022-HSLA RSW stack-ups, a large notch root angle at the weld nugget is beneficial to yield longer fatigue life as less maximum principal strain occurs in the aluminum sheet. It is