PSI - Issue 5

Jidong Kang et al. / Procedia Structural Integrity 5 (2017) 1425–1432 Jidong Kang/ Structural Integrity Procedia 00 (2017) 000 – 000

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also observed. In test specimens with combined fracture modes (sheet plus interfacial fracture), the fatigue cracks in the sheet only grew to the width of the nugget diameter before the joints fractured resulting from interfacial shear fracture. The fracture analysis of the test specimens indicated that the porosity in the AA6022-T4 to AA6022-T4 weld nugget had no major influence on initiating or propagating the fatigue crack. The present findings on the role of porosity on fatigue crack initiation and propagation is consistent with previous studies [10,13]. The better fatigue performance of AA6022-T4 to IF steel RSWs can be attributed to a combination of factors. The primary effect would be the size of the weld nugget. As the diameter of the weld nugget increases, the load carrying cross-section area increases resulting in improved fatigue performance [14,15]. In addition, local differences in joint structure such as the heat-affected-zone properties can affect fatigue performance. To further understand the fracture mechanism, scanning electron microscope (SEM) analyses was performed on fractured joints. SEM images of the fracture surface of the top AA6022-T4 sheet from each representative stack-up is presented in Fig. 6. In AA6022-T4 to AA6022-T4 RSWs, the fracture surface exhibits a slightly mixed fracture (ductile and brittle) mode as can be observed in Fig. 6(a). Recalling Fig. 2(d), the notch like slit terminated close to the periphery of the weld nugget characterized by the presence of a columnar grain structure. The dominant fatigue crack initiated at this notch slit and propagated through the HAZ (ductile failure) as can be observed in Fig. 6(b). The mixed fracture mode was more prominent in AA6022-T4 to IF steel RSWs as can be observed in Fig. 6(c). The fracture surface morphology of the crack initiation and propagation region for the top AA6022-T4 sheet indicates that cracks initiated in the aluminum weld nugget and propagated through the HAZ of the top sheet. As mentioned earlier, the structure of the weld nugget for the top AA6022-T4 sheet was characterized by columnar grains. In addition, the aluminum weld nugget created in the Al-steel joint is typically alloyed with iron due to the extended time that the aluminum nugget is in contact with the steel substrate.

Fig. 6. Fracture surface morphology as observed under the scanning electron microscope (a) surface crack initiation site in top AA6022-T4 sheet from the AA6022-T4 to AA6022-T4 resistance spot weld, (b) typical view of the crack propagation region, (c) surface crack initiation site in top AA6022-T4 sheet from the AA6022-T4 to IF steel resistance spot weld and (d) typical view of the crack propagation region marked by fatigue striations. It is possible that these effects may have provided superior resistance to crack propagation as compared to AA6022 T4 to AA6022-T4 RSW joint. Evidence for this is given by the much smaller spacing between the striations as seen in Fig. 7(d). Thus, improved fatigue crack propagation performance for the Al-steel weld compared to the Al-Al weld is most likely a result of propagation through a fine-grained columnar structure that has been alloyed with iron in the Al-steel weld while crack propagation is contained in the HAZ, which can have variable hardness, for the Al-Al spot welds. The fracture morphologies of the specimens from the two individual stacks in Fig. 7, also support the conclusion that the cracks initially propagated along the sheet thickness and width directions at relatively low rates until they reached the top surface and after which they propagated through the coupon width until the full separation of the top sheet from the weld joint.

3.4. Fatigue life correlation

The structural stress concept is widely used in predicting the fatigue life of resistance spot welds in vehicle structures. The automotive industry uses different parameters to estimate the fatigue performance of spot welds [16]. One of the simpler methods allows determination of the critical stresses by calculating the critical forces and moments around the weld nugget and plotting against laboratory test results. Rupp et al. [17] proposed a structural stress model which is widely used by the automotive industry to correlate the fatigue life of different spot welded joints in