3.1 Al-Al joint
Figures 2 (a&b) illustrate the detailed OM observation of the fatigue fracture with Δσ/σb=0.2. Two fatigue patterns are found: (1) the fatigue striations exhibit circumference direction along the RFSSW joint, where commonly observed in the front of joint, i.e. the fatigue crack initiation region (Fig. 2(a)); (2) the fatigue striations have the path penetrating the thickness direction, which close to the torn fracture region (Figs. 2(b).
With Δσ/σbincreasing to 0.3 as shown in Figs. 2(c&d), the fatigue striations also exhibit two different types with circumference and thickness directions. However, the fatigue striations along thickness direction get more obvious and these striations approach to the fatigue crack initiation region. With Δσ/σb further increasing to 0.4 (shown in Figs. 2(e&f)), the fatigue striations along thickness direction is detected in both crack initiation and torn fracture regions.
The present observation of fatigue behaviors of Al-Al RFSSW supports that the joint significantly influenced by stress amplitude. The higher stress amplitude is, the more obvious fatigue striations, along thickness direction, are. Besides, the fatigue striations along thickness direction cover the fatigue striations with circumference direction located in the crack initiation region. This fatigue behavior transition shall be attributed to the higher stress concentration around the RFFSW joint., rather than the interface between Al-Al layers.
3.2 Al-steel joint
Figures 3(a&b) illustrate the OM observation of both the Al sheet and steel sheet fatigue fractures with Δσ/σb=0.3, respectively. It is found that fatigue striations exist in the edge of the joint, starting from the Al part of the joint. With Δσ/σb increasing to 0.4, the fatigue striations are both found from the Al sheet and steel sheet parts as show in Figs. 3(c&d). The present observation supports that the effect of secrete hardness distribution within the Al-steel joint plays a more important role in fatigue crack initiation position rather than that of the parameter of Δσ/σb does.