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.