13th International Conference on Fracture June 16–21, 2013, Beijing, China -6- Figure 8. Micro mophology of fitigue crack intiation zone of X80 high-strain linepipe material ( / 2 εΔ =1.2%) Micro-morphology of fatigue crack propagation zone of X80 linepipe material under the condition of different strain amplitude is shown in figure 9 and figure 10. It is found that the typical characteristic of propagation zone of specimen is fatigue striations and second crack which induced by cyclic load with the strain amplitude of 0.4%, as shown in figure 9. Comparing with figure 9 (a) and figure 9 (b), it can be found that the amount of second crack and the spacing of fatigue striation of X80 conventional linepipe material is much higher than X80 high-strain linepipe material, which means that fatigue crack propagation rate of X80 conventional linepipe material is much higher. Figure 9. Micro mophology of fitigue crack propagation zone of X80 high-strain linepipe material ( /2 σΔ =0.4%): (a) conventional，(b) high-stain When strain amplitude increases to 1.2%, much more second cracks appear since load enhanced. Since the heavy extruding and grinding of fracture surface, propagation zone turns to be much smoother, tire-shaped pattern presents and fatigue striations almost disappears as shown in figure 10. The emergence of tire-shaped pattern means that fatigue crack speed dramatically increases under the condition of high strain amplitude which can enormously shorten the fatigue lifetime of material [6]. According to figure 10, X80 high-strain linepipe material presents a much obvious tire-shaped patter in crack propagation zone which presents a typical brittle fatigue feature. (a) (b) (b) (a)
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