13th International Conference on Fracture June 16–21, 2013, Beijing, China -8- Figure 7 ECCI pictures show the microstructure under the fatigue crack initiation area in Fig. 3a, Formation of damage at twin boundary by dislocation slipping bands in the annealing twin. The results discussed above are unexpected. High strain localization due to Schmid factor effect will cause an exhaustion of local plasticity of the material and dislocation slip band or persistent slip bands, and consequently formation of localized stress concentrations. This can lead to a fatigue damage or fatigue crack initiation at grain boundaries or twin boundaries by impingement cracking. High strain localization is the result of accumulation of very small strain during each cyclic loading in a much longer life or large number of cycles. This leads to the formation of local “fine grain area” and also increase the local hardness of the material, which can cause quasi-cleavage crack origin. 4. Size of SNDFCO In this investigation, the formation of SNDFCO is treated as a crack propagation process. In the earlier work, the striations or ridges can be observed at the SNDFCO [6]. At small stress amplitudes, the striations can still be observed although the SNDFCO is facet (Fig. 3c). These striations are relatively small (50-100nm). The stress intensity factor ranges at the front of SNDFCO is evaluated using the following equation [15]. area K π σmax 0,5 Δ = (1) In this study, area=L*b, where L is the maximum length of SNDFCO, and b is the shortest length of SNDFCO. This is a rather rough estimation since the actual SNDFCO is usually very irregular. Fig. 7 shows the stress intensity factor range at the front of SNDFCO in the martensitic-ferritic steel after the pulsating fatigue test. Although the size of SNDFCO is different, the stress intensity factor ranges at the front of SNDFCO are comparable, and vary from 2,8MPa√m to 4MPa√m, which is in the range of the fatigue threshold values. For the martensitic-austenitic steel, the stress intensity factor range at the front of SNDFCO is between 3.17 to 3.33MPa√m, which can compare with those of the damage slip band slip band
RkJQdWJsaXNoZXIy MjM0NDE=