Issue 33

M. Sakane et alii, Frattura ed Integrità Strutturale, 33 (2015) 319-334; DOI: 10.3221/IGF-ESIS.33.36 327 Figure 12 : Variation of stress amplitude with cycles in loading mode change tests. Figure 13 : TEM observations for loading mode changes tests at 823K. (a) APT loading followed by P loading, (b) P loading followed by T loading. Regarding the relationship between cyclic stress response and dislocation structure, the findings are as follows. In uniaxial LCF tests at room temperature, a cell structure is formed [2]. At elevated temperatures, however, ladder or maze structures form because of a thermally activated process which rearranges the dislocation structures into more ordered array having lower elastic strain energy. Mura et al. [11] reported that the maze and ladder structures have less strain energy than cell structure by the elastic calculation. In APT loading, on the other hand, the maximum shear strain changes the direction by 45 degrees in each cycle, so that larger interactions between the slip systems occur. In APT loading, the larger interactions result in a cell structure, because it prevents the rearrangement of the dislocations by the thermally activated process. The resistance to the dislocation glide is greater in the cell structure so that larger cyclic strain hardening results. Fig. 12 shows the variation of stress amplitude with cycles in loading mode change tests from P to T and APT to P loadings. Looking at the loading mode change test from P to T, the stress amplitude jumped up in the cycle just switched to T loading, which is well known as the cross hardening effect. The stress amplitude gradually decreased with proceeding cycles after switching the loading mode and finally it reduced to the stress amplitude in the original P loading. This result indicates that once the cross hardening effect occurred by switching the loading mode but the effect disappeared by the following cyclic loading. We have to note that the experiments were performed at high temperature so that the thermal activation assisted the reduction in the stress amplitude after the switch. This result also implies that the microstructure was once disturbed by the switch of the loading mode but it was mostly reformed to the microstructure in the followed loading mode. The stress amplitude in APT loading before the switch was larger than that in P loading and it started to reduce after the switch of the loading mode to P. The stress amplitude after the switch did not reach to that in the original P loading and it stabilized at the higher stress amplitude than that in P loading. The reduction in the stress amplitude after