Issue 33

M. Sakane et alii, Frattura ed Integrità Strutturale, 33 (2015) 319-334; DOI: 10.3221/IGF-ESIS.33.36 329 nonproportionality of the strain path. Fig. 17 correlates the multiaxial LCF lives with the maximum principal stress, where numbers in the figure are the numbers of Cases. The figure clear shows that all the multiaxial LCF lives are overestimated in the correlation with the maximum principal stress based on Case 0 lives. Considering that Mises strain and the maximum principal strain underestimated the nonproportional LCF lives, the stress or the strain only is not a suitable parameter for correlating multiaxial LCF lives but both the stress and stress should be taken into account. We have to note that only the stress or strain parameter is an approximation of the physical damage process in multiaxial LCF from the viewpoint of the simplicity for easy application of the parameter to designs of machines and structures. Figure 15 :14 strain waveforms used in LCF tests. Fig. 18 (a)-(j) show the microstructure observed by TEM. Fig. 18 (a) shows the dislocation structure before testing where the dislocation density is very low and no specific substructure is identified. A cell structure is observed in Case 0, Fig. 18 (b), where the mean cell diameter is around 1  m. Cell formation was also observed in the specimen cyclically loaded at large strain ranges (>1.0%) in Case 0. Dislocation bundles which indicates cluster of dislocations were observed at low strain rages (<0.8%) in Case 0. Cell structures, twins and stacking faults were observed in Case 1, Fig. 18 (c), but only twins and stacking faults in Case 3, Fig. 18 (d). No clear cell formation was found in Case 3 and dislocation bundles were observed. Many stacking faults occurred before cell formation and they appear to hinder the cell formation in Case 3. The phasing of the applied strains produces larger stress and strain ranges for Case 3. Nishino et al. [4] reported that a ladder or maze structure was a common structure for 304SS in proportional loading and a cell structure was primarily found in the nonproportional loading like Case 0 at high temperature as shown previously. At room temperature, however, cell structure formed and no ladder or maze structures were observed in Case 0. This difference of dislocation structure between room and elevated temperatures results from the difference in the thermal