Issue 33

M. Sakane et alii, Frattura ed Integrità Strutturale, 33 (2015) 319-334; DOI: 10.3221/IGF-ESIS.33.36 328 the switch resulted from the rearrangement of small cell structure formed in APT loading to the ladder structure by the cyclic loading with the assistance of the thermal activation but the small cell structure was too stable to rearrange completely. Only some parts of the cell structure developed in APT loading was rearranged to the microstructure in P loading. Dislocation structures are shown in Fig. 13 after the loading mode change from APT to P in (a) and from P to T in (b). As seen in Fig. 13 (a), round shaped cell structure still remains, especially on the right side figure in (b), indicating the cell structure was not completely rearranged to that in P loading. The incomplete rearrangement of cell structure is the experimental background of the incomplete reduction in stress amplitude to P loading mode shown in Fig. 12. Clear ladder and maze cell structures were found in (b), which are the structures in P and T loadings. Especially, the maze structure is a specific cell structure in T loading, and these structures in (b) are the experimental background of the complete rearrangement of cell structure developed in prestraining and complete reduction in the stress amplitude after switching the loading mode. Fig. 14 depicts the stress amplitude ellipse to express the hardening characteristics of the prestrained material in P and APT loadings at a Mises equivalent strain range of 1%. These data were obtained in the test that the material was cyclically strained to certain cycles to obtain the stable stress amplitude and was suddenly switched to the loading mode of combined tension and torsion to get a stress amplitude in Fig. 14. After measuring the stress amplitude, the loading was back to P loading to the cycle of stabilizing the stress amplitude and the loading mode again changed a different combination of combined tension and torsion to obtain a stress amplitude with another combination. The stress amplitude of the prestrained material in APT loading are collapsed on Mises line so the material was hardened isotropically. The isotropic hardening was also microstructurally confirmed from the round shaped cell structure shown in Fig. 10. Round cell structure has a same resistance in any directional loading. The stress amplitude of the prestrained material in P loading, on the other hand, showed anisotropic stress amplitude. The stress amplitudes of the prestrained material in P loading are rather on the line of  2 +2  2 . The prestrained material in P loading had the ladder structure, and it showed the anisotropic resistance to preferential directions. Figure 14 : Stress amplitude ellipse for P and APT loaded materials. M ICROSTRUCTURE IN VARIOUS STRAIN WAVEFORMS AT ROOM TEMPERATURE ystematic multiaxial LCF tests were performed using 14 strain waveforms shown in Fig. 15 on 304 SS at room temperature. The figure represents the shear and axial strain applied to the tube specimen shown in Fig. 1 with a slight modification at gage part to avoid buckling and bulging under severe nonproportional loadings. Case 0 and Case 5 are a proportional loading and the other strain paths are all nonproportional loading. From the test results using the strain waveforms in the figure, various factors which influence multiaxial LCF lives are discussable but the detailed explanation of the factors is not presented here because this paper does not intend to discuss LCF lives in detail. Readers who are interested in how the factors are influential to nonproportional LCF lives can consult to the reference [12]. Fig. 16 illustrates the axial stress-shear stress hysteresis loops at two strain ranges in the respective strain paths. The shear stress-axial stress form the corresponding shapes with the strain waveforms and the stress range enlarged with the S