(ii) This paper studies the crack propagation direction of type 304 steel tube specimen in reversed torsion tests. Strain controlled torsion low cycle fatigue tests were carried out for type 304 steel tube specimens and the number of cycles to failure was obtained. The crack directions were also observed by intermitting the tests. Main cracks propagated in the maximum shear strain directions at high strain ranges but in the principal strain directions at low strain ranges. The transition of the main crack direction from the maximum shear strain direction to the principal strain direction by lowering strain range was discussed based on the observations of micro crack directions. The main crack is the crack that directly leaded to the failure of specimen and the micro crack is the crack whose length is less than 100μm. Most micro cracks initiated in the maximum shear strain directions and they branched in the principal strain directions as they grew. The critical length of branching was related to the strain range. The critical crack length at high strain ranges was longer than that at low strain ranges, which corresponded well with the main crack propagation directions. The aspect ratio of cracks was discussed in relation to the main crack directions and the energy release was discussed regarding the aspect ratio.

This paper discusses crack propagation directions of type 304 steel tube specimens in reversed torsion tests. Strain controlled torsion low cycle fatigue tests were performed on type 304 steel tube specimens and the numbers of cycles to failure were obtained. Crack directions were also observed by intermitting the tests. Main-cracks propagated in the maximum shear directions at high strain ranges but in the principal strain directions at low strain ranges. The transition of the main-crack direction from the maximum shear strain direction to the principal direction by lowering strain range was discussed from the observations of micro-crack directions. The maincrack is the crack that directly leaded to the failure of the specimen and the micro-crack is the crack whose length was less than 100μm.Most micro-cracks initiated in the maximum shear directions and they branched in the principal directions as they grew. The critical length at the branching depended on strain range. The critical crack length at high strain ranges was longer than that at low strain ranges, which corresponded well with the main-crack propagation directions. Aspect ratios of the shear and principal cracks were discussed in relation with the main-crack directions and the energy release in the two cracking modes.