Issue 41

M. Sakane et alii, Frattura ed Integrità Strutturale, 41 (2017) 16-23; DOI: 10.3221/IGF-ESIS41.03 21 distance and branched into the principal directions. Type 3 crack propagated in the principal directions and Type 4 crack is the crack that only propagated in the shear direction. More than 400 cracks were observed on each specimen. Type 1 and Type 2 cracks were most frequently found and Type 3 and Type 4 cracks are scarcely found. This result indicates that the coalescence of micro cracks is not a major mechanism of the shear cracking at high strain ranges and of the principal cracking at low strain ranges because the number of shear cracks should be dominant at high strain ranges and that of principal cracks should be dominant at low strain ranges if the coalescence of cracks is a major mechanism of failure. (a) Δ  =1.73%, Type 1 (b) Δ  =3.46%, Type 2 (c) Δ  =1.73%, Type 3 (d) Δ  =3.46%, Type 4 Axial direction Figure 7 : Four crack type in torsion low cycle fatigue of SUS 304 stainless steel. Figure 8 : Schematic of four crack types in torsion low cycle fatigue of SUS 304. Shear crack lengths before branching to principal cracks for Type 1 and Type 2 cracks (critical crack length) were measured and the average values of the critical crack lengths (average crack length) against shear strain ranges are plotted in Fig.9 [14]. The averaged crack length was obtained by observing 1956 cracks within a gage length of the intermitted specimens. The figure clearly indicates that the average crack length has constant values at the low and high low strain ranges. The transition of the average crack length is found in the shear strain range between 2.0% and 2.5%. This transition strain range well corresponds with the transition strain range of main cracks shown in Fig.6. The correspondence of the transition strain range implies that one of the mechanisms of the crack direction change results from that both a macro shear crack and a macro principal crack microscopically propagate zigzag in the respective directions but the propagation length in shear direction is longer at high strain ranges and that in the principal directions longer at low strain ranges. Makabe et al. [15] made a similar discussion but on the notched specimen. They reported that the critical crack length before branching increased momentously with shear strain range. To investigate the reason of the two propagation directions, aspect ratios of a shear crack and a principal crack were measured by continuing stepwise polishing off the material from the specimen surface and measuring the removed depth and crack length on a respective new surface. The observations are shown in Fig.10 [14]. The figure clearly indicates that the shear crack has a small aspect ratio while the principal crack a large aspect ratio, the shear crack mainly propagating near the specimen surface but the principal crack extending deep into the specimen.