Issue 41

M. Sakane et alii, Frattura ed Integrità Strutturale, 41 (2017) 16-23; DOI: 10.3221/IGF-ESIS41.03 20 Specimen axis 1 mm Figure 5 : Cracking direction in torsion tests of unnotched, notched and precracked specimens. Strain range dependency of cracking direction in torsion low cycle fatigue Cracking direction in torsion LCF is also influenced by strain range, shear cracking at high strain ranges and principal cracking at low stain ranges shown in Fig.6 [14] as a typical case for a SUS 304 stainless steel, but the mechanism of the transition of cracking direction is still an open question whereas many researchers discussed this topic. The strain ranges with shaded area are the strain ranges at which crack transits its direction. Micro-cracks on the specimen fatigued until a half-life shown with the solid circles in the figure were observed in detail with a scanning electron microscope. Figure 6 : Torsion low cycle fatigue life and cracking mode of SUS 304 stainless steel. Four type of cracks were observed on the specimen surface shown in Fig.7 [14]. They are classified according to the propagation behavior as shown in Fig.8 schematically. Type 1 and Type 2 cracks extended in the shear direction in some (a) Unnotched specimen of SUS 304 stainless steel at 923 K (Δσ * =329 MPa, N f =4694) (b) Notched specimen of SUS 304 stainless steel at 923 K (Δσ * =314 MPa, N f =1500) (c) Precracked specimen of SUS 304 stainless steel at 923 K (Δσ * =314 MPa, N f =1320) (d) Notched specimen (0.2 mm through hole) of 1CrMoV at 823 K (e) Notched specimen (0.1 mm diameter, 0.1 mm depth) of SUS 304 stainless steel at 823 K 0.2 mm Specimen axis