Issue 33

D. Shiozawa et alii, Frattura ed Integrità Strutturale, 33 (2015) 56-60; DOI: 10.3221/IGF-ESIS.33.07 60 CONCLUSIONS he mechanisms of crack initiation and propagation during cyclic reversed torque controlled experiment have been studied in situ. Surface observations (optical microscopy and EBSD) show that intragranular cracks initiate in mode II and have a strong cristallographic character. The planes of these cracks correspond to highly activated {111} slip planes. Strong deviations are often observed when those mode II cracks encounter some grain boundaries. The bulk development of those cracks has been imaged inside the sample bulk by synchrotron X-ray tomography. The crack growth rate at the surface (mode II) is faster than in the bulk (mode III) leading to a shallow crack. Friction between the crack surfaces might impede mode III growth in the bulk for low values of the applied torque. When the number of cycles is increased, however, the growth in the bulk resumes but in mode I. The values of the mode I, II and III stress intensity factors computed from a FE simulation cannot account for this transition from mode III to mode I unless some kink appears at the crack tip. Such a kink might be produced by a change in the local crystallography (grain boundary) as observed for the mode II cracks at the surface. R EFERENCES [1] Murakami Y, Takahashi K, Toyama K., Mechanism of crack path morphology and branching from small fatigue cracks under mixed loading. Fatigue Fract Engng Mat Struct, 28(1–2) (2005) 49–60. [2] Sawada M, Bannai K, Sakane M., Crack propagation of type 304 stainless steel in torsion low cycle fatigue, J Soc Mat Sci Jpn, 54(6) (2005) 615–21. [3] Murakami, Y., Takahashi, K., Kusumoto, R., Threshold and growth mechanism of fatigue cracks under mode II and III loading, Fatigue Fract Engng Mater Struct 26(2003), 523–531. [4] Shiozawa, D., Nakai, Y., Murakami, T., Nosho, H., Observation of 3D shape and propagation mode transition of fatigue cracks in Ti–6Al–4V under cyclic torsion using CT imaging with ultra-bright synchrotron radiation, Int. J. Fatigue, 58 (2013) 158-165. [5] PhD I.Serrano Munoz INSA Lyon (2014), http://theses.insa-lyon.fr/publication/2014ISAL0117/these.pdf (in english) [6] Matsunaga, H., Muramoto, S., Shomura, N., Endo, M., Shear mode growth and threshold of small fatigue cracks in SUJ2 bearing steel, J Soc Mat Sci Jpn, 58(9) (2009) 773–80. [7] Campbell, JP, Ritchie, RO. Mixed-mode, high-cycle fatigue-crack growth thresholds in Ti–6Al–4V: II. Quantification of crack-tip shielding, Eng Fract, Mech, 67 (2000) 229–49. T