Issue 40

V. Mazánová et alii, Frattura ed Integrità Strutturale, 40 (2017) 162-169; DOI: 10.3221/IGF-ESIS.40.14 168 (a) (b) Figure 10 : Surface of the specimen cycled in 90°out-of phase straining to fracture: (a) central part of the secondary macroscopic crack, (b) PSMs in a grain and their profiles. Cyclic stress-strain curves were plotted both for both individual channels and in equivalent stress and strain. Equivalent strain could be defined easily for in-phase straining. For 90° out-of-phase (diamond) straining the strain amplitude is reasonably well given by the tension-compression strain amplitude. However, the strain direction in a cycle changes and the strain path is more complicated. The cyclic stress-strain curve in equivalent stress and strain defined above lies above the cyclic stress-strain curves corresponding to tension-compression and torsion and has slightly higher slope. Comparison of the equivalent cyclic stress-strain curves for in-phase and for 90° out-of-phase straining shows that due to the complicated strain path in 90° out-of-phase cycling the cyclic stress-strain curve is well above that of in in-phase straining. It is also apparent from Tab. 1 where the parameters of all cyclic stress-strain curves are listed. The study of the surface of the specimens subjected to biaxial cyclic straining revealed general trend of the cracking in low cycle fatigue domain and also the early damage induced by in-phase and in 90° out-of-phase cycling. Macroscopic cracks grow at the angle around 45° to the specimen axis. The initial stages of fatigue damage were studied in more detail. The study of the PSMs and small secondary cracks on the surface of the specimen revealed the profile of persistent slip markings consisting of extrusions and intrusions. Intrusions represent crack-like surface defect with high stress concentration factor. Fatigue cracks start from intrusions and grow along primary slip plane, usually on the boundary between the PSB and the matrix. This mechanism is the same as observed in uniaxial cyclic loading [2]. A CKNOWLEDGEMENT he present work was conducted in the frame of IPMinfra supported through project No. LM2015069 and the project CEITEC 2020 No. LQ1601 of MEYS. The support by the project RVO: 68081723 and grants 13-23652S and 15-08826S of GACR is gratefully acknowledged. R EFERENCES [1] Alain, R., Violan, P., Mendez, J., Low cycle fatigue behavior in vacuum of a 316L type austenitic stainless steel between 20 and 600 degrees C .1. Fatigue resistance and cyclic behavior, Mater. Sci. Eng., A 229 (1997) 87-94 [2] Man, J., Valtr, M., Petrenec, M., et al., AFM and SEM-FEG study on fundamental mechanisms leading to fatigue crack initiation, Int. J. Fatigue, 76 (2015) 11-18; DOI 10.1016/j.ijfatigue.2014.09.019. [3] Wang, Y., Kimura, H., Akiniva, Y., et al., EBSD-AFM Hybrid analysis of crack initiation in stainless steel under fatigue loading, Key Eng. Mater., 340-341 (2007) 531-536. [4] Jacquelin, B., Hourlier, F., Pineau, A., Crack Initiation under Low-Cycle Multiaxial Fatigue in Type-316l Stainless- Steel, J. Press. Vess.-T. ASME, 105 (1983) 138-143. T