Issue 40

V. Mazánová et alii, Frattura ed Integrità Strutturale, 40 (2017) 162-169; DOI: 10.3221/IGF-ESIS.40.14 163 E XPERIMENTAL he material studied was commercial AISI 316L stainless steel supplied by Thyssen in the form of a round bar of 20 mm in diameter. Its chemical composition (in wt.%) was as follows: 0.017 C, 1.60 Mn, 0.497 Si, 0.029 P, 0.027 S, 16.70 Cr, 10.1 Ni, 2.03 Mo, 0.031 Ca and 0.078 N. The bar was hot rolled;  0.2 = 295 MPa. The microstructure was formed by equiaxed austenite grains with the average austenite grain size was 80  m and some delta ferrite bands. The hollow cylindrical specimens with dimensions shown in Fig. 1 were machined from the round bars. No heat treatment was applied after the machining. Figure 1 : Geometry of the hollow cylindrical specimen The gauge section was mechanically and electrolytically polished in order to achieve perfect surface for observation of the surface relief. All tests were performed in an electrohydraulic axial–torsion computer controlled MTS testing system. Combined axial-torsion extensometer was used to measure and control axial and shear strain. The tests were conducted under fully-reversed straining (R = -1) both in tension-compression and in torsion with constant strain rate. Axial strain rate was 5x10 -3 s -1 and the shear strain rate on the specimen surface was equivalent to the axial strain rate. The strain in the middle diameter of the specimen is reported. Plastic strain was evaluated by subtracting the elastic component from the total strain. Since the fatigue behaviour of the material has been already thoroughly studied in axial testing [1, 6, 7] we have performed torsion tests and in phase and 90° out-of-phase biaxial tension-compression-torsion tests. The biaxial tests are reported here. Surface relief and fatigue crack initiation was studied on cracked specimens using optical microscope, SEM observations and FIB cutting. The surface of the specimen was inspected in FEG-SEM Lyra 3 XMU (TESCAN) equipped with focused ion beam (FIB). In order to protect the surface of the fatigued specimen from the ions during the production of surface craters the area of observation containing the PSMs was first covered by the thin sheet of platinum using electron deposition and later thicker layer was applied using ion deposition. Sectioning using FIB was performed perpendicular to the surface and nearly perpendicular to the direction of PSMs. Final cutting was performed with small intensity to achieve smooth perpendicular surface. The cuts were imaged in secondary electrons under inclination of about 35 degrees. The effect of inclination was compensated in all images of the cuts. R ESULTS Stress-strain response ysteresis loops were recorded during cycling in both channels (uniaxial and torsion). Fig. 2 shows typical shapes of hysteresis loops during in-phase cycling and 90° out-of-phase cycling. Appreciable softening is apparent in in- phase cycling. Much higher stress response has been registered in 90° out-of-phase cycling at approximately the same plastic strain amplitude. Specific shape of the hysteresis loop in 90°out-of phase straining is due to reversion of the strain rate direction in both channels. T H