Issue 40

V. Mazánová et alii, Frattura ed Integrità Strutturale, 40 (2017) 162-169; DOI: 10.3221/IGF-ESIS.40.14 164 (a) (b) Figure 2 : Hysteresis loops in (a) in-phase cycling (b) 90° out-of-phase cycling. (a) (b) Figure 3 : Cyclic hardening/softening curves during in-phase cycling: (a) uniaxial stress amplitude vs. number of cycles, (b) plastic strain amplitude vs. number of cycles. Figs 3 and 4 show in more detail the cyclic hardening and softening behavior. Except the very few initial cycles both tension-compression stress amplitude (Fig. 3a) and shear stress amplitude (Fig. 4a) decrease initially rapidly and either stabilize for the lowest strain amplitude or start to increase again towards the end of fatigue life. During 90°out-of-phase cycling only small changes of the stress and plastic strain amplitude during fatigue life are found (Fig. 5). Early cyclic hardening is followed by mild softening for small strain amplitudes and hardening for high strain amplitudes both in tension-compression and in torsion. The level of the stress amplitude is considerably higher than in in- phase cycling. The cyclic stress-strain curves have been plotted in Fig. 7 for in-phase and 90° out-of-phase cycling. The equivalent stress and plastic strain amplitudes in in-phase straining were calculated using von Misses relations        2 2 , 3 a eq a a (1)              2 2 , 3 a a eq a (2)