Issue 40

V. Mazánová et alii, Frattura ed Integrità Strutturale, 40 (2017) 162-169; DOI: 10.3221/IGF-ESIS.40.14 166 100 1000 200 300 400 500 600 700 800 900 Stress amplitude,  a ,  a ,  a,eq (MPa) 10 -3 10 -2 Plastic strain amplitude,  ap,  ap ,  ap,eq  a vs.  ap  a vs.  ap  a,eq vs.  ap,eq 100 1000 200 300 400 500 600 700 800 900 Stress amplitude,  a ,  a ,  a,eq (MPa) 10 -3 10 -2 Plastic strain amplitude,  ap,  ap ,  ap,eq  a vs.  ap  a vs.  ap  a,eq vs.  ap,eq (a) (b) Figure 7 : Cyclic stress-strain curves in biaxial cycling: (a) in-phase cycling, (b) out-of-phase cycling. and equivalent plastic strain amplitude was evaluated using equation      , , , , a eq a eq p a eq eff E      n a ap K (3) In 90° out-of-phase loading equivalent stress amplitude was evaluated using Eq. (1) but instead of using shear stress amplitude the shear stress corresponding to peak value of the axial stress was used. Equivalent strain amplitude was equal to the tension-compression strain amplitude. The effective modulus E eff =190 GPa was used. Cyclic stress-strain curve in equivalent stress and strain in in-phase straining lies above the cyclic stress-strain curve corresponding to tension-compression but has approximately the same slope. In 90°out-of-phase straining the cyclic stress-strain curve is only slightly above the cyclic stress-strain curve corresponding to tension-compression. All curves could be well approximated by the power law      n a ap K (4) The parameters of all cyclic stress-strain curves determined using least square fitting shows Tab. 1. Type of loading Tens.-comp. stress Torsion stress Equivalent stress K ʼ (MPa) n ʼ K ʼ (MPa) n ʼ K ʼ (MPa) n ʼ in-phase 1262 0.251 483 0.202 1258 0.206 90° out-of-phase 2604 0.279 1261 0.266 3758 0.325 Table 1 : Parameters of the cyclic stress-strain curves Early fatigue damage Fractured specimens were studied in optical and scanning electron microscope. Multiple secondary cracks developed during the fatigue life in addition to the principal crack. The majority of larger cracks were inclined approximately 45 degrees to the specimen axis. The central part of one of these macroscopic cracks is shown in Fig. 8a. It is evident that the crack path is rugged since the crack microscopically often follows crystallographic planes.