Issue 30

M. Da Fonte et alii, Frattura ed Integrità Strutturale, 30 (2014) 360-368; DOI: 10.3221/IGF-ESIS.30.43 367 a) b) Figure 7 : SEM micrographs a) Bs=190 MPa; Ts=0; b) Bs=152 MPa_Ts=71 MPa a) b) Figure 8 : SEM micrographs Bs=152 MPa_Ts=71; a) initial crack growth b) crack growth path C ONCLUSIONS he effects of steady torsion on fatigue crack growth under cyclic bending were studied for long crack growth on aluminium alloy 7075-T6 specimens under multiaxial fatigue loading conditions. For long cracks a superimposed steady mode III loading to a crack growing in cyclic mode I can lead to significant fatigue crack growth retardation, depending of the ratio between maximum bending stress and maximum steady torsion. Crack growth rates decrease with increasing mode III for cyclic mode I (ΔK I ) + static mode III (K III ) loading. These results are consistent with similar ones obtained for Ck45 steels for the same loading conditions, therefore the retardation effect on crack growth can be considered as an effect of the steady torsion on rotating bending. Due to steady torsion effect superimposed to rotating bending, the crack path is in zig-zag which creates a helical surface crack morphology. Fractographic observations lead to the conclusion that this retardation effect is due to a significant higher irregularity on fatigue crack path which is observed when steady torsion is applied to rotating bending. R EFERENCES [1] de Freitas, M., François, D., Analysis of fatigue crack growth in rotary bend specimens and railway axles. Fat Fract Engng Mater Struct, 18 (1995) 171-78. T