Issue 30

M. Da Fonte et alii, Frattura ed Integrità Strutturale, 30 (2014) 360-368; DOI: 10.3221/IGF-ESIS.30.43 366 where ΔK I , ΔK II and ΔK III are the stress intensity factors for mode I, II and III respectively and E and ν are the elasticity modulus and the Poisson’s ratio. Therefore, Eq. (4) can be used to correlate the fatigue long crack growth data in a mixed- mode loading condition. Fatigue crack growth Under rotating or alternate (reversed) bending a stress ratio R=-1 exists, therefore ΔK Imax =-ΔK Imin and according to the recommendations specified on ASTM E647, in this study ΔK I =K I max will be used. For static torsion ΔK III =0 , therefore Eq. (5) is reduced to ΔK I , since only mode I is activated in cyclic loading. Fig. 6 shows the fatigue crack growth rate as a function of the mixed-mode equivalent energy release range which in this case reduces to the mode I stress intensity factor range, determined as mentioned before, for several loading conditions and 10 mm and 12 mm diameter specimens: rotary bending, and bending conditions combined with steady torsion. As observed in Fig. 6, an effect of steady torsion on fatigue crack growth (FCG) rate is observed. Results show that fatigue crack growth in rotating bending decreases with higher levels of steady torsion while for lower levels of steady torsion a small retardation effect is observed. These results represent a similar trend to the one obtained in steels under the same loading conditions that were presented before [9, 13]. a) b) Figure 6 : Fatigue crack growth a) 10 mm specimen diameter; b) 12 mm specimen diameter Fractography Scanning electron microscopic (SEM) observations were carried out for both types of fracture surfaces, pure mode I, due to rotating bending only, and mode (I+III) due to rotating bending and steady torsion, and the crack growth profiles created by two different types of loading are shown in Fig. 7 a) and b) respectively. They correspond to the formation of regular sets of inclined micro facets corresponding to mode I crack growth inclined due to mode III loading, and are similar to the “factory roof” markings observed in previous similar studies. This “factory roof” crack growth is completely “rubbed” by the strong torque imposed during rotating bending. While some “rubbing” is also visible on crack growth due to rotating bending, this is due to the tension/compression loading cycle observed in rotating bending. The rubbing of the fracture facets is very well observed in Fig. 8 a) and b) which shows the fracture surface near the initial phase of fatigue crack growth and much more clearly on Fig. 8 b), where the higher amplification clearly shows two types of fracture facies, one strongly “rubbed” at the tip of “factory roof” fracture followed by more “normal” facies characteristic of fatigue crack growth under mode I.