Issue 31

A.R. Maligno et alii, Frattura ed Integrità Strutturale, 31 (2015) 97-119; DOI: 10.3221/IGF-ESIS.31.08 112 It is understandable that when the crack front approached some 90° on the circumference the crack front started changing its shape and elongated. The external node was likely to enter the compressive zone [24, 25], and the crack growth slowed down, whereas the internal node remained in the tensile-stress zone and continued to propagate (Fig. 22). The number of cycles was monitored for the whole set of crack configurations; the respective results are summarised in Tab. 9. Crack Shape Cycles Corner Crack 2.600E+06 (to reach external surface) TW 1 1.910E+05 (to reach configuration TW 2 ) TW 2 1.450E+03 (to reach configuration TW 3 ) TW 3 No growth TW 4 No growth TW 5 No growth Total number of cycles 2.7925E+06 Table 9 : Number of cycles for different crack configurations. It is interesting to observe that at crack configuration TW 3 the crack propagation can be considered stabilised and no subsequent crack propagation was detected. It is paramount to notice that, while the crack advanced, an inversion of the trend for the stress intensity factor occurred. The consequence of this particular behaviour can be attributed to the position of the crack in relation to the neutral axis, as also described in [24, 25]. In fact, the crack, at its initial stage, was entirely in tension (Fig. 22a), but when it advanced, the neutral axis changed its initial position; as soon as the crack position was between 85° (external surface) and 90° (internal surface), the crack started to be exposed to compressive stresses (Fig. 22b). In particular, from Fig. 22b it is possible to deduce that for the crack positions close to 110° the K value underwent an abrupt reduction. Therefore, the flaw in proximity of the external surface was totally under compressive stress and no crack propagation could be detected (according to the Linear Elastic Fracture Mechanics criteria). (a) (b) Figure 22 : Crack configurations from FE analysis: (a) initial crack configuration; (b) extended crack configuration.