Issue 47

T. Kawabata et alii, Frattura ed Integrità Strutturale, 47 (2019) 416-424; DOI: 10.3221/IGF-ESIS.47.32 422 for which were directly experimentally measured using the method of caustics. Additionally, experimental results using PMMA (Dally et al. [28]) showed that there is a limit on the speed that is considerably lower than the Rayleigh wave velocity and that as K is made extremely large, crack propagation velocity gradually approaches this speed limit. It follows naturally that this relationship can also be applied to the ferrite single phase material used in this study; the relationship between the stress intensity factor and the crack propagation speed is shown schematically in Fig. 11. Accordingly, this experiment should also be consistent with the transition of the stress intensity factor (data of the previous research in Fig. 10). However, the opposite trend is seen (diamond marks in Fig. 10). The authors think that this is an effect of the delay or stopping phenomenon at the grain boundary, which has a significant twist angle. When the number of grains across the thickness of the test piece is low, the crack propagation must proceed directly through the highly misorientated grain boundary, therefore the crack propagation speed is delayed considerably at breakthrough. After breaking through, propagation through the grain continues with relatively low resistance, so the crack propagation speed accelerates quickly and attempts to return to the original speed. Fig. 12 schematically shows the acceleration phenomena that can be seen in very coarse-grained materials, even while the average stress intensity factor is decreasing. Blue line in Fig. 12 indicates the crack propagation history of poly-crystalline material which is frequently experienced in laboratory experiments [29]. The crack propagation history is substantially smooth, according to the conventional K - v relationship. This is because crack can choose a preferential path from many crystal grains so the crack can deviate the grain boundary with a significant twist angle and can choose the path on which the crack easily propagates. Figure 11 : Assumed relationship between stress intensity factor and crack propagation rate based on experimental research by Rosakis et al. [27] Figure 12 : Estimated mechanism of the crack propagation rate history in a coarse grained material