Issue 47

T. Kawabata et alii, Frattura ed Integrità Strutturale, 47 (2019) 416-424; DOI: 10.3221/IGF-ESIS.47.32 420 Figure 7 : Schematic figure of brittle crack initiation in 3%Si steel Investigation of the brittle crack propagation rate in a single crystal A strain gauge including multiple-strain gauge lines was attached to the surface of the grain that was approximately in the centre of the specimen (Fig. 4) to evaluate the crack speed history inside a single crystal. Fig. 8 shows the propagation speed record versus the distance from the grain boundary just above this crystal. Crack propagation speed clearly showed an increasing trend as the crack progressed. On the other hand, when the static stress intensity factor based on continuum mechanics is calculated as the driving force, the K value tends to decrease with crack propagation in the region focused upon (Fig. 9). When the crack propagation speeds from this experiment are plotted versus K static , as shown in Fig. 10, they are located in the polycrystalline region, although the measurements were from a single crystal. This is because the targeted grain did not penetrate the through thickness, and the grains close to the strain gauge likely propagated before the grains on the opposite side, according to detailed observation of the opposite side fracture surface. Additionally, while the crack propagation speed increased as the K value increased for all measurements from the previous study, the crack propagation speed from this study decreased with increasing K value. This discrepancy will be examined in the next section. Please note K static is used for discussion in this study. Authors believe that real K dynamic is better for the detailed discussion. However, actual dynamic K cannot be determined by static SIF and crack propagation rate in actual material. Also this time special observation like caustic method [27] was not employed. So this time we used static K for evaluation of the resistance of crack propagation based on the fact that this time propagation rate is sufficiently lower than Rayleigh wave speed. If dynamic K is calculated by theoretical dynamic fracture mechanics, we think the change of K value is limited. Figure 8 : Crack propagation rate in a single crystal calculated from the record of a multiple-strain gauge.