Issue34

K. Nambu et alii, Frattura ed Integrità Strutturale, 34 (2015) 271-279: DOI: 10.3221/IGF-ESIS.34.29 276 the hardness distribution mentioned above, the hardness of VC material decreases smooth and is in agreement with the hardness of QT material. The depth to converge is mostly in agreement with 2.6 mm of an above-mentioned carburizing layer. The position of the local minimum of the crack progress speed to Δ K =30 MPa√m is near a = 1.25mm. In Δ K =36 MPa√m, the position of the local minimum is near 1.6 mm to it. Moreover, it became clear that a crack progress speed becomes fixed in a depth of 2.6 mm. In addition to it, it was also shown clearly the prohibitive power over crack progress of a carburizing layer and that Δ K followed on going up and had become weaker. The reason the position of the local minimum in high Δ K changes from the above thing is explained. In crack progress, the crack progress resistance accompanying a crack closure formation process decreases with crack progress, and a steady value is shown. It is expected to it that the crack shielding effect by the processing induction martensitic transformation by the crack progress in a carburizing layer is decided by the value of retained austenite and Δ K . In order that retained austenite may show the tendency which decreases in the depth-of-cut direction, the shielding effect accompanying it also becomes the same. Because the assortment of these two effects serves as progress resistance, it is expected that the local minimum changes. Therefore, in higher Δ K, after the high crack progress speed after crack generating is shown, a speed falls gradually, and after a crack progress speed falls to a= 2.6 mm, without showing the tendency to go up after that again, becoming fixed is guessed. Figure 8: Relation between crack length and crack propagation rate under constant Δ K. E FFECTS OF ΔK THAT CAN BE PLACED ON EACH CRACK LENGTH ext, in order to see the influence of Δ K in each crack length, the relationship between crack progress speed da / dN in a crack length and Δ K shown in Fig. 8 as a solid line was shown in Fig. 9. Moreover, it is thought that it is to 2.6 mm to have influence of a carburizing layer also from the 2.6-mm approximated curve and the gradual increase curve of QT material being in agreement. In addition, 0.5mm, 1.0mm, 1.25mm and ΔK do not match. This formation process of crack propagation mechanism or closing action which means different. On the other hand, because 0.5 mm, 1.0 mm, 1.25 mm, and Δ K are not in agreement, it is thinkable that the formation processes of a crack progress mechanism and a crack closure effect differ. Then, in order to consider the influence of the crack closure effect by a carburizing layer, effective stress expansion coefficient width Δ K eff in Fig. 8 was shown in Fig. 10. As shown in a figure, in the case of a= 2.6 mm, it is thinkable that an crack closure effect and a progress mechanism are also almost the same. Moreover, if it arranges by Δ K eff also in 1.25 mm, since an approximated curve is in agreement with QT material, it turns out that the crack progress speed was changing with the transitions of the crack opening loading capacity by the difference in the formation process of the crack closure by a carburizing layer. However, in a = 0.5 mm, since QT material and an approximated curve are not in agreement at all, it is guessed that the difference in a destructive configuration is a difference in a crack progress behavior. N