Issue34

K. Nambu et alii, Frattura ed Integrità Strutturale, 34 (2015) 271-279: DOI: 10.3221/IGF-ESIS.34.29 275 In the case of nitriding processing material, this value is 0.92, and it is clear that there is no big difference in a surface crack and an internal crack length. It is thought that it is because progress of a crack is controlled by internal toughness rather than the surface modification layer as this reason. Fig. 7 showed the observation state in that case typically. As shown in figure, the crack in case progresses in brittleness. However, crack progress is affected by the influence of base metal part, and progress is controlled. If only the hardened layer is observed, a crack progress speed will become early extremely by a brittle fracture, but because it is affected by the influence of a base metal part, it is thinkable to progress with configuration like illustration. Therefore, the surface-crack length and internal crack length of the surface modification material which has a surface-hardening layer are considered to be an almost comparable length. Therefore, it is thought that a stress intensity factor is computable using a surface-crack length. VC material showed bigger crack progress speed da / dN than QT material immediately after crack generating. Then, after the crack growth rate of VC material became a low value and reached the local minimum rather than QT material, it showed the tendency to go up again. This reason is considered that crack progress was controlled because the opening of the crack was barred by the crack closure effect by the compressive residual stress given to the carburizing layer. Moreover, as Sugimoto et al.[11] is also described, when the retained austenite of a carburizing layer carries out a processing induction martensitic transformation with crack progress, a compression stress place is formed at the tip of a crack, and it is thought that crack progress was controlled. However, the area which has on crack progress is only a parts which has influence of a carburizing layer. The influence of a base metal part becomes large with progress of a crack. Therefore, after a crack progress speed shows the local minimum, a crack growth rate rises and shows the same crack progress speed as a base metal. As mentioned above, it became clear that the carburizing layer has affected control of a crack progress speed. Moreover, it is thinkable that the crack progressed in brittleness in the carburizing layer as a reason which showed the value with a big crack progress speed immediately after crack generating. Figure 6: Relation between da/dn and ΔK. Figure 7: Schematic diagram of crack propagation. ΔK CONSTANT TEST RESULTS n the loading-capacity fixed experiment shown for the preceding clause, because Δ K also increases with progress of a crack, it is difficult to consider the influence of a carburizing layer in detail. Then, in order to consider the influence of a carburizing layer, the Δ K fixed experiment was done. The relationship of crack progress speed da / dN and crack length a in each Δ K was shown in Fig. 8. The tendency to go up after a crack progress speed falls by the case of all the Δ K in VC material like the case of a loading- capacity fixed experiment, as shown in a figure. From this, the depth of a carburizing layer is presumed to be about 2.6 mm also from the crack length having been in agreement at about 2.6 mm. This shows that the carburizing layer has occurred until more deeply than an effective carburizing layer. (HV550). Although the value up to 1.2 mm is shown by I