抄録
As for the fatigue strength characteristic, a crack propagating curve (da/dN-⊿K) is mainly used. The curve, which is classified into 3 steps, shows material characteristics in each area. We aimed at the last fracture surface and this research analyzed fracture surface. That is, as for the material which underwent surface modification with high toughness and high strength steel, both the surface crack spread and the inner crack spread exist. Therefore, We analyzed fracture surface where there was fracture in the repeat of the low stress power using the high strength high toughness austempered spheroidal graphite cast iron with the minute flaw for which it is easy to occur inside. We researched for the minute yielding area using the fractal dimension method in the analysis, calculating a destruction toughness value (K_<Ic>) in seeking that case and we got a valid result. The state of which a cleavage surface is formed around the minute flaw, it propagates inside and spreads next from the crosswise direction to the surface were shown. As these, as for the fracture mechanism of the 2-bending curve, that the surface crack didn't become dominant became clear. We think that one with the crack surface, which spreads inside, is caused by the surface hardness gradient.
