Abstract
The fatigue tests were performed to obtain thresholds of stress intensity factor range, ΔKth, by the incremental step method in air at R=0.1 using HY140, HY240, HP9-4-20, 18Ni (200) maraging, 18Ni (250) maraging, MAS-MA-164, and PH13-7 steels. The informations obtained from fracture surfaces were analyzed in the low stress intensity factor range. The results are as follows;
1) The fracture surface consists of unit region, which is a bundle of lath-like-pattern, and flat area just like a grain boundary.
2) The size of the fracture unit incrcases with ΔK.
3) The size of units is about one half of the pre-austenitic grain size at the threshold region.
4) The width of lath-like-patterns is 0.2 to 1.0μm at the theshold region. This width is almost same as the lath width. This width increases with ΔK.
5) The direction of the bundle of the lath-like-pattern is quite randum to the direction of the macroscopic growth. With ΔK increasing, however, the direction of the bundle approachs to the macroscopic direction of crack growth.
6) The maximum intergranular fracture rate generally arises at about 30kg/mm3/2 of ΔK. The maximum values of steels containing no carbon is two or three times as large as thoes of steels containing carbon.
7) The ΔKth of steel scontaining carbon is generally lower than that of steels containing no carbon.
