Effect of fatigue crack closure and geometry on structural sensitivity of near-threshold fatigue in steels

An attempt was made to consider the effect of crack top bluntness (the effect which wasn't considered as the factor of stress relaxation) during evaluation of near-threshold steel fatigue along with crack closure (CC). Industrial iron and 40Kh steel were chosen for experiments. The cyclic crack resistance tests were conducted at installation with rigid loading by bending through a console of beam samples of 12x18x180 mm size at 10 Hz frequency and R=0.05 assymetry factor. Study of crack geometry was reduced to determination of the radius of its rhosub(th) peak, corresponding to Ksub(th) threshold level. It was shown that crack bluntness conditioned by plastic deformation of the material in the vicinity of its peak decreases sufficiently with steel strength growth (rhosub(th)=100, 120 and 140 nm after tempering at 200, 500 and 650 deg C; rhosub(th)=190 nm after annealing). Metallographic analysis revealed the fact that in the case of plastic steel states the marks of plastic strain are observed along one crack bank in the direction parallel to crack front inspite the low level of stress intensity factor. It points to the presence of cut jaws in the fine near-the-surface layer, which could decrease the accuracy of rhosub(th) determination