構造用鋼の疲労亀裂伝播におよぼす鋼の強度および介在物の影響

抄録

Fatigue crack propagation rates were obtained for structural steels having tensile strength levels ranging from 40 to 100kg/mm². The constant m in the Paris' formula, da/dN=C (ΔK) ^m, was found to decrease with increase in the yield strength of the steel. The influence of m on the fatigue crack propagation life was analysed for the case of center through-notched plate as well as part through-notched plate. Both experiments and calculations showed that the higher strength steels endured longer than the lower strength steels when cracks were large and/or stress ranges were high, however, they endured shorter when cracks were small and/or stress ranges were low.
When steel plates were fatigued under through-thickness stresses, m increased with increase in sulphur content, though the strength levels of steels were identical. This phenomenon was quantitatively analysed on the basis of the proposed model that sub-cracks initiating from MnS inclusions near the main crack accelerated the fatigue crack propagation rate.
The endurance limits in through-thickness fatigue were correlated with the projection length of inclusions based on the existance of the threshold range of stress intensity factor, ΔK_th, in the fatigue crack propagation rate ΔK diagram.