塑性疲労下のき裂進展速度に及ぼす平均応力・ひずみの影響とき裂開閉口

抄録

JIN SNCM439, S35C and SUS304 steels were low cycle fatigued with mean stress and strain under load and displacement controlled conditions. The cyclic J integral range, ΔJ and the crack tip opening displacement range, ΔΦ_t(200) were not effective for evaluation of the crack growth rate under test conditions accompanied by a ratcheting deformation. Then, the parameter of ΔJ/{1-(Φ_c^min/Φ_c^max)}^0.2, which can control the acceleration or the crack growth rate due to the ratcheting deformation as well as the crack growth rate under the fully reversed cyclic strain, was usefully proposed here, where Φ_c^min and Φ_c^max are minimum and maximum crack center opening displacements. The ΔΦ_t(200) was correlated with ΔJ by the equation of type of ΔΦ_t(200)=A(ΔJ)^q. In general, the power exponents, q are smaller than the value of 1/(n'+1) or 1/(n+1) estimated by assuming the HRR singular field around the fatigue crack tip where n' and n are the cyclic and monotonic strain hardening exponents of materials.