CORRELATION OF DELTA-KTH-VALUE WITH THE EXPONENT, M, IN THE EQUATION OF FATIGUE CRACK GROWTH FOR VARIOUS STEELS

Literature (mainly in Japan) relating to fatigue-crack-growth-data at R=0 in an air environment for a wide range of steels is reviewed with particular attention to the threshold stress intensity, ΔKth. The collected data are analyzed in terms of the exponent, m(the slope of the linear portion of the log(da/dN)-logΔD relationship) by taking account of microstructure, material strength, fracture toughness and specimen thickness. The mean rate of fatigue crack growth for ductile steels in the range from the intermediate growth rate to threshold level and the relevant threshold values at R=0, ΔKth0, can be represented as da/dN = 1.700 × 10-4(ΔK103.6)m - 10-6andΔKth0 = 103.6(5.88 × 10-3)1/m,where (da/dN) and ΔK are measured by the units of mm/cycle and kgf/mm3/2, respectively. Contrary to this, in the case of extremely brittle steels with KIC-value below 200 kgf/mm3/2 (the fracture occurs by the intergranular separation), the relationships are given by da/dN = 2.893 × 10-5(ΔK/49.94)m - 2.5 × 10-7andΔKth0 = 49.94(8.64 × 10-3)1/m. © 1979 Sijthoff & Noordhoff International Publishers.