疲労き裂伝ぱ法則da/dN=C(ΔK)mにおけるmとCの確率特性

Reliability analysis of structures based on fracture mechanics requires knowledge on statistical characteristics of m and C in the fatigue crack propagation law da/dN=C(ΔK)m. However, rather few experimental data have been reported on variability of m and C. In the present paper, axial fatigue tests were conducted on 30 specimens of 2024-T3 Al alloy under the same one condition. The values of m and C were determined for each specimen, and all the data were analysed statistically. The method for predicting the distribution of fatigue crack propagation life was also examined. The principal results are summarized as follows. (1) m follows a normal distribution approximately. (2) C follows a log-normal distribution approximately. (3) A strong linear correlation exists between m and log C, which is approximated by C=C0K0-m. (4) On the log da/dN-logΔK coordinates, the so-called pivot point (K0, C0) is considerably apart from the centroid of the data points, and therefore, it seems that the variation of m and C as well as their correlation do not arise from the intrinsic nature of the regression line. (5) Strictly speaking, C0 in the relation C=C0K0-m is not a fixed value but a random variable which is independent of m, and in the first approximation C0 follows a log-normal distribution. (6) The distribution of fatigue crack propagation life was calculated using the equation da/dN=C0(ΔK/K0)m on the assumption that m and log C0 are normally distributed random variables. The calculated distribution was in good agreement with the experimental distribution.