長期使用クロム・モリブデン鋼劣化材の疲労き裂進展寿命予測 : 第1報, 長期使用劣化材での疲労き裂進展特性

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To investigate the effect of material degradation on life prediction, fatigue crack propagation in a degraded steel was studied. Fatigue crack growth rate, da/dN, vs stress intensity factor range, ΔK, relationships were obtained for 72 crack tips in degraded 2 1/4 Cr-1 Mo steel due presumably to temper embrittlement during long service, a recovered one made from the above degraded steel and an unused carbon steel S50C. The main results are : (1) High intergranular fracture surface ratio and preceding cracks ahead of main cracks are remarkable characteristics of the degraded steel. (2) The da/dN vs ΔK relationship was not well represented by a single line for the degraded steel. Above 13 to 15 MPa √<m>, da/dN in the degraded steel was higher than that in the other two steels. (3) The m and C in the da/dN = C(ΔK)^m for each steel tested followed normal and log normal distributions, respectively. (4) The coefficients of variation in the values of m and C were larger in the degraded steel than in the other two steels. (5) Almost the same linear relationship was obtained between log C and m for the three steels tested. (6) The predicted fatigue crack growth life given by Monte Carlo simulation was shorter at higher stress levels, and the scatter of lives was larger in the degraded steel than in the recovered steel.
1986-08-25