腐食環境下での疲労き裂成長速度に及ぼす応力上昇時間，保持時間，下降時間効果の定量的評価

Abstract

The corrosion fatigue tests were carried out for Cr-Mo steel (SCM440) by varying the stress rising time (tR), holding time (tH) and descending time (tD). In the present paper, for the case of symmetrical stress wave (tR=tH), the constitutive equation of corrosion fatigue crack growth rate, da/dN|CF, was derived explicitly in terms of tR and tH as follows:da/dN|CF=Dda/dN|air, D=1/1-dtRetHfΔKbexp{a(lnΔK)2+c} (1)where da/dN|air is the fatigue crack growth rate in air, ΔK is the range of stress intensity factor, a, b, c, e, and f are constants. The acceleration coefficient with holding time D(=[D]tH) is this equation was derived by the following method; First, the acceleration coefficient under the condition without holding time, [D]tH=0 was obtained as:[D]tH=0=a(lnΔK)2+blnΔK+c (2)Next, under the condition with holding time, the holding time effect was estimated by the following equation, [D]tH-[D]tH=0/[D]tH=dtRetHf (3)The experimental results of corrosion fatigue crack growth rate under various stress rising and holding time conditions were found to be well expressed by eq. (1). On the other hand, this equation does not satisfy the critical condition that da/dN should be proportional to 1/tH for tH→∞, which is characteristics of stress corrosion cracking. Thus a more satisfied constitutive equation was derived as the equation of the following type: da/dN=F0(ΔK, tR, 2tR+tH). From this equation, it can be found that the corrosion fatigue crack growth rate is affected by f=1/(2tR+tH)) which concerns the time dependent effect and tR which concerns the fatigue effect under the symmetrical stress wave condition.Furthermore, it was deduced that the mechanism of the tR process is controlled by the fatigue effect coupled with the time dependent effect controlling the tH process, while the tD process is controlled mainly by the time dependent effect although it includes some fatigue effect.