金属材料の高温における動クリープおよび疲労

Abstract

The analysis by the author, which aims to predict the dynamic creep and fatigue strength from the information of static creep tests, is presented in a summarized form and discussed in comparison with the experimental results for various metallic meterials. The analysis is classified into the following four categories:(a) Dynamic creep rupture: In the case of the dynamic stress with small stress ratio (ratio of alternating stress to mean stress), rupture strength can be estimated from static creep rupture data by analysing on the basis of the idea of cummulative creep damage. In conducting the analysis, the equivalent static stress, which yield the same rupture life as the dynamic stress, has been determined asσ'e=σm[1/2π∫2π0(|1+Asinωt|)αsd(ωt)]1/αs, where σm, A, t and ω are mean stress, stress ratio, time and frequency of alternating stress, respectively, and αs is a constant which is obtained as the inclination of log-log plots of stress vs. rupture time diagram of the static creep rupture test.(b) Dynamic creep: In the range of small stress ratio, the creep deformation also can be predicted from test results under static stress. The analysis was carried out on the basis of the strain hardening theory, and the equivalent static stress, which causes the same creep as the dynamic case, had been defined asσe=σm[1/2π∫2π0(1+Asinωt)α/βd(ωt)]β/α, where α and β are the stress index and the time index, respectively, in the case that the creep strain in the transient stage is expressed as a power function of stress and time.(c) Fatigue: In the case of large stress ratio, the material is likely to fracture due to the accumulation of fatigue damage, and the strength can be estimated from reversed stress fatigue data by assuming that the alternating stress determines the life independent of the mean stress.(d) Fatigue deformation: In the range of stress ratio, in which the fatigue fracture occurred, the deformation was sometimes larger than that predicted on the basis of strain hardening theory. The deformation in such case was called as fatigue deformation.The analysis stated above could be applied to materials of relatively stable structure, that is, a carbon steel, some ferritic and austenitic stainless steels and a commercially pure titanium.In the case of super alloys of precipitation hardening type, however, considerable descrepancy was observed between theory and experiments, and this descrepancy seemed to result from acceleration of precipitation hardening or other strengthening effect by alternating stress. As an attempt to overcome this difficulty, a correction factor, called as degree of strengthening, was introduced into the analysis, such asχ=[(σm/σe)exp-(σm/σe)th]/(σm/σe)th, where subscript exp and th denote experimental and theoretical values, respectively. This factor could be expressed as a function of stress ratio and time, that is, χ=χ0θμtν, where θ=tanA; χ0, μ and ν are constans, and μ≅1.5, ordinarily. By using this formulae, the strength of such materials under dynamic stress condition also could be estimated from static creep and reversed stress fatigue data.