動クリープ強度解析における二, 三の問題

Abstract

The authors have conducted some analyses on the dynamic creep for several years with the aim to estimate the dynamic creep strength from the informations on static creep, and verified the applicability of the analyses to several materials. In these studies, the analyses were confined to the case of the dynamic creep under an axial varying stress which was uniformly distributed over the cross-section of specimen. In the actual service conditions, however, there would be the cases of non-uniform stress distribution such as occurring in the members subjected to bending or torsion and also in the turbine blade, which is assumed to be under the combined stress state of axial static tension and alternating bending. Taking up the above-mentioned combined stress condition for turbine blade as a typical example of the cases of non-uniform stress distribution, the authors have carried out experiments as well as analyses, which brought the possibility of predicting the dynamic creep strength under this combined stress condition from static creep data. In this paper, the analyses are extended to the cases of bending dynamic creep, torsional dynamic creep and also of dynamic stress relaxation.The outline of the analyses is as follows:The assumptions which were used and verified in the previous study on the dynamic creep under combined static tension and alternating bending is utilized in this study also, that is, (1) The equivalent static stress σe introduced for the purpose of predicting the axial dynamic creep strength in the transient stage of creep is also applicable to the second stage of creep.(2) The distribution of alternating stress σa may be regarded as elastic, since the alternating component of strain is almost purely elastic as a result of the sufficiently high speed of stress alternation.(3) The distribution of the equivalent, static stress σe under a dynamic stress condition is the same as that of the static stress which would produce the same creep.If these assumptions are adequate the distribution of the dynamic stress which will produce the creep strain same as that under any static stress condition can be determined easily in the following way. The distribution of the equivalent static stress σe is obtained as a static creep problem according to the assumption (3). The distribution of the alternating stress σa is known as an elastic stress distribution from the assumption (2). On the other hand, the relation of σe, σa and σm is obtained from static creep data, and, finally, the distribution of the mean stress σm is determined from the relation, by inserting the above obtained value of the equivalent stress σe and the alternating stress σa.According to the results of analyses on the bending dynamic creep and torsional dynamic creep, the relation between Mm/Me and Ma/Me in these cases (where Mm, Ma and Me denote the mean moment, the alternating moment and the equivalent static moment, respectively, as in the case of stress) is similar. to the relation between σm/σe and σa/σe, provided that the latter relation for the case of the dynamic creep under an axial stress condition may be expressed approximately in a straight line within a sufficiently wide range of the variables.In the case of stress relaxation, the differential equation for dynamic stress relaxation becomes as1/Edσm/Idt+f(σm, ε0-σm/E)=0,