パルセイティングストレスクリープ特性に関する研究(I)

抄録

Most of machines, vessels and others for high-temperature service are not static in their load or stress in actual service but vary in some regular or irregular manner depending on the operating conditions. In designing and manufacturing these products, therefore, the flow or creep properties and rupture characteristics of used materials and their welds under varying stresses at elevated temperatures are very important.
Such varying stresses as mentioned above may be classified into two types; one is the alternating static stress. which has as long period as several minutes or several hours and another is the continuous and dynamic stress which has as short period as less than one second. The latter stress condition was adopted in this study.
Both static and tensile pulsating stress rupture tests were carried out on mild steel at 450°C as a fundamental research before the investigations on heat resisting alloys and their welds were made.
Pulsating stress rupture tests were conducted in which four values of stress amplitude ratio A (σa/σm) were selected for a given mean stress; stress amplitude ratio A=5, 10, 20 and 30%. The frequency of pulsating stress was 380 rpm.
The mean stress versus time for rupture diagram in log-log plots showed that the rupture life decreases as the stress amplitude ratio A increases for the same mean stress. For a given mean stress, therefore, the static creep has longer rupture life than pulsating stress creep in which the rupture life decreases with increase of stress amplitude. But it is not appropriate to consider that decreases in rupture life mentioned above are only caused by the dynamic stress or fatigue behaviour due to alternating stress.
In view of fatigue behavior, a stress versus time for rupture diagram was plotted for maximum stress (crest stress) in the stress cycle on log-log scale. It was shown that the rupture life in pulsating stress creep became longer as the stress ratio A increased and the static creep had the shortest life. This is incompatible with fatigue behavior.
Rupture characteristics in pulsating stress creep were also investigated from creep behavior, assuming that ruptures under pulsating load were caused by accumlation of static creep damage produced by the instantaneous stress and the dynamic effect did not influence the rupture behavior. Consequently, the rupture strength of pulsating stress creep, regardless of stress ratio A, turned out approximately equal to that of static creep.
Ductilities of test specimens, determined by true strain at the fractured section, were compared in both static and pulsating stress creep. But no remarkable difference was found.
For the mild steel at 450°C, ruptures under the stress conditions of a relatively small pulsating stress being superimposed on the static stress which is sufficient to produce a creep rupture may be caused by static creep damage, and dynamic stress effects may scarcely affect the rupture behaviors.