A Static-to-Dynamic Transition in Creep of Metallic Materials under Cyclic Stress Conditions

Abstract

A transtition from static creep to dynamic creep was observed in the range of frequency of stress cycling from 1 cycle per hour to 1 cycle per minute with metallic materials of different crystal structures and compositions; the creep rate decreased drastically with the increase of frequency of stress cycling from the rate which is equal to the constant stress creep rate to a very low value. This transition was shown to be the result of creep recovery during the low stress period in each stress cycle. In the case of intermittent stressing, the frequency at which the transition occurs is determined by a critical creep strain which is, roughly speaking, common for different materials and independent of stress and temperature; if the creep strain increment during the load-on period in a stress cycle is less than the critical creep strain, it recovers during the following load-off period and the resultant creep rate becomes very low. In the case of cyclic stressing of non-zero lower stress, the transition frequency increases with the increase of the lower stress level. It is suggested that the transition might be related to small amount of impurity in the materials.