Creep Strain Increments Research Articles

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

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.

Computation of creep effects in prestressed concrete pressure vessels using dynamic relaxation

The paper describes a method developed for time dependent stress analysis of prestressed concrete pressure vessels. Different creep functions for concrete are discussed at first in order to select the best basis for the computation. The full solution is obtained in a step-by-step process. The time is divided into short intervals during which the stresses are supposed to remain constant. According to this assumption the creep strain increments are calculated after each time interval and then the incompatibility is corrected by an elastic solution. The complete previous stress history which has to be known for the computation of the new step is stored on the tape. This enables the free choice of the creep function as well as of the length of all time intervals. The so-called dynamic relaxation method is used for the elastic solution at any stage. The application of dynamic relaxation for the purpose of time-dependent stress analysis is also presented. The structure of the computer programme compiled in ALGOL language is briefly discussed. At the end of the paper some results of the computation for an actual cylindrical vessel subjected to the steady thermal gradient are presented.