Mechanics of Creep For Structural Analysis

Under certain conditions in a stressed structural material, a continuous plastic deformation occurs with time for a constant stress value. This deformation is called “creep.” The importance of creep in a particular design depends upon various factors as, for example, the material considered, the allowable stress used, the temperature of operation, and the desirability of maintaining a small deformation. For this reason the designer must consider the effect of creep in many applications, such as occurs in the turbine, oil-refining and automotive industries. In cases where high temperatures occur, changes in the stress distribution and deformations are sometimes so great that the need for considering effects of creep has been realized for several years. For normal temperatures, on the other hand, little has been done to determine the influence of creep as a factor in structural design. This paper is an attempt to analyze this problem. In developing methods of analysis for statically indeterminate structures it was necessary first to select a creep-stress law that could be used in place of Hooke's law. For this purpose a new creep-stress law is proposed in this paper. With a basic creep-stress relation for simple tension, theories for bending stresses and deflections in beams are developed. These theories form the basis for the slope-deflection and moment-distribution procedures developed for the analysis of statically indeterminate structures in the case of creep.