CREEP IN PLATES AND SHELLS

Abstract

Creep is the designation of slow deformation, which occurs in many materials within certain ranges of stress and temperature. In every day experience metals are known as firm sdids. When a piece of metal is loaded by small forces, it will deform elastically. This means that the piece will regain its original shape, when the load is removed. This deformation process is independent of time in the sense that a certain force causes a certain instantaneous deformation that remains constant as long as the force remains constant. In some metals, the deformation is found to proceed at a certain slow rate. When unloaded, these metals show an instantaneous reverse deformation followed by a slow recovery of part of the remaining deformation. Such time-dependent deformation is termed CREEP. Experience shows that the pheno menon is strongly temperature-dependent, the creep rates increasing markedly with rising temperature. In many sectors of modern technology high temperatures are present and even aimed at. The thermal efficiency of a heat power-machine or plant rises with the temperature of the heat source. A study of temperatures employed in steam power plants shows a steady temperature increase during the last two hundred years. Many chemical processes, such as cracking in oil refinery, require high temperatures in addition to high pressures*, parts of nuclear power reactors operate at very 3 high temperature*, in supersonic aircraft the adiabatic heating at leading edges of wings may be very severe*, and, in space vehicles the heating caused during reentry may be enormous. At such high temperatures, the creep deformation would occur. The presence of creep in engineering structures poses a number of problems to the designer. Pirst, the deformation caused due to creep may cause malfunctioning of machine components. Second, the stress distribution in such components during a creep process is different from that in the elastic state and may also vary with time. Third, rupture may occur abruptly after a certain amount of creep deformation has developed. Although observations of creep deformation had been reported in the literature of the nineteenth century, the first methodical study was not published until the early part of <1>* the twentieth century by Andrade (1910). He investigated creep in lead wires subjected to deadweight loading at room temperature. The general character of the creep process from a macroscopical point of view was established. Subsequent efforts took two different paths - one aiming at an engineering theory of creep to be used in design work, the other aiming at a physical theory capable of describing the creep process in terms of already established concepts in physios.