SOME PR0BL;EMS ON THE THEORY OF CREEP*

Section 1. The term creep of metals i s applied t o the phenomenon i n which, at temperatures beyond a certain l i m i t, the metal subjected t o a load slowly undergoes deformation w i t h time. For the case of s tee l, the creep phenomenon must be taken in to account a t temperatures above 4000 C. Very slow deformations for a prolonged period are cumulative and lead e i ther t o inadmissible changes i n the dimensions of a s t ruc tura l par t or t o i t s f a i lu re. It i s important t o note tha t fa i lure due t o creep occurs fo r very small s t r a ins considerably l e s s than those i n s ta t ic rupture. b In the design of steam power units, boi lers and turbines, creep is a basic factor which determines the choice of the admissible s t resses. On account of the extreme urgency of the problem, the creep phenomenon has claimed the widest a t ten t ion of metallurgists, physicists, and t o a lesser extent, technicians. A t the same time, however, the theory of creep consti tutes par t of the mechanics of dense media and the mechanical formulation of the pro-blem may be given as the following: A body i s subjected t o the action of a given system of forces, or i n i t i a l displacements are prescribed on i t s surface. f ind the s t r e s s dis t r ibut ion i n the body and the changes of i t s defor-mations with time. It i s required t o Such a statement of the problem immediately raises the following question: character is t ics of creep (cer ta in constants or functions) may be deter-mined? accepted methods of tes t ing, or is it necessary t o supplement them? What t e s t s should be se t up i n order tha t the mechanical Is it suf'ficient for t h i s purpose t o make use of the generally For the solution of the problemof creep as thus formulated, a mechanical theory of creep i s required. s t a t e of knowledge of the physics of the process, must necessarily bear Such theory, a t the presen