The Initial Stage of Plastic Deformation in Lead Under Compression

Abstract

In the steady-creep stage in metals, stress (or resistance to deformation) is related to the velocity of deformation, in accordance with the hyperbolic-sine law, or the exponential law. These laws, however, fail completely to describe the initial stage of plastic deformation in lead. The present series of tests is an investigation of this initial stage in compression. An optical lever system and high speed kymograph have been employed. The photographic records obtained show the progress of deformation with time. The duration of the initial stage increases with the compressing load. For a load of 120.0 kg the duration is about 0.16 sec. It is found that a unique, initial deformation, independent of time, is associated with each load. From measurements on the deformation curves, resistances as functions of time have been calculated. These data indicate a linear relationship between the resistance R and time t: R=l+mt,where l=a parameter of small value, evidently a correction term, and m=rate of increase of resistance. Using this equation and the dynamic constants of the apparatus, deformation curves in good agreement with experiment have been derived. Resistance in the initial stage appears to be only approximately a function of deformation alone