The effect of the magnitude of the stress exponent on the time span of logarithmic stress relaxation and creep

Abstract

representing the dependence of the shear strain rate y on the shear stress T is extensively used in the study of plastic deformation. It has been found that the stress exponent n is a quantity which characterizes several plastic properties of theoretical and engineering interest. 1,2,3,4 For example, Johnston has shown1 that the yield drop exhibited in constant strain rate tests and the delay time observed in constant stress creep is controlled by the magnitude of the stress exponent n. Because both tests are controlled by the same thermally activated process the two effects are interrelated.5 Studies by Faucher, Aggarwal and Krausz6 have indicated that the behavior exhibited in constant loading rate tests is also strongly affected by the magnitude of the stress exponent. It was found that when the stress exponent is large the sow stress increases only slightly over a long period of loading time while when n is sm all the stress increase is very steep. Experimental observations indicate that superplastic behavior is also characterized by the stress exponent. It has been established empirically7,8 that the smaller is n the greater is the elongation. The relation between the stress exponent and superplasticity was explored in a theoretical study9,lO and it was shown that when both deformation and fracture are controlled by Newtonian sow (n = 1) the material is ideally superplastic. The stress exponent is, therefore, a me aningful characteristic quantity. It is the purpose of this communication to report the results of a study in which the effect of the magnitude of the stress exponent on the extent of the logarithmic stress relaxation and creep period was investigated.