Stress as a Reduced Variable: Stress Relaxation of SBR Rubber at Large Strains

Abstract

Abstract Stress relaxation measurements on SBR were carried out at temperatures from −5 to +60° C and at initial strains of up to 550%. The effects of strain and time were found to be factorable, so that the isochronal stress-strain curve may be written as a modified Hooke's law with a time-dependent modulus: S=E(t)eƒ(α), where ƒ(α) is an appropriate function of the strain. By defining a strain-reduced stress S*=S/∫(α), i.e., a strain-reduced modulus E*(t)=E(t)ƒ(α), it can be shown that Ferry's method of reduced variables may be extended to large deformations. An appropriate strain function was obtained from the empirical Martin-Roth-Stiehler equation as ƒ(α)=α−2 exp A(α−α−1) with A=0.40. Although it cannot yet be certain that A is truly a constant and the same for all elastomers, this equation has the advantage of being valid right out to the breaking strain.