CHARACTERIZATION OF THE BEHAVIOR OF RUBBER FOR ENGINEERING DESIGN PURPOSES .1. STRESS-STRAIN RELATIONS

The stress-strain behavior of a range of black filled rubbers has been studied in extension, compression, pure shear and simple shear. The data have been analyzed to examine the validity of Gregory's hypothesis that the stored energy function U of filled rubbers can be expressed solely in terms of the strain invariant I-1, ignoring I-2, to an accuracy adequate for most engineering design requirements. Our results confirm his suggestion. An analytical form for U is proposed which gives a very good fit to the experimental data for strains from less than 0.1% to somewhat greater than 100%, which cover the range of interest for most engineering applications. The dependencies of the parameters in the expression for U on filler level and degree of crosslinking have been examined. It has thus been demonstrated that, for a given material, the form of U can be determined from the measured stress-strain relation in any simple deformation mode, shear or tension for example, without the necessity of relatively difficult biaxial measurements, and that this function should then be applicable to any deformation mode, complex or simple.