Relaxation Behavior of Elastomers under Large Deformation

Abstract

Stress relaxation measurements were conducted in three different modes: after an “instantaneous” deformation, after a constant rate of deformation, and after the cessation of steady‐state flow. In addition, dynamic oscillatory measurements, stress‐rise measurements at constant rates of deformation, and steady‐state flow measurements were made. A Rheometrics mechanical spectrometer was used at 100°C with a cone‐and‐plate fixture. Samples were an ethylene‐propylene copolymer (EP) Mooney index 40 and two butadiene‐acrylonitrile copolymers (BN) Mooney index 35. The EP sample exhibited melt flow behavior at the testing temperature and the BN samples were in the rubbery region at the testing condition. The viscoelastic behavior observed with the above measurements were correlated systematically. In the correlation the EP sampler obeyed linear viscoelasticity theory. With the BN samples the applicability of a strain‐time shift is demonstrated, whereby the nonlinear data were reduced to their linear equivalents. A method is presented for estimating strain and the strain‐time shift factor for the stress relaxation after the cessation of flow.