Video-controlled tensile testing of polymers and metals beyond the necking point

Abstract

A novel technique has been developed to record the intrinsic plastic behaviour of ductile materials by monitoring the effective strain and the effective stress in the mid-plane of hour-glass-shaped tensile specimens. The method utilizes a computer-aided video extensometer which analyses the sample profile in real time. The effective strain is computed automatically from the minimum diameter, and the effective stress is deduced from the applied load by taking into account the stress triaxiality corresponding to the local radius of curvature of the sample profile. Furthermore, a digital closed-loop system controls the ram speed of the hydraulic tensile testing machine in such a way that the local effective strain rate is maintained at a constant value. It is shown that most polymeric and metallic materials are entitled to be investigated by this method, which gives access in real time to the constitutive plastic equation, up to strains far beyond the necking point. The capabilities of the technique are illustrated and discussed critically, with more details for two polymers of different structures: polyethylene and polycarbonate.