LOCAL-SCALE ANALYSIS OF THE LONGITUDINAL STRAINS IN STRONGLY NECKING MATERIALS BY MEANS OF VIDEO-CONTROLLED EXTENSOMETRY

A novel method of determination of the longitudinal strains at a very local scale in the case of materials that develop a plastic neck under tensile drawing is presented. It relies on the analysis of the distortions of a linear target drawn on a thick thermoplastic polymer film by means of computer-assisted video-controlled extensometry. An equation is given for the target slope as a function of the local principal strains. True-stress and true-strain determinations can be performed along the neck as it forms and propagates. In situ measurements of the volume variations are afforded as a function of strain during the necking process