Semi-automated image analysis of the true tensile drawing behaviour of polymers to large strains

Abstract

An image analysis system has been developed using commercially available hardware with custom software to investigate the deformation behaviour of solid polymers in uniaxial tension. This technique provides a rapid, semi-automated non-contacting method for determining true process stress-strain-strain-rate behaviour for both homogeneous and inhomogeneous deformation. The relative displacements of printed transverse grid lines are determined from images captured during a standard monotonic tensile test, providing local measures of strain. The examination of a time series of images allows the generation of true strain-rate data, and concurrent monitoring of the total draw force from the load cell allows the generation of true stress data at those times when the images are captured. Therefore, it is possible to produce a series of process uniaxial true stress-strain curves for individual “elements” of material within the gauge length of the specimen. Synthetic elastomers drawn at ambient temperature have been found to display relatively homogeneous deformation, resulting in a simple process axial stress-strain curve for the single-speed test, whereas in the case of inhomogeneous deformation (“necking”) exhibited by polypropylene, it is verified that each element of material experiences a slightly different deformation process. This spatially variant deformation is related to the original location of the particular element with respect to the point of neck initiation.