An experimental study of end effects in the extensional deformation of polymers

Abstract

The sample geometry and the constraints of grips used in measurements of mechanical properties affect the values of the elastic constants obtained. This is known to be more serious for anisotropic materials. Three polymers have been examined in extension: oriented polyethylene terephthalate, isotropic low-density polyethylene, and oriented nylon 6. The results all show a linear increase of compliance with increasing reciprocal sample length. Subtracting the displacements of samples so short that the grips were initially in contact (“zero-length” samples) indicates that the effect is caused mainly by shear in the grips. Extrapolation of the results to zero reciprocal length gives the true sample compliance, and this removes discrepancies in comparing shear compliances derived from extensional and torsional methods.