Thermomechanical effects accompanying the localized necking of semi-crystalline polymers

Abstract

This paper presents an application of quantitative infrared thermography to the analysis of the mechanical behavior of materials. We particularly focus on the thermomechanical behavior of a semi-crystalline polymer below its glass transition temperature. During a quasi-static tensile test, the temperature distribution at the surface of a thin flat sample is recorded by an infrared camera. Using a local expression of the heat balance, the distribution of heat sources is derived from thermal measurements. These calorimetric data are then correlated with strain and stress fields. Indeed, a second optical device gives displacement fields using correlation methods on speckle images. Strain and strain-rate are derived from displacement data by numerical differentiation. The load-displacement curve shows three stages: first the load increases with the stretching, then a significant softening of the sample occurs until it flows at constant load. Both calorimetric and kinematic measurements indicate that the sample softening is associated with a progressive localization of heat sources and strain-rates, while the loading plateau corresponds with a regular expansion of the neck. A local construction of stress-strain diagram is then proposed. Several stress-strain curves are finally analyzed taking into account the loading stage characteristics and the expansion mode of the necking region.