Thermomechanical effects accompanying the localized necking of semi-crystalline polymers

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 quasistatic 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. (C) 2002 Editions scientifiques et medicales Elsevier SAS. All rights reserved.