Calorific Analysis of the Mechanical Response of Granular Materials Composed of Ellipsoidal Rubbery Particles.

Background: Infrared thermography (IRT) techniques and full-field deformation calorimetry approaches have profoundly impacted the experimental mechanics community. The way in which granular materials are tested is currently excluded from this trend, as evidenced by the small number of papers in the literature mentioning the use of thermographic cameras. Objective: The objective of this work was to perform a thermomechanical analysis of soft granular systems by distinguishing temperature changes associated with thermoelastic coupling (TEC) and mechanical dissipation (MD). Methods: Cyclic confined compression was applied to different granular systems consisting of ellipsoidal cross-section cylinders made of thermoplastic polyurethane. The temperatures measured using IRT were processed to identify the heat associated with TEC and MD, based on considerations of adiabaticity and thermodynamic cycle completion. Various granular configurations and spatial resolutions of the thermal maps were compared. Results: Strong TEC is revealed around the contact areas between all particles, which can be explained by stress concentrations. Strong MD is found around specific contacts, as well as within some particles, which can be explained by viscosity and friction. TEC data was processed for a granular system comprised of about 600 interparticle contacts, providing statistical information. Conclusions: TEC appears to be a strong coupling in the sense that its intensity is much higher than that of the MD. It is shown that IRT provides valuable information at the local scale, namely the signature of reversible (TEC) and irreversible (MD) mechanical phenomena, both at the interparticle contacts and inside the particles. [ABSTRACT FROM AUTHOR]