Mechanical Response and Energy Stored During Deformation of Crystallizing TPU

Abdelmonem Lachhab,Yvon Tirel,Frédéric Canévet,Jean-Benoît Le Cam,Eric Robin,Frédéric Mortier

doi:10.1007/978-3-319-95074-7_10

Abstract

AbstractThe present study investigates the thermomechanical behavior of closed-cell TPU foams. The effects of the density and the loading conditions on the softening, the residual strain and the hysteresis have first been characterized. The thermal responses exhibit numerous particularities. First, a threshold effect in terms of the density on the self-heating has been highlighted. Second, entropic effects are strongly weighted by energetic effects (internal energy variations) during the deformation. Typical changes in the thermal response highlight that SIC and crystallite melting occur during the deformation. The characteristic stretches of this phenomenon evolve with the maximum stretch applied. The lower the density, the lower the crystallinity. In the second part of this study, a complete energy balance is carried out during cyclic deformation of compact and foamed crystallizing TPUs. Results show that viscosity is not the only phenomenon involved in the hysteresis loop formation: a significant part of the mechanical energy brought is not dissipated into heat and is stored by the material when the material changes its microstructure, typically when it is crystallizing. Some of this energy is released during unloading, when melting occurs, but with a different rate, which contributes to the hysteresis loop. The part of the mechanical energy stored by the material has been quantified to investigate the effects of the loading rate and the void volume fraction on the energetic response of TPU. These effects cannot be predicted from the mechanical responses and the present study provides therefore information of importance to better understand and model the effects of the density and the loading conditions on the thermomechanical behavior of closed-cell TPU foams.KeywordsTPU foamIntrinsic dissipationEnergy storedStrain-induced crystallizationInfrared thermography

Highlights

The thermoplastic polyurethanes (TPUs) are subjected to many anelastic phenomena such as mechanical hysteresis, residual stretch and softening [1–6], which strongly depend on the material formulation
The energy dissipation capacity of TPU is classically measured as the mechanical hysteresis area, but no study investigates the physical origin of this dissipation, while three phenomena could be responsible for this mechanical hysteresis: the intrinsic dissipation (d1), due to internal friction and/or damage, which leads to selfheating; the thermal dissipation (d2) and the part of the mechanical energy used by the material to change its microstructure (Wstructure), see for instance recent studies on polyurea [11], natural rubber [12] and nitrile rubber [13]
The present study investigates the thermomechanical and calorific behavior of TPU foams

Summary

Introduction

The thermoplastic polyurethanes (TPUs) are subjected to many anelastic phenomena such as mechanical hysteresis, residual stretch and softening [1–6], which strongly depend on the material formulation. The energy dissipation capacity of TPU is classically measured as the mechanical hysteresis area, but no study investigates the physical origin of this dissipation, while three phenomena could be responsible for this mechanical hysteresis: the intrinsic dissipation (d1), due to internal friction and/or damage, which leads to selfheating; the thermal dissipation (d2) and the part of the mechanical energy used by the material to change its microstructure (Wstructure), see for instance recent studies on polyurea [11], natural rubber [12] and nitrile rubber [13]. Such analysis is classically carried out in metallic materials [14–17]. To further discuss on the relative contribution of the energy stored in the hysteresis loop of rubbers, they proposed a ratio γ se, written in terms of energies over one mechanical cycle as follows:

Methods

Results

Conclusion