Energy stored during deformation of crystallizing TPU foams

Abstract

The hysteresis loop observed in the mechanical response of elastomers is classically assumed to be due to viscosity. In this study, a complete energy balance is carried out during cyclic deformation of compact and foamed crystallizing thermoplastic polyurethanes. 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 through the γse ratio (Loukil et al. (2018) European Polymer Journal, 98, 448–455) to investigate the effects of the loading rate and the void volume fraction on the energetic response of thermoplastic polyurethane.

Inverse identification of constitutive parameters from heat source fields: A local approach applied to hyperelasticity

“…The mechanical response of rubber-like materials is governed by numerous physical phenomena: rate-dependent effects, [4] permanent set, [5] stress softening, [6][7][8][9][10] mechanical hysteresis, [6,9] strain-induced crystallization (SIC), [11][12][13][14] energy storage, [15][16][17] anisotropic effects, [5,9,[18][19][20] change in volume, [21] and the competition between entropy and isentropy at low strains. [22] It should be noted that most of these phenomena are induced or amplified by adding fillers in the rubber matrix.…”

Recent progress in the energy characterization of the mechanical behaviour of rubbers

“…• the heat power density at any time during the deformation s(t), named heat source in the following. All of these quantities and their calculation are presented in Lachhab et al (2018) and Loukil et al (2018).…”

“…Three materials are considered in the discussion: an acrylonitrile butadiene rubber (NBR) unfilled and carbon black filled, an unfilled natural rubber and a polyurethane (TPU). Further information on the materials and the specimens used is provided in Loukil et al (2018), Le Cam (2017) and Lachhab et al (2018), respectively. The mechanical tests consist in applying sets of several cycles at an increasing maximum stretch λ max .…”

“…Polymers have then benefited from this approach (Rittel 2000, Rittel and Rabin 2000, Benaarbia, Chrysochoos, and Robert 2014, Benaarbia, Chrysochoos, and Robert 2015. Concerning elastomers, three recent studies investigate the energetic behavior and the energy storage during deformation (Le Cam 2017, Lachhab, Robin, Le Cam, Mortier, Tirel, andCanevet 2018, Loukil, Corvec, Robin, Miroir, Le Cam, andGarnier 2018). From these studies, the energy storage in different types of elastomers, filled and unfilled, crystallizing or not, can be discussed.…”

A comparison between X-ray diffraction and quantitative calorimetry to evaluate the strain-induced crystallinity in natural rubber

“…This technique has been successfully employed to characterize the calorimetric signature of the main phenomena involved in the rubber deformation (Samaca Martinez, Le Cam, Caillard 2013, Le Cam, Samaca Martinez, Balandraud, Toussaint, andCaillard 2015) and at the crack tip where the mechanical and calorimetric fields are strongly heterogeneous (Samaca Martinez, Berghezan 2014, Samaca Martinez, Toussaint, Balandraud, Le Cam, andBerghezan 2015). Furthermore, such technique was used to carry out energy balance and to identify the intrinsic dissipation (Le Cam 2017, Lachhab, Robin, Le Cam, Mortier, Tirel, andCanevet 2018, Loukil, Corvec, Robin, Miroir, Le Cam, andGarnier 2018). Recently, Le Cam (2018) proposed to apply this technique for measuring the strain-induced crystallinity in an unfilled natural rubber.…”