Comparison of direct and indirect measurement of the elastocaloric effect in natural rubber

The directly measured temperature change Delta T upon deformation (elastocaloric effect) of natural rubber was compared with indirect method, which is deduced from the Clausius-Clapeyron factor (partial derivative sigma/partial derivative T)(epsilon),where sigma is the stress and e is the strain. The factor (partial derivative sigma/partial derivative T)(epsilon) be measured by two different methods. One is to measure the stress vs. strain behavior at different static temperatures. It is found that the Delta T deduction is underestimated or even of opposite sign compared with the directly measured one. These behaviors are different from elastocaloric effect of shape memory alloys. An interpretation based on strain-induced crystallite is proposed. The other characterization is to measure the stress vs. temperature at constant strain. It results in a prediction, which is in good quantitative agreement with the directly measured one. The stress appears then to be a non-state variable, thus questioning the ergodicity of the material. (C) 2016 AIP Publishing LLC.