Hervé Louche

Thermal and dissipative effects accompanying Lüders band propagation

This study deals with an experimental investigation of thermal and calorimetric effects induced by Luders band propagation during monotonic and quasi-static tensile tests. One grade of steel is considered. Temperature variations were measured with an infrared camera on the surface of flat samples. Heat sources were deduced from thermal data using the local heat diffusion equation. For elastoplastic materials, when no phase transition occurs, the heat source is essentially due to intrinsic dissipation. The experiments revealed that a dissipation wave accompanied Luders band propagation. Some characteristics of the wave, such as its shape, orientation and velocity, were derived from the experimental measurements and compared to previously published results. Finally, comparison of the calorimetric and kinematic data revealed a dissipation function proportional to the absolute value of the longitudinal plastic strain rate.

Heat Estimation from Infrared Measurement Compared to DSC for Austenite to R Phase Transformation in a NiTi Alloy

Heat source estimations from temperature field measurements deduced from infrared imaging are increasingly used to study thermo-mechanical coupling during materials’ deformation. These estimations are based on approximations of the derivative terms with respect to time and space which are involved in the heat diffusion equation. This paper proposes a first experimental validation of this method by applying it to an experimental uniform air cooling of a NiTi Shape Memory Alloy thin plate. In the studied cooling temperate range, heat sources are due to Austenite to R phase transformation. Transformation temperatures, heat sources, and energies are estimated from infrared temperature measurements and compared successfully to differential scanning calorimetry results.

Experimental estimation of the Inelastic Heat Fraction from thermomechanical observations and inverse analysis

A new method to estimate the inelastic heat fraction (Taylor and Quinney Beta coefficient) during the deformation of a titanium material is proposed. It is based on (1) thermomechanical full field measurements during the loading and on (2) an inverse analysis. First, two cameras (a visible and an infrared) are used to measure the kinematic and the thermal fields on each face of a notched flat sample loaded in tension. Second, two coupled finite element simulations (a mechanical, then a thermal one) of the same tests are conducted. Associated with a Levenberg-Marquardt optimization algorithm, they are able to give, in a first step, optimized values of the anisotropic elastoplastic model parameters. Then, in a second step, parameters of four strain dependent Beta models are identified. Finally, the thermal responses of these models are compared to the experimental values.

Terahertz thermometry system to measure temperature in the thickness of a solid polymer

Abstract This paper deals with thermal attenuation of a transmitted THz signal (165 GHz) across a POM polymer sample subjected to a thermal gradient in the thickness direction. The paper describes the experimental setup, including a THz sensor, a system to impose thermal loading and an infrared camera used to measure temperature variations at the surface. The thermal dependence of the transmitted THz signal through the sample was studied along a spatial profile. A simple polynomial model, validated through finite element analysis and thermal imaging measurement, was used to estimate temperature variations in the thickness direction. The correspondence between the 2D transmitted THz signal and the 3D temperature distribution allowed us to estimate the thermal sensitivity of the absorption coefficient in the THz range. This study showed that the THz sensor was sensitive enough to measure THz signal variations due to small temperature variations. The mean temperature may be determined along the thickness direction once the thermodependence of the transmission and reflection coefficients are known.

Numerical study on the effect of the paint layer used for infrared thermography on heat source estimation

This paper investigates the effect of the paint layer used for infrared thermography on the heat source estimation for thin plates. The thermal behaviour of two plates is studied using finite element modelling. The first one is composed of the studied homogeneous material. The second one is a sandwich plate composed of a central material covered on each side with a high emissivity paint layer. For the two plates, two thermal loading cases are simulated. Case 1 corresponds to natural cooling from a initial uniform temperature to room temperature. Case 2 corresponds to a situation where the initial uniform temperature is equal to room temperature. In both cases, the central material is loaded with internal heat sources leading to temperature field variation. From the computed external surface temperature, heat sources and energy are then estimated using homogeneous plate and sandwich plate models derived from the heat diffusion equation. Then, the estimated heat sources are compared to the one input in the finite element model. It is shown that the paint layer significantly isolates the central material from the external environment. In case 1, it is shown that the error is reduced from 10% with the homogeneous plate model to 5%, with the sandwich plate model. In case 2, heat sources are estimated with an error of less than 8% with the homogeneous plate model while it is below 2% with the sandwich plate model. The sandwich plate model is thus significantly more accurate for heat source and energy estimations than the homogeneous plate model.