An infrared image processing to analyse the calorific effects accompanying strain localisation

Abstract

This paper presents an infrared data processing developed to analyse the calori®c manifestations accompanying elastoplastic transformation during tensile tests. The surface temperature images are provided by an experimental set-up essentially made of a testing machine coupled with an infrared camera equipped with a home-made numerizer. The`inverse' passage from temperatures to heat sources is detailed in the case of¯at and thin parallelepipedic samples. The infrared image processing, based on FourierÕs techniques, was checked using spectral solutions of the heat equation in the case of realistic examples close to experiments. Numerical simulations are shown which attest coherence and eciency of the method for several heat source distributions and dierent sets of noisy data. The method is then applied to experimental data ®les coming from tensile tests on mild steels at the room temperature. Sudden dissipative eects due to the propagation of the L uders bands during the plastic plateau can be observed. Then, during the strain hardening, gradual and precocious concentrations of dissipation are shown; they herald the local necking of the sample. Finally, the interest of such experimental results is brie¯y discussed by referring to the specialised literature dealing with localisation phenomena and behaviour identi®cation.

Fields of stored energy associated with localized necking of steel

“…Using different experimental arrangements (calorimeter [Shenogin et al 2002], thermocouples [Zehnder et al 1998], IR sensors [Guduru et al 2001;Chrysochoos and Louche 2000]), these studies gave similar results, showing that a variable amount of mechanical energy is converted into heat during inelastic transformation. Such techniques generally provide a macroscopic estimate of the TaylorQuinney coefficient that links mechanical and dissipated energies.…”

“…In standard conditions, we estimated that the peak-to-peak thermal noise was about 200 mK (that is, before data filtering) and the range of the thermal noise dropped to 20 mK for standard filtering parameters. Moreover, the order of magnitude of the spatial resolution (in terms of pixel size) was about 0.4 mm and the temporal resolution was considered to be equal to the 0.04 s. By integrating the heat Equation (4) over the depth of the sample [Chrysochoos and Louche 2000], and defining the mean thermal disequilibrium over the thickness between the sample and its surroundings by = θ − θ ref , we obtained the following 2D differential equation:…”

“…DIC and IRT image processing algorithms have been widely presented and checked in previous works [Wattrisse et al 2001a;Chrysochoos and Louche 2000]. In what follows, we tested the reliability of the energy balance construction by comparing the overall heat sources w • h with the mechanical energy rate w…”

Preprocessing of temperature measurement by IR techniques using POD truncated basis for heat source estimation

“…However thermal measurements are always noisy and the method to calculate the derivatives may become highly unstable. Various methods were used in the literature in order to limit the noise effects: Gaussian convolutive filtering [4,5], spectral methods [1,6], and local least-squares fitting [2].…”

Fatigue limit by thermal analysis of specimen surface in mono axial traction test

“…Lazzarin et al [36] studied the local energy density in welded joint and the value of the strain energy averaged over a well-defined control volume was given in closed form. Chrysochoos and Louche [37] analyzed the temperature surface by infrared camera during a static traction test of a flat and thin parallelepiped sample. By using an infrared image processing based on Fourier's techniques, they observed a sudden dissipative effect due to the propagation of the Luders 38010-p. 3 band during the local plastic deformation.…”