Christian Bissieux

Thermoelastic stress analysis under nonadiabatic conditions

Thermoelastic stress analysis is a full-field stress measurement technique complementary to local techniques like strain gages. Generally, the heat transfer inside the material is neglected with respect to the frequency of the cyclical loading. An adiabaticity criterion is established to assert this simplification as a function of the thermal diffusion length and the spatial stress gradients. Under nonadiabatic conditions, heat diffusion attenuates the spatial temperature gradients, which leads to an underestimation of stress concentrations. Analytical and numerical considerations allow for the quantification of the spatial resolution. Finally, several inverse techniques can restore the thermally attenuated contrasts.

Focal plane array infrared cameras as research tools

Temporal, thermal and spatial performances of some FPA cameras have been tested. Different disturbing behaviors related to this recent technology have been pointed out, especially with a view to research and development applications.

Statistical treatment applied to infrared thermoelastic analysis of applied and residual mechanical stresses

Very small temperature variations are quantified by a statistical treatment of standard infrared equipment images. This procedure determines the signal amplitude value, comparing the noise and the noisy signal dispersions characterized by their variances. This robust and simple method has the advantage of needing no link to the reference signal and of treating any kind of signal shape. It is applied here to thermoelastic analysis of applied and residual stresses.

Diffusion harmonique de la chaleur appliquée au contrôle non destructif par méthodes photothermiques

Abstract The thermal behaviour of a sample submitted to a sinusoidal optical irradiation is modelled in axixymmetrical two-dimensional geometry. The solution by integral transforms and by Greens functions or harmonic responses points out characteristics heat diffusion behaviours, according to the ratio between the thermal diffusion length and the dimensions of the beam or of the sample. With a view to parameter identification, these particular behaviours provide model reduction opportunities. The spherical behaviour, mainly involved with a micrometric excitation, contributes to the spatial resolution of the photothermal techniques.

Parameter identification in the Hankel space

The Hankel integral transform is used to interpret the radial temperature distribution at the surface of a black paint coating on a steel substrate, under irradiation by a modulated laser beam. An inverse method based on the Gauss-Newton algorithm allows the identification of the coating thermal conductivity. To avoid difficulties due to the great number of thermographic data, we present estimations realized directly in the Hankel domain. The black paint coating reveals to be significantly insulating, with a thermal diffusivity of 7.5 10-8 m2.s-1.

Non-contact photopyroelectric method applied to thermal and optical characterization of textiles. Four-flux modeling of a scattering sample

Abstract The radiative transfer equation is solving considering the textile material equivalent to an absorbing and scattering slab and using a four-flux approach. Three macroscopic values of radiative parameters are accessible from a spectrophotometer used with an integrating sphere: the directional transmittance and the hemispherical reflectance and transmittance. The four-flux model for scattering with a linearly anisotropic phase function was used to identify the absorption and scattering parameters and the geometrical factor of the textile sample. For the nine investigated cotton samples, we showed that these parameters are related to the yarn, to structure and to water content. Then, the previously developed theoretical model for the photopyroelectric method in a noncontact configuration was modified for the case of a scattering sample using the same four-flux approach. The method was used for water content measurement and thermal parameters characterization of textile materials. Full description of the correlation analysis and inversion procedure and a synthesis of the results concerning the thermal parameters are given in a second article [Duvaut et al., Rev. Sci. Instrum. 73 (2002) 1299–1303].

Thermomechanical Modelling of a Steel Plate Impacted by a Shot and Experimental Validation

This work presents an experimental and numerical study of the thermo-mechanical problem of a steel plate impacted by a shot. The temperature rise is estimated and its effect on the compressive residual stress is analyzed. The simulations show that the value of the compressive residual stresses at the surface of the plate is modified when thermo-mechanical effects are included in the model as compared with simulation including hardening effects only. To validate this numerical study, an experimental device has been developed to measure the temperature rise after the impact. The experiment consists of the impact of a shot on a metallic plate. The temperature measurement is performed by an infrared camera located on the side of the plate opposite to the impact. Comparison between these experimental measurements and the numerical solution gives good agreement (to within 5%).

Resolution of an inverse heat conduction problem with a nonlinear least square method in the Hankel space. Application to photothermal infrared thermography

Integral transforms (Laplace, Fourier, Hankel) are widely used to solve the heat diffusion equation. Moreover, it often appears relevant to realize the estimation of thermophysical properties in the transformed space. Here, an analytical model has been developed, leading to a well-posed inverse problem of parameter identification. Two black coatings, a thin black paint layer and an amorphous carbon film, were studied by photothermal infrared thermography. A Hankel transform has been applied on both thermal model and data and the estimation of thermal diffusivity has been achieved in the Hankel space. The inverse problem is formulated as a non-linear least square problem and a Gauss-Newton algorithm is used for the parameter identification.