Measurement of the thermal radial diffusivity of anisotropic materials by the converging thermal wave technique - Application to NDE : first results in the detection of normal cracks

International audienceThe Converging Thermal Wave (CTW) technique is used for measuring the thermal radial diffusivity of orthotropic materials. In this technique, the surface of the sample is heated on an annular pattern, and the temperature rise at the center of the annulus is recorded. From this temperature-time history, the radial diffusivity is identified using a novel procedure. Application to a carbon-epoxy composite material is presented. The ability of the CTW technique to detect normal cracks to the heated surface of the sample is also demonstrated

Measurement of the thermal radial diffusivity of anisotropic materials by the converging thermal wave technique

International audienceThermal non-destructive techniques are increasingly being used for testing and characterizing various materials. The technique we discuss here is quite appropriate for laminated materials, which are often highly anisotropic, with a thermal radial diffusivity ar (parallel with the laminae) that is very different from the thermal normal diffusivity (normal to the sheets).The radial diffusivity ar is measured using a method proposed by Cielo et al. A pulsed laser heats the sample through an axicon (lens of conical cross-section), so that the heated surface is annular. The surface temperature is monitored by an IR detector focused on the center of the annulus. From the experimental temperature-time history of this point, one can plot an apparent radial diffusivity versus time curve, under the assumptions that: (1) the medium is either semi-infinite or extremely thin or translucent; (2) there are no convection heat losses at the surface of the medium; (3) the energy deposit on the medium is of short duration in comparison with the characteristic radial diffusion time. This apparent radial diffusivity curve can be obtained either by direct inversion of the experimental thermogram using the mathematical expression of the theoretical thermogram, or by a technique using its logarithmic derivative with respect to time.In the case of a semi-infinite medium, these two methods are compared here from the point of view of the error in the evaluation of ar while assuming an absolute error in the temperature (due to the noise of the experiment). Then the general case of the sample of finite thickness with heat losses at its surface is studied numerically. First the errors in the calculation of ar are evaluated with the two methods, then mathematical corrections are proposed to reduce the effects of finite thickness and heat losses in the experimental thermogram. Finally, several sources of error in the evaluation of ar are presented and studied numerically, namely the non-zero width of the annulus, the offset and the radial extent of the detection area respectively

Photoacoustic microscopy by photodeformation applied to thermal diffusivity determination

International audienceIn this paper, an original technique is proposed to measure the thermal diffusivity at local scale using a photoacoustic microscopy set-up. Several experimental results collected on thin metallic layers are presented

Imaging of the interface between fibers and matrix in the yarns of three-directional carbon-carbon composites by a photoacoustic method

International audienceThe performance of composite materials such as three-directional carbon-carbon composites depends greatly on the quality of the interface between the fibers and the matrix in the yarns. A set-up was built to examine the quality of this interface using a thermal method: photoacoustic method by photodeformation. Comparison of photoacoustic and electron beam microscope images, on the same area of a sample confirms that fibers 5 to 7 µm in diameter can be seen with the set-up. Moreover, the thermal phase images show discrepancies around some fibers which could be due to abnormal local thermal resistances