We propose a methodology to determine the thermal diffusivity of both isotropic and anisotropic samples when they move at constant speed, using laser spot thermography with continuous illumination. In the case of anisotropic samples, the method does not require knowledge of the thermal principal directions and is able to provide the orientation the principal axes with respect to the direction of motion. We show analytically that, once the steady state has been reached, the natural logarithm of the temperature along any temperature profile crossing the center of the laser spot features a linear dependence with the distance. The slopes of these straight lines are related with the speed of the sample, the principal thermal diffusivities and the orientation of the principal directions. We present experimental data taken on both, isotropic and anisotropic reference materials moving at constant speed. From the fitting of the slopes of the radial profiles in multiple directions, we are able to assess the values of the principal thermal diffusivities as well as the orientation of the principal axes with high accuracy. These results are promising regarding the quality control of industrial products in a production chain as, for instance, for fiber orientation monitoring of carbon fiber reinforced composites.
Thermographic nondestructive techniques with focused laser excitation have proven as very efficient tools for the detection of narrow cracks. Moreover, it has been shown that in the ideal case of infinite cracks, the width of the crack can be assessed quantitatively using laser spot thermography, both in lock-in and pulsed regimes. In this ideal case, the surface temperature of the cracked material can be obtained analytically. However, real cracks feature finite penetration and length and, in these conditions, the calculation of the surface temperature needs to be performed numerically. In this work, we combine laser-spot lock-in thermography with finite elements modelling (FEM) to perform a full characterization of the local values of the width and depth of narrow cracks along the whole crack length in two Alalloys plates after fatigue test. First, in order to locate and image the crack, we combine the squares of the spatial derivatives of the amplitude thermograms along two perpendicular directions for different positions of the laser spot. Then, we place the laser close to the crack and we fit the numerical model to the amplitude data, so as to obtain the values of the width and depth of the crack at the current position of the laser. By displacing the laser spot at different positions along the crack length, we fully characterize the width and depth of the crack, whose resulting values are of the order of 1 µm and 0.5 mm, respectively.
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