The flash method is the most used technique to measure the thermal diffusivity of solid samples. It consists of heating the front face of an opaque slab by a short light pulse and detecting the temperature evolution at its rear surface, from which the thermal diffusivity is obtained. In this paper, we extend the classical flash method to be used with rods, tubes, and spheres. First, the temperature evolution of the back surface of solid cylinders, hollow cylinders, and spheres is calculated. Then, experimental measurements of the thermal diffusivity on a set of stainless steel samples confirm the validity of the method.

Up to now, research in photothermal techniques has been mainly restricted to samples with flat surfaces. In this work the surface temperature oscillation of multilayered cylindrical samples which are heated by a modulated light beam is calculated by using the quadrupole method. Different illumination geometries have been studied. Moreover, the lack of adherence between layers, as well as heat losses at the surface, has been considered in the model. Following this theoretical approach, photothermal techniques can be used for the quantitative thermophysical characterization of cylindrical samples with continuously varying in-depth thermal conductivity.

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In this article, we present a general solution for the ac temperature field of an opaque material containing aligned subsurface cylinders produced by a modulated line illumination parallel to the cylinders. This work completes the case of plane illumination treated in a previous paper ͓J. M. Terrón, A. Salazar, and A. Sánchez-Lavega, J. Appl. Phys. 91, 1087 ͑2002͔͒. The model includes the multiple scattering effects suffered by the cylindrical thermal wave, generated by the line heat source, when interacting with the cylinders and with the sample surfaces. Numerical calculations illustrate the effect of buried cylinders on the sample surface temperature. This model is then used to calculate the in-plane effective thermal diffusivity of unidirectionally fiber-reinforced composites. We also calculate the decrease of this effective thermal diffusivity as a consequence of the presence of a thermal resistance between fibers and matrix. Experimental measurements using modulated photothermal techniques on calibrated samples confirm the validity of the results.