Thermal diffusivity evaluation for carbon-carbon composites using infrared thermography

This paper presents a flash infrared and a step-heating thermography technique for the determination of the whole-field thermal diffusivity of anisotropic Carbon/Carbon (C/C) brakes. Quick set-up and a portable system provide a powerful tool that can realize on-site non-destructive measurement of thermal diffusivity. In the flash thermography method, heat was provided by two flash lamps and was applied to one side of the sample (e.g. fraction surface of the C/C brake), while the temperature response of the opposite surface was monitored by an infrared camera. Thus, by analyzing real-time digital infrared thermal images and the temperature evolution on one of the two surfaces, one can obtain the whole-field through-thickness thermal diffusivity in a single experiment. The experiments were conducted on two types of nonplate like sub-scale C/C and Al-6061 brakes that have an inner step along the axial direction. The influence on the evaluation of the thermal diffusivity caused by the specific geometry of the brakes was studied based upon the Finite Element Analysis (FEA) modeling. The FEA software ANSYS 8.0 was used to generate and analyze models of the composite disks, which helped to predict the measurement bias and determine the inspection procedure such as estimation of experimental time. The diffusivity evaluation was then corrected based on FEA results. Moreover, the resulting diffusivity value was compared with the value measured from the same brake by the conventional laser flash method. This flash infrared technique also was used on full-scale C/C and Al bulk samples for through-thickness measurement.;For the measurement of in-plane thermal diffusivity, the proposed method uses infrared thermography with a periodic mask. The principal characteristic is that a thermal gradient across the testing plane has to be generated. Here, heat is still applied to the front face of the sample by flash lamps and the periodic mask is added between the specimen and the lamps. After flashing, the heat is periodically distributed over the image area with such a grid pattern. The masks were specially designed to determine the radial and circumferential diffusivities of carbon disk by generating heat diffusion along these two orthotropic directions on the carbon disk sample. The period was carefully chosen as a function of sample thickness and an estimation of the ratio of through-thickness and in-plane diffusivity. The in-plane thermal diffusivity profile is then obtained by doing a Fourier transform analysis over one period of the grid mask. The results represented the average diffusivity value in that one period area. The tests have been conducted for aluminum plate and half C/C specimen. The resulting in plane diffusivity then compared with the value obtained from conventional laser flash method and literatures.;Another method, called step heating, is an alternative technique to the known flash method, which is based on the application of an instantaneous constant heat flux on the front face of the sample and measurement of the temperature response at the rear face. This work will extend the applicability of the step-heating technique to large dimension of the thick sample. The through-thickness thermal diffusivities were determined for 3-D C/C disk and Al 6061 alloy samples.