Accounting for penetration of laser heating in flash thermal diffusivity experiments

Since the early 1960s, the laser flash method of thermal diffusivity measurement has been used on a large variety of materials. Several parameter estimation methods have also been used in analyzing such experiments, employing various levels of sophistication. Estimation of thermal parameters, using the models developed as part of this research, is performed on experimental data from the Oak Ridge National Laboratory in Oak Ridge, TN. The material used is carbon bonded carbon fiber (CBCF) which is designed as an insulating material for atmosphere re-entry applications. Ambient temperatures in the experiments range from 800 °C to 1200 ° C. The approximate thermal diffusivity of the material is 0.3 mm₂/sec. This research investigates the penetration of the laser flash beyond the surface of the material being heated. Three heat transfer models are presented, each with different assumptions about the initial temperature distribution inside the material. An evaluation is made of the response of the methods to factors which may enter into the experimental process. This is done in quantitative terms so as to assess the adequacy of the models in comparison to one another.