Simultaneous measurement of the orthogonal components of thermal diffusivity in PVC sheet

An extension of the flash method is described that measures simultaneously the three orthogonal components of thermal diffusivity in specimens of mode rate thermal diffusivity. Only part of the top face of the specimen is illu minated a,ld the temperature histories are recorded at three points on the bottom face. A Marquardt parameter estimation algorithm coupled with a fini te difference model of the diffusion equation analyzes these temperature hi stories to determine the components of thermal diffusivity. Illustrative me asurements in stainless steel, glass, and PVC demonstrate that accurate thr ee-dimensional thermal diffusivity measurements can be made in this way. Th e in-plane components of thermal diffusivity of as-supplied PVC sheet are s hown to be about 20 percent greater than the out-of-plane component. This a nisotropy appears to be due to the manufacturing process and exemplifies th e need for such measurements.