Measurements of Thermal Expansion of Nonmetallic Materials at Low Temperature using ESPI

Abstract

The electronic speckle pattern interferometry (ESPI) is applied to measure the thermal expansion for several nonmetallic materials (GFRP, A12O3, PTFE, and SiO2) during heating from low temperatures to ambient. Between two temperatures, the thermal strains were acquired by ESPI and the temperature of the surface by thermocouples. The results allow for the determinative of the thermal expansion coefficient. Since nonmetallic materials like fiber reinforced plastic (FRP), plastics and ceramics are translucent to light, a speckle pattern is formed by the interference between the light not only scattered at the surface but also multiple-scattered in the inner part of the specimen. As the temperature changes, the totality of the scatter movements exceeds a certain limit to keep the correlation between the speckle patterns. Loss of ESPI fringes results in the image process. Satisfactory results were obtained by coating the specimen surface by a vacuum-evaporated aluminum film. The thermal expansion coefficient of a SiO2 specimen could be acquired by calculating the phase map caused by the transformation of the object using ESPI with Fourier transform method (FTM). The thermal expansion coefficients obtained by ESPI were in the range of 1 x 10-7 to 1 x 10-4(l/K) between 100K and 300K.