Ultra-high temperature mechanical property test of C/C composites by a digital image correlation method based on an active laser illumination and background radiation suppressing method with multi-step filtering

Carbon/carbon (C/C) composites have been widely used in aerospace engineering because of their high temperature resistance. However, the commonly used non-contact deformation measurement method based on digital image correlation (DIC) still has some problems of speckle preparation and background radiation in an ultra-high temperature environment above 2000°C. An ultra-high temperature mechanical property test method based on active laser illumination and a background radiation suppressing method with multi-step filtering is proposed. In the proposed method, a laser with good coherence is used as the active illumination source to generate laser speckles that will not fall off or discolor as temperature increases. Also, a multi-step filtering device is devised with the combination of a spatial filter, plano-convex lens, polarizing filter, and two different bandpass filters, which can successfully suppress the background radiation and retain the characteristics of laser speckle patterns at the same time. Tensile tests of C/C composite had been carried out at ultra-high temperatures of 2200°C, 2400°C, 2600°C, and 2800°C. The experimental results showed that high-quality laser speckle images were obtained and the deviations between the elastic moduli obtained by the proposed method and by the high temperature contact extensometer were all less than 7%.