Dynamic Compressive Properties of 2D-C/SiC Composites under Low Temperature

The dynamic compressive experiments of 2D-C/SiC composites under low temperature were performed on the SHPB system. The low temperatures at 173K and 103K were obtained by controlling the ratio of the absolute alcohol to the liquid nitrogen. The wave shapers were designed to meet the requirements of stress uniformity and constant strain rate in SHPB setup. The experimental results show that the 2D-C/SiC composites exhibit pseudo-plastic behaviors under dynamic loading and low temperature conditions for the micro cracks in the composite. The significant strain softening of the specimens occurred instead of the typical brittle failure when the stress exceeded the compressive strength in the test, and the specimens have experienced a large deformation before the final failure. The dynamic compressive strength of the composites increased with decrease of temperature, however the failure strain decreased. The strength of the interface between internal fibers and matrix in the 2D-C/SiC composites enhanced at low temperatures, resulting in the higher compressive strength of the composites. Introduction C/SiC composites have been widely used as the outside thermal protection structures of aerospace vehicles for its low density and high anti-thermal shocking properties . The thermal protection structures in service are always threatened by the increase in space debris. If the anti-thermal structures are damaged by the high velocity impact, the primary structures of aerospace vehicles will be destroyed by ultrahigh temperature when returning from the space. In order to predict the impact of the debris at ultrahigh velocity on the C/SiC structure, dynamic compressive properties at low temperature is desired. From literature, studies on mechanical properties of C/SiC composites are concentrated on static properties. Jiao et al [4-8] obtained the static properties of C/SiC composites by tension, compression and shear experiments. The deformation mechanism and failure models of C/SiC composites have been analyzed. The results show a good linear relationship between the compressive stress and strain. The angle between the fractured face of the specimen and loading direction is 13°, and interface failure is a main failure mode. Garland et al [9] investigated the effect of the properties of the interface between the reinforced fibers and matrix with the damage development. He found the magnitude of the damage angle is depended on the strength of the interface. Narayanan and Schadler also discussed the relationship between the damage angle and the compressive strength of the composites. They found that the magnitude of the damage angle increased with the increase of the compressive strength. Yotte and Quenisse concluded that the nonlinear properties of 2D-C/SiC exhibit pseudo-plastic mechanical behaviors, because there were lots of internal micro cracks in the composite material. The stress concentration occurred around the initial micro cracks during the loading procedure. However, the stress concentration can not be released under high loading rate, thus the damage evolvement was accelerated and the failure strain decreased. In this study, the dynamic mechanical properties of 2D-C/SiC composites under low temperature are investigated. The dynamic compressive experiments of these composites were performed by the 2nd Annual International Conference on Advanced Material Engineering (AME 2016) © 2016. The authors Published by Atlantis Press 65 SHPB system. The testing temperature was from 173K and 103K and the strain rate was 1000 s. Experimental Method Specimen The 2D-C/SiC composite specimens were made by the Science and Technology on Thermostructural Composite Material Laboratory in Northwest Polytechnical University. The carbon fibers (T300-1K, made by Nippon Toray Corporation) were firstly woven into carbon lamina with 0.16mm thickness, and then the carbon laminas were prefabricated to panel in 0°/90° direction. The PyC layer with 100-200nm thickness was deposited on the surface of the fibers, and the SiC matrix was deposited by the CVI technology. The volume percentage of the fibers was 45% and the porosity of 2D-C/SiC composites was 5%~8%. Finally, the 2D-C/SiC plate was cut into test cubes by the diamond. The specimens’ dimension is 4mm×4mm×3mm approximately. Figure 1 shows the structure of 2D-C/SiC composites and the loading direction in test. Figure1 The structure of the 2D-C/SiC Experiments The compressive experiments were performed using SHPB system. The shape and the size of the wave shapers were designed to meet constant strain-rate loading. The low temperatures of 173K and 103K were obtained by controlling the ratio of absolute alcohol to liquid nitrogen. The incident wave, reflection wave and the transmission wave were measured by the strain gages glued on the surface of the bars. The stress, strain and the strain rate of the specimen can be calculated by the following equations.