Effects of hypervelocity impact on thermal expansion behavior of 2.5D carbon/carbon composites from 850 °C to 2500 °C

Effects of impact defects on thermal expansion behavior of 2.5D C/C composites were studied in this work. A 6 mm steel ball driven by solid explosive was used as a projectile to simulate hypervelocity impact of debris or flying stones. The coefficients of thermal expansion (CTEs) of post-impacted samples located in different areas were investigated from 850 to 2500 °C. The results show that impact defects are primarily focused on crater regions. These defects can significantly decrease the CTEs of C/C composites under 850-2350 °C and have a little influence in the range of 2350-2500 °C. Post-impacted samples near the crater have a hysteresis than that of edge samples at the minimum of CET-XY and the maximum of CET-XY. There is less thermal stress produced in samples with a large amount of damage than samples having a little damage during CTE tests. Cracks produced by the explosive impact not only offer spaces for thermal expansion but also supply channels for releasing thermal stress. Specifically, fiber breakages and matrix cracks perpendicular to fiber direction are the major factors of CTE decrements in X-Y direction. Sub-layer laminations, fiber/matrix debonding, and circumferential cracks lead to the decreases of CTEs in Z direction.