Damage Behavior Of Hypervelocity Impact On C/C Composites

Recently,more than 500,000 pieces of debris or“space junk”are tracked as they orbit the Earth.They all travel at 7.8 km/s?22.9 M?fast enough for a relatively small piece of orbital debris to damage a satellite or a spacecraft.It is inevitable for spacecraft to bear hypervelocity impact of missile debris,which makes structures damaged and threatens the safety of the shuttles.In particular,the thermal protection system of the spacecraft is more susceptible to such threats,it is necessary to explore the damage behavior of hypervelocity impact on materials used in the thermal protection system,providing design suggestions.Carbon/carbon?C/C?composites have been widely used in the the thermal protection system of the spacecraft.But,the knowledge about hypervelocity impact behavior of C/C composites is still incomplete,and the effects of impact damage on the properties of C/C composites during their service time are rarely reported.Therefore,it is significant for the design of the thermal protection system to study hypervelocity impact behavior of C/C composites and the changes of properties after impacted during their service life.C/C composites with different preforms and pyrocarbon,SiC coated C/C composites and ZrB₂-SiC coated C/C composites were the targets in this study.Projectiles were driven by solid explosive of pentaerythritol tetranitrate(C₅H₈N₄O₁₂)to conduct hypervelocity impact tests.The effects of impact velocity,the hypervelocity impact behavior of C/C composites were investigated the projectile types?dimensions,number,shapes and so on?and the dimensions of targets on.The main damage types of C/C composites under hypervelocity impact tests include:perforation,penetration,spallation and flexural deformation.And the main damage images include:one big impact crater,cracks and“little impact crater group”on the front surface,and cracks,fiber breakages and delamination on the rear surface.In addition,the maximum depth and diameter of the impact crater can be used to quantitatively evaluate the damage degree on the front surface.2D cross-ply preforms with high texture?HT?pyrocarbon?PyC?matrix,and 2.5D preform with HT PyC and low texture-high texture?LT-HT?matrices and 2.5D needle felt and were impacted under the same condition,Results showed that the anti-penetration ability of C/C composites can be improved by increasing interfaces the strength of materials.The spallation resistance and integrity of surface structure of C/C composites can be improved by continuous long fibers,pores and weak interfaces.Impact tests of 2.5D C/C composite samples under different velocities have been performed to detect correlations between the impact direction and damage distribution and the residual flexural strength of the composites.Results showed that the impact resistance of the C/C composites was affected by the impact velocity.Damage modes evolved from matrix cracking and fiber breakages to spallation and delamination along the impact direction.Furthermore,delamination occurs delamination occurred in mat plies and other plies?0°and90°plies?,resulting from poor load-bearing capability of short mat layers and weak adhesion between fiber bundles.In addition,the residual strength of the damaged C/C composites in the upper side was higher than that of bottom side,implying that spallation and delamination could severely weaken the flexural strength of C/C composites.And the failure modes changes from with brittle fracture to pseudo-plastic fracture with the velocity increasing.Furthermore,the residual bending strength and fracture modes of the coated samples after hypervelocity impact tests were similar to C/C composites.And the impact damage had little effects on the bonding strength between the coating and the substrates.In order to simultaneously monitor damage evolution of C/C composites,bending-bending fatigue tests were carried out by unidirectional C/C composites and 2D cross-ply C/C composites.The results indicated that the electrical resistance change?ERC?methods can be as a method to monitor the damage evolution during their service life.Effects of impact defects on thermal expansion behavior of 2.5D C/C composites were studied and the relationships between damage and coefficient thermal expansion of C/C composites were obtained.These defects significantly decreased the CTEs of C/C composites under 850–2350? and have a little effect in the range of 2350–2500?.Post-impacted samples near the crater have a hysteresis?50??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.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.Besides,for damaged C/C composites,the d?002?decteases and L_c increases after heat treatment.After the impact tests,C/C composites,SiC-C/C composites and ZrB₂-SiC-C/C composites were ablated by oxyacetylene torch for 100 s at 2500?.Results showed that the ablation rates increased with the increase of impact velocity.Ablation areas,the integrity of ablated boundary and ablation rates of all samples increased with the increase of impact velocity.Due to the“second damage”of SiC coating prepared by pack method,linear ablation rates of coated composites were higher than those of C/C composites after the same impact tests.These results demonstrated that morphology of interfaces between coatings and substrates,the hardness and distribution of the inner coating were the major factors for the use of coated C/C composites.