The initiation and propagation of damage ultimately results in failure of aircraft structural components. Often, impact damage is difficult to identify in-service, and hence design of continuous carbon fiber reinforced polymer (CFRP) composite structure involves up to a 50% knockdown in the undamaged failure strength allowable. If damage is identified in a composite structure, the vehicle must be grounded for structural repair. This involves the grinding away of damaged regions and drilled holes to secure patches. By providing a polymer matrix with the ability to self-heal after impact damage is incurred, vehicle safety is greatly improved by increasing the design allowable for strength, resulting in more efficient CFRP structure.
Self-healing polymeric materials have been defined as materials that have the built-in capability to substantially recover their load-transferring ability after damage. Such recovery can occur autonomously or be activated after application of a specific stimulus (e.g. heat or radiation). Effective self-healing requires that these materials heal quickly following low- to mid-velocity impacts, while retaining structural integrity. At Langley Research Center, an amorphous thermoplastic has been identified that self-heals at ~50 °C after through-penetration by a 9.03-mm-diameter bullet at 400 m/sec. Continuous CFRP composites have been processed with this thermoplastic, and the damage tolerance of this material in comparison to reported values for thermosetting-toughened epoxy CFRP have been determined.