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.
Polybutadiene graft copolymer (PBg), the previously mentioned amorphous thermoplastic, was processed with unsized IM7 carbon fibers to fabricate reinforced composite material for evaluation. Temperature-dependent characteristics — such as the degradation point, glass transition (Tg), and viscosity of the PBg polymer — were characterized by thermogravimetric analysis (TGA), differential scanning calorimetry (DSC), and dynamic parallel plate rheology. The PBg resin was processed into unidirectional prepreg tape. Data from polymer thermal characterization guided the determination of a processing cycle used to fabricate quasi-isotropic laminate panels in various dimensions up to 30.5 × 30.5 cm in a vacuum press. Damaged IM7/PBg coupons were subjected to a non-autonomic healing cycle at elevated temperature/pressure, similar in heat and pressure magnitude to the developed composite processing cycle. Ultrasonic inspection of these coupons both before and after the healing cycle indicated that the delaminations at the impact site had been non-autonomously healed or, at least, were no longer visible. Compression testing of these healed coupons demonstrated significant improvement in retention of strength compared to coupons having damage.
Structures utilizing a self-healing thermoplastic matrix may provide the following advantages: 1) improved damage tolerance compared to industry state-ofthe- art thermoset CFRP; 2) a route for recovery of a large proportion of the pristine mechanical properties, thus extending the life of the structure; 3) the potential to be directly substituted for conventional thermosetting matrices that do not possess self-healing characteristics, since conventional thermoset matrix composites already suffer a knockdown of up to 50% due to inherently low damage tolerance; and 4) repeated healing from multiple damage events as long as there is no loss of matrix material incurred in the event.