Fatigue endurance is critical for the airworthiness of civilian and military aging aircraft and for long-duration flight and deep space missions. NASA has developed a new metal matrix composite (MMC) that can repair itself from large fatigue cracks that occur during the service life of a structure. This novel liquid-assisted MMC recovers the strength of the structure after a healing cycle.
The MMC contains both shape memory alloy (SMA) reinforcements and some low-melting phase components which, when heated, essentially clamp the crack edges back together and flow material into the crack’s gap for a high strength repair.
The system is comprised of an Al metal matrix with high-performance SMA reinforcements. The combination of the unique matrix composition and SMA elements allow for this material system to self-repair via a two-step crack repair method. When a crack is present in the matrix material, the MMC is heated above the austenite start (As) temperature. This initiates shape recovery of the SMA, pulling the crack together as the SMA reinforcements return to their initial length. Concurrently, the increased temperature causes softening and liquefaction of the eutectic micro-constituent in the matrix, which enables the recovery of plastic strain in the matrix as well as crack filling.
Combined with the crack closure force provided by the SMA reinforcements completely reverting to their original length, the MMC welds itself together and, upon cooling, results in a solidified composite able to realize its pre-cracked, original strength.
The research team has demonstrated and tested the new materials. The team induced cracks in prototype materials based on Al-Si matrix with SMA (NiTi) reinforcements and demonstrated the recovery of tensile strength after healing. Data from tensile and fatigue tests of the samples before and after the fatigue crack healing shows a 91.6 percent healing efficiency on average under tensile conditions.
While current crack repair methods exist, such as doublers or welding overlays, these methods require complex surface prep and bonding, which can be difficult and may result in decreased strength. The new material allows for the repair of fatigue cracks without additional materials or human interaction.
The system has applications in aircraft structural components such as fuselage skin, stingers, frames, ribs, longerons, stiffeners, doors, tanks, wheel wells, fuel lines, shock struts, and floor beams; spacecraft structural components for longer missions where current repair technologies like welding and bonding are not an option; as well as in oil and gas industry for repairing cracks in oil-well casings.