Self-Healing Aluminum Metal Matrix Composite (MMC)

materials and coatings
Self-Healing Aluminum Metal Matrix Composite (MMC) (KSC-TOPS-80)
Repairs Large Cracks and Reverses Fatigue Damage in Structural Metal
Fatigue endurance is critical for the airworthiness of civilian and military aging aircraft and for long-duration flight and deep space missions. Estimates are that 90% of structure failures are due to fatigue. 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. 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 a region of decreased strength. The new material allows for the repair of fatigue cracks without additional materials or human interaction.

The Technology
This materials 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 SMA's 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% healing efficiency on average under tensile conditions.

Copyright by RPXTech. Permission to use freely granted by RPX Tech via email from Steve Sennet on 11/17/2020, attached. Fig.  The healing cycle for liquid-assisted self-healing metal-matrix composites. The system consists of a metallic matrix with a eutectic micro-constituent shown in black and reinforcing SMA wires shown in green (I). After catastrophic failure, the SMA wires deform to bridge the crack (II). To heal the sample, a high temperature  healing treatment is initiated, during which the eutectic component melts and SMA wires close the crack (III). During cooling, the eutectic component freezes, welding the crack surfaces and eliminating the crack (IV).
  • Enables Safety: can potentially heal fatigue cracking, helping avoid catastrophic failure of a structure
  • Improves Fatigue Endurance: enables crack repair during flight or service to extend the life of structures
  • Solves Hard-to-Access Repairs: works where a crack location is difficult to reach or common repair techniques are not applicable
  • Reduces Labor and Materials Requirement: works with applied heat; no additional materials or human interaction/labor are needed.

  • Aeronautics: aircraft structural components such as fuselage skin, stingers, frames, ribs, longerons, stiffeners, doors, tanks, wheel wells, fuel lines, shock struts, and floor beams.
  • Commercial Space: spacecraft structural components for longer missions where current repair technologies like welding and bonding are not an option
  • Oil & Gas: for repairing cracks in oil-well casings
Technology Details

materials and coatings
Charles R. Fisher, M. Clara Wright, et. al. 2018. Repairing Large Cracks and Reversing Fatigue Damage in Structural Metals; Applied Materials Today 13 (2018). Open access article; online version available here,
Similar Results
Innovative Shape Memory Metal Matrix Composites
Shape memory alloys (SMAs) are metals that can return to their original shape following thermal input. They are commonly used as functional materials in sensors, actuators, clamping fixtures and release mechanisms across industries. SMAs can suffer from dimensional/thermal instability, creep, and/or low hardness, resulting in alloys with little to no work output in the long term. To combat these deficiencies, NASA has developed a process of incorporating nanoparticles of refractory materials (i.e., carbide, oxide, and nitride materials with high temperature resistance) into the alloys. Using various processing methods, the nanoparticles can be effectively mixed and dispersed into the metal alloys as shown in the figure below. In these processes the SMA and refractory material powder is mixed and the refractory nanoparticles incorporated through extrusions, melting, or directly used in additive manufacturing to create parts for applications across the aerospace, automotive, marine, or biomedical sectors. The nanoparticle dispersion is a controllable method to strengthen the SMAs, increasing the hardness of the alloys, reducing the impact of creep, and improving the overall dimensional and thermal stability of the alloys. The related patent is now available to license. Please note that NASA does not manufacture products itself for commercial sale.
MMC blade stiffener bonded to aluminum plate during hot rolling.
In-Situ Selective Reinforcement of Near-Net-Shape Formed Structures
This innovation allows the incorporation of metal matrix composite reinforcing material into a metallic structure as part of the structures fabrication process. It does not require secondary processing that may affect the structures mechanical properties. It does not require bonding agents that would limit the benefits of the reinforcing material. It adds reinforcement to only the specific regions of the structure that need enhanced strength, stiffness, and/or damage tolerance, thereby allowing for more efficient design and reduction in structural weight.
Concept composite aircraft
Healable Carbon Fiber Reinforced Composites
A composite fabrication process cycle was developed from composite precursor materials developed at LaRC to fabricate composite laminates. The precursor material is a pre-impregnated unidirectional carbon fiber preform, or prepreg. In the pre-pregging process, the high strength, structural reinforcing carbon fiber is wetted by a solution containing a self-healing polymer. The resulting material is of aerospace quality and exhibits a significant decrease of internal damage following impacts tests (using ASTM D 7137 standard).
Open Pit Mine
Shape Memory Alloy Rock Splitters (SMARS)
Glenn's revolutionary SMARS device is fabricated from nickel-titanium-halfnium (NiTiHf), nickel-titanium-zirconium compositions, or a combination. These compositions contain a secondary, nanometer-sized precipitate phase, which is produced through processes of compositional control and ageing heat treatments. Glenn's novel materials and processes have yielded a SMA composition that produces much higher stresses than other SMAs on the commercial market. The SMARS device is composed of 1) SMA material as the actuating member; 2) a casing heater placed around the SMA member; 3) a DC or AC power source to provide current through the heater; 4) pointed tips for acute penetration into rock formations; and 5) a hand-press to reset the SMA element after each use. In the rock-splitting process, a hole equal to the diameter of the SMA element is drilled in the portion of the rock where the fracture is desired. Next, the pre-compressed SMA is inserted into the hole, and AC or DC current is applied to energize the devices heaters. Once the heater achieves the critical transformation temperature, the SMA will begin to expand within seconds. Since its expansion is constrained by the rock walls, the SMA will eventually exert up to 1500 MPa of stress, splitting the rock apart. When the current is removed and the heater cools, the SMA material returns to its pre-compressed state. At this point, the material can be recovered, so the process is repeatable after reshaping. The SMA actuating members were also designed to achieve displacement greater than the materials strain output. Glenns SMARS device provides high-powered rock fracturing that is controllable, reliable, and comparatively simple without the use of explosives, hydraulics, or chemicals.
Robotic Arm
How to Train Shape Memory Alloys
Glenn researchers have optimized how shape memory alloys (SMAs) are trained by reconceptualizing the entire stabilization process. Whereas prior techniques stabilize SMAs during thermal cycling, under conditions of fixed stress (known as the isobaric response), what Glenn's innovators have done instead is to use mechanical cycling under conditions of fixed temperature (the isothermal response) to achieve stabilization rapidly and efficiently. This novel method uses the isobaric response to establish the stabilization point under conditions identical to those that will be used during service. Once the stabilization point is known, a set of isothermal mechanical cycling experiments is then performed using different levels of applied stress. Each of these mechanical cycling experiments is left to run until the strain response has stabilized. When the stress levels required to achieve stabilization under isothermal conditions are known, they can be used to train the material in a fraction of the time that would be required to train the material using only thermal cycling. As the strain state has been achieved isothermally, the material can be switched back under isobaric conditions, and will remain stabilized during service. In short, Glenn's method of training can be completed in a matter of minutes rather than in days or even weeks, and so SMAs become much more practical to use in a wide range of applications.
Stay up to date, follow NASA's Technology Transfer Program on:
facebook twitter linkedin youtube
Facebook Logo Twitter Logo Linkedin Logo Youtube Logo