Tech Briefs Magazine - December 2022 - 26

Materials & Coatings
Self-Healing Aluminum Metal Matrix Composite
The material can repair large cracks and reverses fatigue damage in structural metal.
Kennedy Space Center, Merritt Island, FL
F
atigue 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
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). (Image: NASA)
100
A
20
40
60
80
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
26
0.01
0.02
0.03
Strain [mm/mm]
This graph shows the stress/strain data for the mechanical
testing of the sample. Note the inset macro-images
correspond to the before and after healing heat treatment
of a crack along the dog bone-shaped specimen.
(Image: NASA)
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
www.techbriefs.com
0.04
Composite
Tensile Behavior
Before
Healing
After
Healing
2 mm
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.
2 mm
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 oilwell
casings.
NASA is actively seeking licensees
to commercialize this technology. Please
contact NASA's Licensing Concierge at
Agency-Patent-Licensing@mail.nasa.
gov or call at 202-358-7432 to initiate
licensing discussions. For more information,
visit https://technology.nasa.
gov/patent/KSC-TOPS-80.
Tech Briefs, December 2022
SMA
Liquid
I: Monolithic
Composite
II: Matrix
Fracture
III: High
Temp Healing
Partial Liquid Matrix
IV: Crack
Bonding
Liquid Bonds Interface
Solid
Eutectic
Solution
SMA Wires Close Crack
Strength Recovery
Internal
Crack
Stress [MPa]
SMA Wire

https://technology.nasa.gov/patent/KSC-TOPS-80 https://technology.nasa.gov/patent/KSC-TOPS-80 http://www.techbriefs.com

Tech Briefs Magazine - December 2022

Table of Contents for the Digital Edition of Tech Briefs Magazine - December 2022