"This would significantly drive down costs and labor associated with replacing damaged composite components, and reduce the amount of energy consumed and waste produced by many industrial sectors - because they will have fewer broken parts to manually inspect, repair or throw away," says Jason Patrick, corresponding author of the paper and an associate professor of civil, construction and environmental engineering at North Carolina State University.
At issue are fiber-reinforced polymer (FRP) composites, which are valued for their high strength-to-weight ratio and are commonly used in aircraft, automobiles, wind-turbines, spacecraft and other modern structural applications. FRP composites consist of layers of fibers, such as glass or carbon fiber, that are bonded together by a polymer matrix, often epoxy. The self-healing technique developed by the NC State researchers targets interlaminar delamination, which occurs when cracks within the composite form and cause the fiber layers to separate from the matrix.
"Delamination has been a challenge for FRP composites since the 1930s," Patrick says. "We believe the self-healing technology that we have developed could be a long-term solution for delamination, allowing components to last for centuries. That is far beyond the typical lifespan of conventional FRP composites, which ranges from 15-40 years."
The self-healing material resembles conventional FRP composites, but with two additional features. First, the researchers 3D-print a thermoplastic healing agent onto the fiber reinforcement, creating a polymer-patterned interlayer that makes the laminate two to four times more resistant to delamination. Second, the researchers embed thin, carbon-based heater layers into the material that warm up when an electrical current is applied. The heat melts the healing agent, which then flows into cracks and microfractures and re-bonds delaminated interfaces - restoring structural performance.
To evaluate long-term healing performance, the team built an automated testing system that repeatedly applied tensile force to an FRP composite producing a 50 millimeter-long delamination, then triggered thermal remending. The experimental setup ran 1,000 fracture-and-heal cycles continuously over 40 days, measuring resistance to delamination after each repair. In other words, the researchers cracked the material in the exact same way, healed it, and then measured how much load the material could handle before delaminating again. And they did that 1,000 times, an order-of-magnitude beyond their prior record.
"We found the fracture resistance of the self-healing material starts out well above unmodified composites," says Jack Turicek, lead author of the paper and a graduate student at NC State. "Because our composite starts off significantly tougher than conventional composites, this self-healing material resists cracking better than the laminated composites currently out there for at least 500 cycles. And while its interlaminar toughness does decline after repeated healing, it does so very slowly."
In real-world scenarios, healing would only be triggered after the material is damaged by hail, bird strikes or other events, or during scheduled maintenance. The researchers estimate the material could last 125 years with quarterly healing or 500 years with annual healing.
"This provides obvious value for large-scale and expensive technologies such as aircraft and wind turbines," Patrick says. "But it could be exceptionally important for technologies such as spacecraft, which operate in largely inaccessible environments that would be difficult or impossible to repair via conventional methods on-site."
The study also shed light on why recovery slowly declines over time. With continued cycling, the brittle reinforcing fibers progressively fracture - creating micro-debris that limits rebonding sites. In addition, chemical reactions where the healing agent interfaces with the fibers and polymer matrix decline over time. Even so, modeling suggests the self-healing will remain viable over extremely long time scales.
"Despite the inherent chemo-physical mechanisms that slowly reduce healing efficacy, we have predicted that perpetual repair is possible through statistical modeling that is well suited for capturing such phenomena," says Kalyana Nakshatrala, co-author of the paper and the Carl F. Gauss Professor of Civil and Environmental Engineering at the University of Houston.
Patrick has patented and licensed the technology through his startup company, Structeryx Inc.
"We are excited to work with industry and government partners to explore how this self-healing approach could be incorporated into their technologies, which has been strategically designed to integrate with existing composite manufacturing processes," Patrick says.
Research Report:Self-healing for the Long Haul: In situ Automation Delivers Century-scale Fracture Recovery in Structural Composites
Related Links
North Carolina State University
Space Technology News - Applications and Research
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