Researchers at North Carolina State University have developed a self-healing composite material that can repair itself more than 1,000 times, demonstrating increased toughness compared to conventional materials. This innovation has the potential to significantly extend the lifespan of fiber-reinforced composite materials, which are commonly used in various industrial and aerospace applications, from decades to centuries. The technology aims to address interlaminar delamination, a prevalent issue in these materials, and reduce associated maintenance costs, labor, energy consumption, and waste.
Material Development and Mechanism
The self-healing composite material was developed to target interlaminar delamination, a process where cracks form between the fiber layers and the polymer matrix within fiber-reinforced polymer (FRP) composites. FRP composites are utilized in sectors such as aircraft, automobiles, wind turbines, and spacecraft due to their high strength-to-weight ratio. The challenge of interlaminar delamination in FRP composites has been recognized since the 1930s.
The healing mechanism involves two primary additions to conventional FRP composites:
- Healing Agent Layer: A thermoplastic healing agent is 3D-printed onto the fiber reinforcement. This forms a polymer-patterned interlayer, which has been shown to increase the material's resistance to delamination by two to four times.
- Activation System: Thin, carbon-based heater layers are embedded within the material. When an electrical current is applied, these embedded layers generate heat, melting the healing agent. The molten agent then flows into existing cracks and microfractures, re-bonding the delaminated interfaces and restoring the material's structural performance.
Performance and Longevity Testing
The research team conducted extensive testing using an automated system designed for long-term healing performance evaluation. The system subjected the material to 1,000 fracture-and-heal cycles over a period of 40 days. Each cycle involved inducing a 50-millimeter delamination, followed by triggering thermal remending, and then measuring the material's resistance to delamination after the repair.
Key findings from the testing include:
- The self-healing material exhibited significantly higher fracture resistance initially compared to unmodified composites.
- It maintained superior crack resistance for at least 500 cycles.
- While interlaminar toughness experienced a gradual decline after repeated healing, the material remained effective.
- Statistical modeling suggests the possibility of perpetual repair.
- In practical applications, researchers estimate the material could last 125 years with quarterly healing activations or 500 years with annual healing, a notable increase from the typical 15-40 year lifespan of conventional FRP composites.
Factors Affecting Long-Term Efficacy
Researchers investigated the reasons for the observed gradual decline in recovery over time. Findings indicate that continuous cycling leads to the progressive fracture of brittle reinforcing fibers, which produces micro-debris that can limit rebonding sites. Additionally, a decrease in chemical reactions at the interface between the healing agent and the fibers and polymer matrix was noted over time. Despite these factors, modeling suggests the self-healing process can remain viable over extended periods.
Potential Applications and Commercialization
This technology holds potential for large-scale and costly applications, including aircraft components and wind turbine blades, where conventional repairs can be expensive and labor-intensive. It is also considered particularly relevant for inaccessible environments such as spacecraft, where traditional on-site repair methods are impractical or impossible.
Jason Patrick, a corresponding author and associate professor, has patented and licensed this technology through his startup company, Structeryx Inc. The company's objective is to integrate this self-healing approach with existing composite manufacturing processes, collaborating with industry and government partners.
Research Publication and Support
The research findings were published in the Proceedings of the National Academy of Sciences under the title “Self-healing for the Long Haul: In situ Automation Delivers Century-scale Fracture Recovery in Structural Composites.” Key authors involved in the paper include Jack Turicek, Jason Patrick, Zach Phillips, and Kalyana Nakshatrala. Support for this work was provided by the Strategic Environmental Research and Development Program and the National Science Foundation.