Schematic of: (a) enhanced mechanical performance of easy-repair interleaved composites that outperforms basic carbon-fiber laminates even after a few repair cycles; (b) location, damage and healing process of phenoxy interleaves, showing re-bonding and crack filling of phenoxy phase during healing and repair process. Inset in (a) are pictures of specimens after test and repair cycles.
Schematic of: (a) enhanced mechanical performance of easy-repair interleaved composites that outperforms basic carbon-fiber laminates even after a few repair cycles; (b) location, damage and healing process of phenoxy interleaves, showing re-bonding and crack filling of phenoxy phase during healing and repair process. Inset in (a) are pictures of specimens after test and repair cycles.

Intelligent materials that can monitor their structural integrity in real time and repair damage would extend the life of composites used in transport and renewable energy applications. Now researchers from Queen Mary University of London, Loughborough University, University of Warwick, and Imperial College London in the UK have revived a long-standing approach to create a new self-sensing and self-healing composite [Thorn et al., Composites Part A 165 (2023) 107337, https://doi.org/10.1016/j.compositesa.2022.107337].

“Through a simple ‘old’ approach of interleaving composite laminates, we have demonstrated ‘new’ added functions of repairing and structural health monitoring with the aim of prolonging composite components’ service life,” points out Han Zhang of Queen Mary.

Most material self-healing strategies rely on extrinsic approaches using healing agents or intrinsic systems based on reversible chemical bonds or supramolecular interactions. Adding liquid or solid healing agents to structural composites, however, can impact their load-bearing performance and add complexity to the manufacturing process. Instead, the researchers interleaved thin thermoplastic layers in between carbon fiber composite plies. To repair damage, heat and pressure are applied, enabling the thermoplastic to diffuse into damaged regions and fill in cracks. While the thermoplastic layers provide the capacity to self-repair, they do so without affecting the mechanical properties.

“Our work addresses two drawbacks of continuous fiber-reinforced thermoset composites with a simple and scalable solution,” explains Thomas D. S. Thorn, first author of the study. “We demonstrated increased laminate toughness alongside an easy-repairing functionality, solving the current challenges of composite materials’ relatively poor out-of-plane performance and inability to sense and repair internal damage.”

The properties of the thermoplastic layer and curing conditions can be tailored to enable both easy repair and improved fracture toughness, even after multiple cycles of damage and repair. Moreover, monitoring the electrical resistance through the composite provides a means of detecting delamination.

“Repairing with this thermoplastic in thin continuous interleaves rather than a thermoset/thermoplastic blend resin is novel and allows for a different repairing mechanism,” explains Zhang.

The proof-of-concept smart sensing composite material could extend the service life of composite components, reducing the carbon footprint and environmental impact. The researchers now plan to maximize the repairing efficiency and sensing capabilities without impacting the thermomechanical properties.

“The most useful applications would be where routine maintenance and repair to composite structures are required but logistically challenging or impractical [such as] off-shore wind turbine blades or aircraft,” says Zhang. “This simple and scalable approach could also be taken up in other sports or marine composite applications.”