In the early 20th century, materials engineers faced many challenges as ships built of sturdy steel plates were failing and breaking apart, sometimes under forces a thousand times smaller than predicted.
An English engineer, Alan Arnold Griffith, came to the rescue by redefining the theory of fractures. Griffith realized that every material is filled with tiny cracks and fractures, whose size can be less than the millionth part of a meter. The fractures can spread and grow quickly, causing the material to disintegrate.
While there are many techniques for manufacturing a material with a minimal amount of fractures, none of them can grant immunity from future cracks. That is, until now.
Recently, aerospace engineers at Bristol University devised a new composite material for aircrafts that can self-repair and restore its structural integrity. In a nutshell, when a crack appears in the aircraft – whether as a result of wear and tear, fatigue, or even a tiny gravel stone hitting the wing – epoxy resin ‘bleeds’ from hollow glass fibers near the hole or fracture and seal it rapidly. The sealing of the cracks prevents the tiny fractures from expanding and weakening the entire aircraft.
“This approach can deal with small-scale damage that’s not obvious to the naked eye but which might lead to serious failures in structural integrity if it escapes attention,” says Dr. Ian Bond, who led the project. “It’s intended to complement rather than replace conventional inspection and maintenance routines, which can readily pick up larger-scale damage, caused by a bird strike, for example.”
By mixing ultra-violet dye into the resin, the ‘self-mended’ fractures can be easily pinpointed during ground inspections, and a full repair session can ensue if the damage is assessed to be too extensive.
The project, which was funded by the Engineering and Physical Sciences Research Council (EPSRC), has the potential to be applied wherever fiber-reinforced polymer (FRP) composites are used. Since these composites are lightweight and high-performance materials, they have a good chance of gaining popularity not only in the manufacturing of aircrafts but also in cars, wind turbines, and even in spacecrafts.
In addition to their other impressive advantages, aircrafts containing more FRPs will be significantly lighter than current-day models, which are largely based on aluminum. Even a small decrease in the weight of the aircraft equates to substantial fuel savings over an aircraft’s lifetime.
“This project represents just the first step,” says Dr. Bond. “We’re also developing systems where the healing agent isn’t contained in individual glass fibers but actually moves around as part of a fully integrated vascular network, just like the circulatory systems found in animals and plants. Such a system could have its healing agent refilled or replaced and could repeatedly heal a structure throughout its lifetime. Furthermore, it offers potential for developing other biological-type functions in man-made structures, such as controlling temperature or distributing energy sources.”
The new self-repair technique is expected to be available for commercial use within four years.
More information on the project can be found on the EPSRC website.