The demand for composite materials is increasing worldwide. Their light weight, combined with exceptional strength, makes them ideal for use as wind turbine blades and major aircraft components, such as wings. However, they have low fracture toughness and a low resistance to crack propagation. In other words, they don’t tolerate impact damage well.
All structures in service deteriorate over time. Designers build in over-capacity to cater for the stresses from normal operation when designing structures and components. However, impacts from clumsy handling or bird-strikes can cause damages that may be severe but almost impossible to detect without expensive testing methods and equipment. Many researchers are looking to nature; more specifically to the ability of living systems to bruise and to heal after injury.
In nature, bruising occurs when tiny blood vessels near the surface are ruptured by an impact. Blood leaks from the damaged vessels into the surrounding tissues and the subsequent discoloration is visible through the skin.
Bruising in composite materials is achieved in a similar fashion. Microcapsules containing dye chemicals are incorporated into a surface coating, typically a gel coat. An impact ruptures the capsules and releases the chemicals. In the simpler types, the ruptured capsules just spill their colored contents, while other types use a chemical reaction to create the bruised effect. Where cosmetics are important, such as the surface of an aircraft, the bruise may be actually colorless, but will fluoresce under ultraviolet light to indicate areas where deeper inspection may be required.
Healing in nature similarly involves bleeding at the wound site, and continues with clotting and scabbing of the blood in the wound. To reproduce this process in composite materials, researchers are experimenting with incorporating hollow glass tubes and microcapsules into the material itself during manufacture. Some of the tubes or capsules contain a resin, and others contain a catalyst or hardening agent. The theory is that an impact that causes cracks in the material will cause the tubes to rupture, releasing their contents into the crack. The resin and catalyst mix and harden into an epoxy plug, thereby healing the damage. Further, the chemical action can be made to cause a color change or to fluoresce, “bruising” the material.
As with any new technology, there are problems to solve. Incorporating glass tubes or microcapsules into a composite material is bound to cause some reduction in the structural integrity of the material. There must be a sufficient number of hollow tubes or capsules within the material to create the healing effect, but not so many that it destroys the integrity of the original material.
Other factors include the sort of damage that is anticipated. If you have the glass fibers nearer the surface, they are more likely to be damaged in an impact, but it may be that it is the deeper damage that is structurally more compromising. The arrangement of the tubes or microcapsules is very dependent on the type of risks that are likely to occur in service.
This research and development exemplifies a growing interest in the wider field of biomimicry for engineering materials and structures. The challenge is to understand the functional characteristics of natural systems to produce systems that work with engineering structures, feasibly, and economically. Read more
