SHAPE FORMING
Could aero turbines like this Rolls-Royce UltraFan one day be self-healing?
is fully self-contained and requires no external input; and non-autonomic, which requires external stimuli to trigger the healing process. In addition, there are three core mechanisms used to promote self-healing in materials, and they come in the form of encapsulated healing agents, microvascular healing networks, and intrinsic healing. Let’s look at each in turn.
ENCAPSULATED HEALING Encapsulated healing is arguably the most common technique for incorporating self-healing properties into materials such as polymers and polymer composites. This mechanism incorporates micro- or nano-capsules of healing agent into the material’s structure, during the manufacturing stage.
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Materials that use this mechanism
will feature catalysts dispersed throughout the material matrix, alongside the microcapsules of healing agent. When a crack occurs, it will lead the microcapsule to rupture and release the healing agent into the crack. The released agent reacts with the catalyst and undergoes a process that causes it to harden and fill the crack.
One of the earliest practical
examples of this was demonstrated in 2001, by Professor Scott R. White and his colleagues, in a polymer epoxy material. In the study, White et al. developed an epoxy composite that mixed 100 parts Epon Resin 828, a blend of bisphenol-A and epichlorohydrin, with 12 parts diethylenetriamine (DETA). As White reported, “the reaction polymerises dicyclopentadiene at
room temperature in several minutes to yield a tough and highly crosslinked polymer network.” This mechanism is, however, limited
by design. Each capsule of healing agent is microscopic to prevent it from compromising the overall integrity of the material. It would be counter- intuitive to employ self-healing mechanisms that repair damage to a material with lower fracture toughness than its non-healing counterparts. The size constraints of the capsules limit the amount of healing agent that each contains. This therefore limits the amount of damage it can adequately repair. There is also only a finite number of capsules, each of which can only heal the material once. If an application is likely to experience repeated localised damage – such as aircraft components that are frequently exposed to extreme
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