Inspiration for Self-Healing The interest in developing synthetic self-healing in metal
castings primarily has been inspired by existing biological systems with auto-repair capabilities. Natural self-healing is seen in many organic systems, such as bones. Certain advances in fabricating self-healing systems have
been made in the past few decades. The healing process can be autonomous (without human intervention) or non- autonomous. The latter requires external intervention, such as heating the material to trigger the repair process. Following is a look at eight healing mechanisms that have been explored in fields other than metalcasting. 1. Researchers at the Univ. of Illinois have developed autonomous healing in polymers by embedding microcapsules filled with a monomer healing agent and a catalyst in a polymeric epoxy matrix. When the healing agent contacts the catalyst, it is converted into a solid polymer, sealing cracks as they develop.
2. At the Univ. of Bristol, researchers used previous work to develop fiber reinforced polymer compos- ites. By adding hollow fibers containing healing resin and hardener among the reinforcement fibers, they were able to demonstrate recovery from damage caused by impact. When damaged, the filled hollow fibers fracture locally, releasing resin and hardener.
3. A single, localized healing event has been achieved through the development of an interconnected micro-vascular network of encapsulations of heal- ing agent, imitating the circulatory system present in
biological organisms. The network delivers the healing agent continuously and repeatedly to damaged sites.
4. Groups at the Univ. of California Los Angeles and the Univ. of Southern California have developed a polymer in which about 30% of the cross-links disconnect at 248F (120C) and reconnect upon cooling. This allows the unlinked polymer chains to flow into the crack or void caused by damage, and upon cooling, the chains link the crack closed.
5. Self-healing concrete composites are being developed with glass fibers containing an air curing sealant em- bedded in the concrete matrix. This composite exhibits self-healing behavior but it suffers from a significant (10%-40%) loss of stiffness compared with standard concrete due to fibers.
6. Researchers have studied the crack-healing behavior and mechanical properties of a mullite com- posite toughened by the inclusion of 15% (by volume) SiC whiskers.
7. Self-healing ceramic materials can be produced where an oxidative reaction causes the volume of oxide to exceed the volume of the original material, allowing products of these reactions to fill small cracks.
8. Scientists at the Dalian Univ. of Technology in China have developed self-healing concrete using shape memory alloy wires to close cracks and hollow fibers containing adhesive to bind crack faces back together.
METAL
July/August 2011
MetAl CAsting Design AnD PurChAsing
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