INSPIRATION FOR SELF-HEALING
Fig. 2. This scanning electron microscope image shows the crack surface of the NiTi short fiber reinforced Sn-20% Bi matrix.
Once the potential directions for developing self-healing
metal castings were identified, computational simulation was performed to aid in predicting the types of cracks that could be healed using the third system being explored. While the precise microstructure of self-healing metal castings has not been deter- mined, the examination of the systems at UWM was intended to help drive the development of casting methods in the future. Te techniques and concepts developed for solder and Mg-based castings can be extended to aluminum castings. In the case of SMA-reinforced systems, a metalcaster or designer would need to identify a suitable aluminum alloy matrix and an interfacing material that will liquefy and become sealed under the clamping force needed to heal the crack. For systems involving a low melting point-encap- sulated healing agent, the healing agent must be selected based on its flow and solidification characteristics. Other off-eutectic systems also can be identified based on the thermodynamic phase diagrams of aluminum alloys, leading to self-healing in the selected system. Self-repairing metal castings would be ideal for use
in applications such as high temperature turbine blades, automotive engines, fatigue and failure prone structures and aircraft castings. Te success of self-healing concepts when used with polymers, ceramics, solders and magnesium-based alloys could pave the way for the success of production cast- ings in the future.
The Three Methods Following is an exploration of the three self-healing
systems under consideration at UWM.
SMA Inserts When a casting with embedded SMA wires or parti-
cles is cracked, the reinforcements deform. Upon heating the composite to the austenite transformation tempera- ture of SMA, the wires or particles recover their original shape. This shape change provides a clamping force that
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 micro- capsules 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 composites. By adding hollow fibers containing healing resin and hard- ener 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 healing 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 devel- oped with glass fibers containing an air curing sealant embedded 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.
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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.
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