We've talked a lot on the comment boards about the problems of repairing fiber-reinforced composites, especially pricier composites like the ones used in aerospace. Wouldn't it be cool if they could just repair themselves? That may not be such a wild idea. Engineers at the Beckman Institute for Advanced Science and Technology think they've found a way to do that.
A team led by professors Nancy Sottos, Scott White, and Jeff Moore in the Beckman Institute's Autonomous Materials Systems Group at the University of Illinois at Urbana-Champaign have invented a system of three-dimensional capillary microchannels carrying two different reactive fluids throughout fiber-reinforced composite materials such as fiberglass. The two fluids (an epoxy resin and a hardener) are contained in two vascular networks isolated from each other. When the composite is damaged by delamination, sections of the two networks at the damage site break apart, and the liquids mix and polymerize to heal the composite.
A system of three-dimensional capillary microchannels carries two isolated reactive fluids (red and blue) throughout a fiber-reinforced composite laminate. Delamination damage begins at the interface between the composite's layers (1). Sections of the two networks at the damage site break apart, releasing the liquids, an epoxy and a hardener (2). They mix and polymerize to heal the composite (3), making it even stronger than before. (Source: Advanced Materials/University of Illinois)
As the team points out in an Advanced Materials article describing their work (subscription required), one of the biggest obstacles to greater use of fiber-reinforced composites in automotive and aerospace structures is the difficulty of detecting internal damage that can quickly propagate and cause delamination problems. There's been a lot of progress in self-healing polymers, such as the ones used for a composite's matrix, but that hasn't been translated into self-healing techniques for an entire fiber-reinforced composite structure.
The researchers inserted sacrificial polylactic acid monofilaments infused with the catalyst fluids into the interior layers, or plies, of an aerospace-grade fiber-reinforced textile structure -- a process compatible with composite manufacturing. The resulting preform was turned into a structural laminate using a vacuum assisted resin transfer molding process. Vaporizing the monofilaments created the microchannels. Surprisingly, the team reports, this process does not degrade the composite's inherent fracture properties.
The team tested the vascular network system over multiple delamination fractures and healing cycles, using two architectures: a herringbone-patterned network and an isolated parallel configuration. In the herringbone pattern, the system delivered greater than 100% recovery of the material's resistance to fracture after delamination. The researchers were surprised to discover that, after each cycle of damage and healing, a greater load was required to propagate a crack in this architecture. The isolated parallel configuration fared much worse. The researchers wrote that the herringbone pattern's better performance was probably due to better dispersion of fluid throughout the fracture plane.
This research was supported by the Air Force Office of Scientific Research; the Department of Homeland Security Center of Excellence for Explosives Detection, Mitigation, and Response; and the Army Research Laboratory.
"a much wider operating temperature range is also needed in space, and also in military environments. OTOH, notice where the funding comes from: three different military-related sources, so you can bet they expect an ROI."
Ann, you are right. Wait and see is my answer for the second part.
J. Lombard, thanks for your thoughtful and thorough comments. I had the same initial thought about repairing the matrix layers vs the fibers within them: this may work for delam issues but what about breaks in fibers? OTOH, as the authors say in their article's introduction, delam problems are among the most common, hardest to detect, and most difficult to repair, and these problems are at the top of the list for why composites haven't been adopted more widely for structural components. That's one of the main reasons why the researchers tackled these issues first. Some of your other questions may be answered by the article itself. (Hint: a free copy is located on the website of one of the team's leaders: http://sottosgroup.beckman.illinois.edu)
That's a good point, Mydesgn--a much wider operating temperature range is also needed in space, and also in military environments. OTOH, notice where the funding comes from: three different military-related sources, so you can bet they expect an ROI.
"Materials used in spacecraft, however, must withstand a lot more than those used in commercial planes, such as meteor strikes and radiation. This type of self-repair system might not do so well in spacecraft unless it was redesigned to accommodate those specific needs. Similar issues apply for use in military applications."
Ann, you are right and apart from that heat is also a major issue when the flight moves across different atmospheric level.
Mydesign, there are definitely problems repairing fiber-reinforced composites in both aircraft and spacecraft -- because of the material's failure charactertistics and because of access issues -- and this material could be potentially used for both applications. Materials used in spacecraft, however, must withstand a lot more than those used in commercial planes, such as meteor strikes and radiation. This type of self-repair system might not do so well in spacecraft unless it was redesigned to accommodate those specific needs. Similar issues apply for use in military applications.
fdos, this is a commercial aerospace-grade composite, meaning it's aimed at commercial aircraft, not military aircraft or any other military vehicles. Materials made to withstand missiles, bullets, etc. are made to a different set of standards.
"New generation large aircraft are designed with all composite fuselage and wing structures, and the repair of these advanced composite materials requires an in-depth knowledge of composite structures, materials, and tooling. The primary advantages of composite materials are their high strength, relatively low weight, and corrosion"
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