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.
Thanks for the feedback and clarification. Since the whole point of this self-healing material is to deal with hard-to-detect, difficult to-repair delam problems (the most common ones), I don't see how a thicker non-self-healing material can last longer. The more layers there are (that's how you'd make it thicker), the more potential fractures there'd be. Plus it would make it heavier, which is contra the point of using composites in the first place. Anyway, let us know what you think after checking out the article.
Ann, I could easily continue the discourse on the non-relavant language issues, but this is not the forum for that.
Yes, it was the "So there's no added weight, except as compared to a composite with no self-healing system." comparison that I was asking about. All I wanted to know was the weight difference. Knowing it would allow me to safely suggest making a bigger non-healing composite material of the same weight as the self-healing material, not smaller as you suggest, and compare its longevity to the self-healing stuff. I think that one might be surprised which one wins out in a particular application. I would bet it won't always be the self-healing material.
davidmac, thanks for the explanation. D'oh! What you meant seems obvious now, and I think those are good questions. To clarify, the voids in the filaments are filling with epoxy that was separated into two parts in the microchannels. All of the epoxy-making material was already present in the composite, as the illustration shows. So there's no added weight, except as compared to a composite with no self-healing system. Many other self-healing methods, and even similar methods to this one, have been tried and have problems this one doesn't have, according to the journal article. Which I suggest you check out for more detail (see my answer to J. Lombard below). If you're proposing that a self-healing material could weigh less and be thinner than this one, how would you design it?
davidmac, I agree in principle about the desirability of a single correct way of doing things, such as spelling or grammar. The problem in maintaining such a standard is the pluralistic nature of human societies: we live in different regions of the world with many different languages, which change over time. Even English is different, with different conventions, from one English-speaking country to another. Within the editorial world, conventions change over time, even within the same publication. Which is my point--these are conventions of how to do something made by humans, unlike the principles of physics or chemistry, which are more like laws made by nature. So it's not really about selling anything, it's just the way human-generated systems work.
One more point I wanted to make is that this material would not be good for anything under constant over-stessing situations. The epoxy would not have time to set before more breaking took place. For normal aircraft wings, I could see this being an advantage. For fighters, it would seem that thicker material with unfilled channels would make the whole plane stronger at the same weight it would have been using older material without the microchannels. Of course, I am assuming that they take up a significant portion of the material when in fact it could be a small percentage. That is why I am asking; you know what assuming does.
First, it is unfortunate that we can't have a single correct way of doing anything. Even the standards people change things so they can sell you new books.
Now to the important stuff. They are creating voids in the filaments, which they are then filling with epoxy. It would seem that a trade-off could be made between making the overall parts bigger and stronger, or weighing down smaller parts with epoxy. Thinner self-healing material would be an advantage in some, but not all cases. The question is, how much does an unfilled and hence unaltered material weight compared to the new material with epoxy added?
davidmac, re an ROI vs a ROI, there's no single way that's correct for all publications or contexts. You may not realize this, but every publication has its own individual style guide, and what's in a dictionary is not the final word--even they don't always agree.
Regarding your other question, I don't follow. The extra weight of what? Replacing empty space with what fluid? Please clarify and I'll do my best to respond.
Ann, maybe I missed it, but I didn't see anything about how much extra weight this process required. After all, you are replacing empty space with a relatively heavy fluid.
On the nitpicking side, since it is a common mistake that I used to make, it is a ROI, not an ROI. The reason is that you must treat acronyms as if they were the full words. However, it would be corect to say "You are missing an r from the word correct in the first part of this sentence".
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