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.
"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."
Ann, self healing process is bit complicated in machines. In human self healing mechanisms works well because of the running blood and healing contents inside the blood. But in machines, its bit difficult because of the constituted participles, but to an extent now researchers are confident that machines can also be in self healing process. Hope this will be good for space, avionic and remotely controlled machines.
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.
"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.
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.
"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.
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".
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.
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, 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.
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?
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.
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.
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.
Ann, this is really cool. I really think self-healing materials are the way forward, even if it's a bit strange to imagine that this is possible and also trust that materials can fix their own cracks (especially in vehicles like airplanes, which are risky for human life is there is even a bit of damage to a crucial piece of equipment). This reminds me of a story I wrote awhile back about a self-healing polymer--you probably read it already: http://www.designnews.com/author.asp?section_id=1386&doc_id=267960
This is certainly interesting technology and a novel approach to hidden damage mitigation. However, there are a few issues that come to mind related to practical application:
1. This approach addresses delamination propagation by re-bonding the material layers, but does not address damaged fiber structure. Hopefully, if it is sub-surface damage, the fibers are not directly damaged, else surface damage would be visible, requiring conventional scarfing and material replacement.
2. It would be interesting to see what an ultrasonic NDT scan looks like - both of an undamaged structure and a "self-healed" structure. My understanding is that these scans look for anomalies in the reflection waveform. Would the micro-channels look like anomalies or just raise the noise level of the total scan? Would the self-healed structure look like just another discontinuity, indicating a damaged section?
3. In solid laminate construction, we are seeing more use of Double Vacuum Debulking processes (out of autoclave repair) that are intended to eliminate voids to replicate autoclave strength repairs. Do the micro-channels introduce weak areas where the composite materials are no longer homogeneous? Is there a weight penalty to pay, as more material (layers) are required to obtain the same strength as a composite structure without micro-channels and embedded uncured resin?
4. Resin systems used in aircraft fabrication have a shelf-life. What is the shelf life of the embedded resin system?
5. This application would require a room-temperature cure resin system. At this time, there are no room-temperature structural resin systems used in aircraft construction (although a lot of work is going into developing a suitable material). This self-healing approach is more suited to the marine fiberglass application mentioned in earlier posts.
I wonder how much damage those polymers could sustain before complete failure to heal occurs. Could they handle an impact from an AMRAAM missile or just structural damage and fatigue from wear and tear?
"Im kind of confused here, why do you feel its not a suitable material for battlefield purposes ? "
Fdos, its suitable for all kind of applications, in battle field and days to day using engines, it can be get repaired physically. But in space and avionic applications physical repair is quiet difficult, especially when they are in voyage.
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.
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)
Thanks, Liz, I did see that article. Of course, self-healing composites is much more difficult to do than the type of self-healing plastics we've both written about. Fortunately, anything made for commercial aircraft has to go through a very tight series of controls before being adopted, somewhat parallel to medical-grade materials.
"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"
This is a HUGE breakthrough in composite structures. Very innovative and creative application of engineering and materials technology. I have a little experience in repairing fiberglass boats and can state unequivocally --I'm TERRIBLE. It's a real pain and even if successful, the repair looks awful! It looks like a patch. The methodology you describe could certainly provide needed solutions to composite materials. It seems the repair is every bit as durable as the original material if not more so. I love hearing that, in some cases at least, our tax $$$$$$$ are used for efforts that provide value-added to components and structures. Excellent post. Really appreciate the continued updates relative to composites.
bobjengr, I totally agree: thanks for recognizing the potential magnitude of this discovery. I've seen repairs to fiberglass boats--you're right they look awful and it's hard to believe they're seaworthy.
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