Friday, September 05, 2008

Fabric Pictures

Normally, I work with unidirectional CF, and sometimes a cosmetic layer of plain weave. Let me explain. In a plain weave, there are threads going both up and down, and side to side, each go over one thread then under the next. It's about as simple a weave as can be imagined. If you know that CF only exhibits strength in tension, then this might sound like a great idea, because one layer can deal with tension in two directions, each at a 90 degree angle to the other. Unfortunately, this doesn't quite work. In weaving the fabric, the threads get bent up and down - they don't lie in a straight line. So we can say that they are 'crimped' and these reduces their strength. In fact, the strength to weight ratio of CF is negatively impacted by the need add epoxy, to align with the vectors of stress (adding layers), and because of crimping. This latter factor is the reason that a woven fabric is usually reserved for an outer 'cosmetic' layer.

Uni-directional CF is an interesting beast and comes in several form
s. Because it is uni (one) directional, it doesn't have crimps. Also, it's fairly easy to align with the force vectors, although multiple layers may be needed to pick up all the force vectors. Multiple layers aren't really a problem for us because we'll use enough CF to require multiple layers anyhow.

CF is held together in one of several ways. The first is to run a thread (generally of something other than CF) across the width of the fabric (which is sometimes narrow enough to be called a tape, or wide enough to be a cloth). The cross thread is held in place by some sort of glue. Another approach is to glue a veil (very thin layer of randomly aligned threads) of fibers to one or both surfaces of the uni. The later approach tends to be less visible afterward, but often doesn't allow as much bending of the uni to follow the shape of a structure.

The uni with the cross threads is more common, but the cross threads are thick enough that, even when they are turned in towards the work, they tend to print through somewhat as a ridge in the finished material. Also, its easier to damage the uni carbon threads by rubbing something (anything) across
their surface. Each thread is made of very fine strands of CF, and abrasion starts to pull the individual strands away from the thread. With the veil, the surface is better protected, and often on both sides. However, with wider uni fabrics, the veil so limits bending that it doesn't seem to be used much. However, there is an interest form of CF tape made with a veil backing. A one inch tape will be divided into three strips of CF, each with a space between them. The veil runs edge to edge and so crosses these empty strips. Using this tape, it can be split length-wise along these gaps, allowing it to better follow contours. So where one tube joins another, the end of the tape can be split (as an example) allowing the middle CF strip to bend up along the length of the intersecting tube, while each of the other strips angles off and wraps around the intersecting tube. I'll probably have to add a picture of these later to illustrate this clearly.

Woven CF fabrics can come in a number of different weaves, which I won't try to describe here. But, some of the fancier weaves offer more flexibility than a plain weave, and often less crimping as well. This makes them better structural solutions, especially for more complex shapes. In a few moments we'll see more regarding this.

It should be noted that some of the challenges of handling uni-fabrics can be overcome using pre-preg CF. However, pre-preg requires a freezer for storage and an over for curing. Currently, my little shop has room for neither of these appliances - so pre-preg is out.

There are some other venues were pre-preg isn't the chosen solution. And for these
variations on uni-fiber have been developed. Most of these solutions are v
ery high-tech, at least in terms of how they are manufactured. Only a few manufacturers have the facilities to create the best of these fabrics. And, these fabrics aren't generally available except on special order in very large quantities. Thus, they haven't readily been available to custom frame builders.

Recently I spotted someone selling 150 yards of such a fabr
ic - which is much more than I can use over the course of several years - so the purchase was out of the question. But I made contact with the individual who is in the aero-space industry. He works for a major company that you've worked for, and my best guess is that his work is in the defense sector. But that's all I can share.

Anyhow, I asked questions because I wanted to know more about this stuff and it's applicability to framebuilding. He offered to sell me a small lot, so I bought 4 linear yards. Here is a picture of some in the raw:

If you look closely, you will see that there a
re two layers of uni CF. The top layer is clear, but look at the edge and see that there is a layer behind running at 90 degrees. What is this? It is non-crimped +/- 45 degree uni-directional carbon fiber. The stiching you see helps to hold its shape or body, but it allows the fabric to be very flexible and drape wonderfully around complex shapes (think of a bottom bracket where 3-4 tubes join together around the BB shell). Moreover, each layer of this fabric is about as heavy as one lay of my normal uni-CF - so only half as many layers need to be cut and applied. Naturally, each layer soaks up more epoxy, and more work has to be spent workign the epoxy thoroghly through the cloth. Also, I still need normal Uni to add a third major force vector plus occassionally other lesser vectors. This is very cool stuff and I hope to have some pictures for you soon of it in use.

My new friend also sent me samples of a couple of other interesting fabrics. Look at this:
When I first looked at this, I thought it was a 1x1 plain weave using 12K bundles - this is the now fashionable large checkerboard effect seen on a number of new bikes. Closer examination revealed that it is something else. First note that there is a veil above the upper surface. The backside has the same veil. Looking at the edges, this is 2 layers of CF, with the treads running at right angles. Because there is no stiching, I'm guessing that there is veil between the layers to glue them together. Also look at the weave, it doesn't make squares, but instead forms rectangles. This is a twill weave. Both directions of fabric run over 2, under 2 patterns, and adjacent bundles are staggered by one thread creating the diamond like pattern. My best guess is that when wet out, the veil becomes week and that this should form nicely over bends and curves. It has fewer crimps than a plain weave, but should still offer a nice cosmetic finish. Once I've experimented with it, I'll fill you in.

The next and last fabric is a bit more of a mystery to me. Here are two pictures, one where it has unraveled a bit, and another where the fabric is intact.
From the unravled edge, we can see that there's a whole lot of CF going on. Also, that the fabric is stiched through and has a veil on the top.

My best guess is that this has from 4 to 6 layers of CF. Looking at the edge in the second picture, you can get the sense of all the layers. Also, it looks like there may be some intermediate layers of veil. Without trying to take this apart a layer at a time, it's hard to get a clear picture of the internal structure. And because I only have a limited sample, its hard to investigate in a destructive fashion. Nonetheless, it looks like it has at least 3 directions of uni (0, +45,, -45 degrees) and possibly 4 directions (0, +45, -45, 90 degrees). With all of these layers in one cloth, very few pieces of this should need to be laminated together. Also, most force vectors should be addressed by this one piece of cloth. So it could speed construction significantly. It will take more effort and care to work the epoxy through this cloth. And, it may not drape as well as the other examples that we have. But like the rest, it's going to make for some fun play.

Well that's it for tonight. I'll probably take a cut at editting this a little tomorrow - meanwhile you can enjoy the pix.

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