Passaretti wrote:I'm all ears! Do you have a reference for the 55-degree number? I heard something similar before but could never find a source. I would also be very interested to understand how you are estimating your case strength and how this is stacking up against test results. One of my big questions right now is - what is the actual strength (vs calculated) of the material/tubes I am building? I am planning on a hoop strength tests on samples and hydrostatic pressure tests of sealed tubes. I should be in a position to start stressing the CF tube sections soon - but I'm a sponge for information from anyone that has made composite pressure vessels...
Any info would be helpful. Thanks!
Here are a couple of references on winding angles:http://iopscience.iop.org/article/10.1088/1757-899X/50/1/012061/pdfhttp://www.polymerjournals.com/pdfdownload/994244.pdf
We are really not doing any calculations in terms of trying to work out the strength before hand. Since we are limited to the available CF sleeves, we just make a number of test pressure chambers and test them. There are too many factors in terms of materials and construction techniques to consider that ultimately doing an actual test gives us the most accurate results. We are trying several different sleeves to get the strength to weight ratio that will work for our application. Though we do hydro tests on these pressure chambers it is only a part of the whole picture because it doesn't take into account the temperature increase during air compression heating which can weaken the composite, and also the longitudinal compression forces during acceleration. (we're looking at around 70G for these ones)
We build our pressure chambers in two stages. The first one involves making the inner pressure chamber out of several sections that are glued together. A tube and two end closures can be easily made on mandrels and molds (plugs). These sections are thin walled (~0.6mm) and essentially form the internal liner and also give the pressure chamber its shape. Each piece is made from Carbon Fiber biaxial sleeve. When all the glue is cured we pressurise this inner liner to around 50 psi and check it for leaks. If there is a leak then its easy to fix with a small patch or a dab of epoxy. When leak free we pull one heavy Carbon sleeve over the entire thing again making sure the fiber angle is close to the optimal (at least on the widest part of the pressure chamber). The good thing about the sleeve is that it nicely conforms to the shape over which you pull it. We normally put it on dry and then work the epoxy into it using a roller. Putting it wet on a 2.5m long inner liner would be very difficult.
Here are the end closures before gluing into the inner tube.
Here it is after gluing, and prepping to put the outer sleeve on.
Here is the outer sleeve being put onto the inner liner
We also put 1 layer of 85gsm fiberglass over the top of that to make the surface much smoother. We've used peel ply before and it works quite well too. The extra thin layer of glass is mostly going to be sanded through though in order to get a nice smooth finish without cutting through the outer carbon fiber layers.
To increase pressure containment you just pull on more sleeves as needed. We get the sleeves from Soller Composites. http://www.sollercomposites.com/NewSoller/index.html
They have a good selection and reasonable prices.
We have used this techinque a few times in the past and seems to be working well. Last time we made the inner liner with just 85gsm fiberglass, but this time we are using CF for that as well.
We then stick the whole thing on a rotiserrie and let it rotate in front of heat lamps for a couple of hours to make sure that the epoxy doesn't pool on one side. Ideally you would probably want to stick it in a vacuum bag but we don't have the set up. You would probably also want to pressurise the inside while its in the vacuum bag so it doesn't crush it.
Although your end closures are going to be very different to what we use, this is how we make ours:
We used to machine the end closure molds/plugs by hand out of wood, but now we just 3D print them as it is much more accurate and faster to get the shape we want. We just give them a sand to make them smooth but that's all the prep they need. We then pull a balloon over the top of it to give a nice smooth finish, coat it with silicone grease and then pull another balloon over the top of that. Then we give it a very light coat of silicone and you can then directly put the sleeve and epoxy on that. To get it off the mold is very easy because the two baloons just slide over one another and the end closure comes right off. I do like how you used the heat shrink though, I may give that a go especially on concave shapes. For larger end closures we used silicone swimming caps instead of balloons.
I am putting together a video of this technique so will post it when complete.
Here are the final test chambers with threaded inserts for hose connection for hydro test:
Crop Circles: ... just a bunch of guys looking for their rockets ....