Tube Strength - practical vs theory

General Composites Discussions Engineering Talk
#1

Hi,

I’ve been making some tubes and the stiffness is not matching theoretical calculations so I wanted to ask the forum what should be expected as a normal variation from theory.

The tubes are prepreg roll wrapped on mandrels, shrink wrapped and then cured as per the manufacturers cure schedule. The calculations are done using composite pro and using my own spreadsheets, with input from the datasheet. Composite pro and my calculations match.

I’m measuring the tube stiffness by fixing one end and applying force at the other and then calculating the stiffness from the deflection.

I’m fairly confident in the theory, and I’m fairly confident in the measurements, but the results are about 20% lower than theory.

What I’m wondering is
1: Is this a normal variation for this method or should the results be better?
2: What results are achievable for other methods for example filament winding?
3: If I need a tube of stiffness X from theory, should I over build it to take into account the variation?
4: How realistic are datasheet figures likely to be in real world situations

Any thoughts would be very welcome.

Thank you

#2

I’m not qualified to even comment but how confident are you the prepeg you have is what it states. There could be some variation from the manufacturer I guess but with all things being equal are the testing points attached and distributed evenly? I guess that’s the only thing I can think of and again apologize if you’ve already considered this.

#3

Could the reduced performance be partly down to the shrink wrapping? My thinking is the prepreg performance will be based on autoclave curing.

#4

What fabric is being used ?

5 degree fiber misorientation could result in up to a loss of 20% in part strength

#5

thanks for the replies.

I’m using UD fabric. Parts are coming out at about the right thickness and weight as what I would expect but other than that I have to trust that it is the material the manufacturer claims it is.
In terms of the layup, I’m sure there is a bit of variation in the angles, but probably 1 or 2 degrees at most. I’m using between 7-11 layers of 150gsm (depending on the part). I suppose using heavier material and less layers would be better. I’ve done a fair number of tests on different parts so statistically I’m comfortable with the measurements but there will be some margin of error. The datasheet figures are based on autoclave curing and a standardised test protocol, which is different from my method. And the theoretical predictions will be, well, theoretical :slight_smile:

So based on all of that I would expect a variation. But I’m fairly new to composites so I’m trying to work out if the 20% difference is about acceptable or expected for shrink wrapped tubes, or if it means I’ve got something wrong and I should be working towards much closer figures.

I think I’ll make a few more tubes and do a bit more testing - any more thoughts would be very welcome.

PS: The autoclave pressure is 5bar. If I am right vac bagging is around 1 bar. Does anyone know what typical pressure is applied by shrink wrapping?

#6

First thing that comes to my mind is that with that many layers, you’re not getting sufficient compression with shrink wrap.

Second thing (maybe more importantly) is that compressing the reinforcements around a mandrel is resulting in fibers that are sort of “relaxed” relative to the path or direction you intend them to lay. Think about it, even if the reinforcements are tightly wrapped initially, when things soften during the cure cycle they will be trying to squeeze together into a smaller wall thickness. Distortion of the fiber positions seems likely.

I thought about this when coming up with a process for the tubing I use and ended up using a female mold and internal bladder. My theory was that this method would lessen the chance of fibers being “wrinkled” during the cure process. Molding tubes this way may also cause some altering of fiber angles for bias plys, so it’s not without tradeoffs.

I think filament winding tubes is the best method to ensure accuracy of individual fibers. Especially if done in some way as to keep the fibers under tension during winding and curing. Sort of like embedding prestressed rebar or cable into concrete.

If you can’t alter your process to improve stiffness, you may have to alter your layup schedule to compensate.

#7

I agree with petey.
When the srinking film pushes inside you end up with a a slightly deformated prepreg and does not let the cf to come to action maybe by this you make the operate before the fibers as a sequence …and that is not good

#8

I saw this thread about shrink tape…
http://www.compositescentral.com/showthread.php?t=9463&highlight=shrink+tape+psi

if the PSI is less than the autoclave’s 70psi, this could be part of the mismatched results.

Also what is your laminate ply schedule? I think if you move the desired orientation to the outside plys, it would increase stiffness. The outer ply orientation will tend to dominate the resulting stiffness.

just a guess… could be many things.

I would also look at the numbers you’re using for your simulations. Numbers from the manufacturer might not be exactly the data you want. We do our own versification on the materials through destructive testing to make sure that our simulations are as accurate as possible.

#9

Thanks everyone, I’ll report back when I’ve done some more testing. All the comments have given me a lot of very useful information to think about.

#10

So, I’ve been making more tubes and doing more testing. I’ve improved the layup though practice to get better fibre orientation, and I’ve made incremental improvements to they way I’m doing most parts of the process. I’m now somewhere in the region of 10-15% which I think is probably close enough (for now) using the techniques I’m using (shrink wrap/roll wrapping), and I can take this into account when designing my layups.

Thanks for all the advice.