Mechanical Properties of Non-vacuum Bagged CF

I did a lot of hunting on the internet, but with no results.

I understand that doing a hand layup and leaving it sit to harden will produce a product that does not have mechanical properties (strength and stiffness), as good as a vacuum bagged part that is well consolidated.
But has anyone quantified the differences?

I ask because I see parts being made this way (in the telescope community) where stiffness in particular is most important- and the smallest deflection in optics is bad. The modulus of epoxy is relatively low, and the composite will have to bend before the fibers straighten out and take the load. I also wonder if anyone has studies these “micro” deflections as I call them?

When you speak of “… the fibers straighten out and take the load.” I gather you are speaking of woven cloth laminates. If you want the fibers to take up the load without “straightening out” you should laminate your telescope tube using high modulus unidirectional carbon tapes in a properly designed layup.

Stiffness of a laminate is also greatly determined by thickness. To maximize stiffness, you should consider a cored laminate. Just for the record, stiffness increases as the cube of the thickness, so a little core goes a long way in the properties of your tube.

Can’t help with “micro deflection” studies. I do know that compaction (or consolidation if you prefer) reduces thickness and therefore normally decreases stiffness. [Good compaction is often important for other reasons, and required stiffness is addressed by other parameters, most often thickness.]

Im not familiar at all with the design parameters of telescopes, but I think JumpingJax nailed the core issue with his last paragraph.

A wet-laid laminate can often provide better stiffness at a reduced cost compared to a compacted laminate, simply due to the thickness of the laminate. If the laminate doesn’t require a great deal of actual strength (in terms of tensile strength), then a wet-laid laminate probably provides the required stiffness without sacrificing too much in weight and strength. In the end, I’m sure it all comes down to costs vs what the actual performance properties are.

i think your idea of micro deflection or having independently measurable properties of a cured laminate are off.

A composite part is considered to be one material… it’a s composite of two, the matrix and the reinforcement. While the two materials do have measurably different properties, they are considered one material for testing and measurement. Once cured, they are considered one material, one laminate. You would test a wet laid part vs a vacuum consolidated part and compare the numbers on the two processes… you’d test a bunch of parts though, as process can vary, especially with a wet layup.

though all in all… yes vacuum bagged is better, but many many many things are made without it. Ultimately you design for whatever strength is required with some safety margin. On something like a telescope, just use more material, as weight isn’t going to be a big issue. As said before, using a core will give much greater increase in rigidity than having a super thick laminate, with a lighter weight. you can also use different materials that have higher rigidity, like unidirectional materials.

Also a big issue with a wet layup part is going to be resin content. I think you had hinted at this in your concerns about the different properties. If you do have a wet lay part, chances are it will have a higher amount of resin. Having closer to ideal resin content will give you the best numbers for the given fabric. Exceeding that, having a rich laminate, will give you a bit weaker laminate. Again this should be considered into the design. Also while you layup, you want to try to use the proper amount of resin, which is something that can take a bit of practice and experience to get right. Especially if you just mix up what you think is ‘enough’ and then keep adding and adding. This is where vacuum also helps as you can control resin content through a bleeding schedule in your bagging. I’d be this is what gives the biggest improvement in wet layup; more than the consolidation.

I think that covers most of it… seems like i’m starting to ramble :wink:

I appreciate everyone’s answers. Adding a core does not really address the original question, so I will not address it here.

Hanaldo makes an interesting point about the thicker laminate possibly being as stiff as a well consolidated one. The moment of inertia increases by the cube of the thickness, while increasing the resin content will decrease the fiber dominated properties such as tensile modulus. So you gain on one variable, and you give up on another.

I was able to find a paper from Gurit that states that the fiber volume fraction of typical hand layup, such as used by boat builders, is approx 30%-40%. We know that 60% or better is easily attained for high performance composites. Helius Composite software, which uses micromechanics to predict laminate properties, calculates a lamina value of E of 1.63E+07 psi for a 40% fiber volume and a value of 2.4E+07 for a fiber volume of 60%. This was done by a friend using IM7 with 8551 resin. So this is a 147% increase in the modulus by reducing the resin content 20%. This shows the benefits of lowering the resin content- but that should be a given. A CF/epoxy laminate is a combination of the properties of both constituents.

It is clear that a well consolidated laminate offers better mechanical properties than a poorly consolidated one. I was interested in trying to roughly quantify it, though I understand that there are a lot of variables, especially in a laminate that is wet out and set aside to harden.

And, yeah, I was referring to woven fabrics regarding the fact that the fibers in tension try to straighten out when loaded, and this reduces the modulus of the laminate. Of course, there is uni or non-crimp fabrics available that don’t have this issue. But my thoughts still envision a resin heavy composite as having a low modulus at lower loads, and increasing under heavier loading- mainly because the epoxy has a modulus in the range of 1/20th that of an IM carbon fiber, and there is so much of it between the fibers. This would be the same reason that woven fibers perform poorer than non-woven, that is the epoxy allows the fibers to actually move in the matrix as they have tension increased. This movement in the matrix has been studied in woven cloths as it is a common failure point.

Just guessing a little here but the possible reason they use use carbon fibre in the telescopes could also be it’s thermal stability compared to metal or other materials that will expand/contract with the temperature??

Yes Fasta, having a low CTE is a good characteristic. 8.4 um is the focus range of a f/3.3 Newtonian optic, so on medium size (0.7 meter optic) structure, it only takes a degree change in temperature to put it out of focus. Using CF for truss poles has become common, but there are advantages to having the whole structure made of CF.

Having a low mass is important for mounting considerations and inertial problems which will affect fast tracking and slewing. The old way of thinking was to throw mass at the tube and mount structure to make it as stiff as possible- but that comes with a price due to thermals. The thermals are the result of the telescope/mount/building cooling down and attempting to equalize to the dropping night time temperature. The thermal distortions are similar to the distortion you would see coming off of a hot road in the summer. They are a real problem that have a strong impact on all telescopes- even the smallest backyard ones. Lightweight optics along with lightweight structures allow better thermal tracking of the scope and night time air temperature.