Determine amount of layers in a composite

[stars25]

Hello,

I am new to the forum, I did some searching but couldn't directly find an answer to my question.

I want to determine howmany layers of carbon layers I need to use for my design. To start easy for me lets assume my design is just a plate from 22cm x 22cm

I need to determine/calculate howmany layers I need to use to withstand around 2900 N, when one side is fixed and the other side may not move more then 0.1mm

Is there a rough way to determine this?

Thanks for any help.

 

[RPstress]

Your question is almost certainly one of very basic stressing. It sounds like the sort of thing that Roark (for instance) would be useful for.

A simple ASCII-diagram of how the load is applied, or just a better description, would help immensely.

Also necessary is some idea of what material you want to use. It's probably some sort of glass fibre or carbon fibre/polymer laminate. If so we would need to now more about the fibre, its weave (if it's woven) its thickness and how much resin there is compared with the fibres. A material spec would probably be helpful if one is available.

Some sort of context such as the reason for the limit on deflection would probably also help.

 

[stars25]

Hello, thank you for your answer.

The material will be carbon 3K, produced via RTM probably with epoxy.

Things as layers, lay-up and volume of matrix and fiber are things that I wish to determine with the calculations.

 

[RPstress]

That partly answers as to the nature of the material. Since it's RTM epoxy it'll likely be 50–60% fibre volume fraction. Since the tow size is mentioned it's quite likely to be woven. If it's plain weave that would make a likely cured ply thickness maybe 0.008"/0.2 mm, maybe a dry fabric areal weight of 200 gsm.

These are very largely guesses. You really need to find this sort of thing out. However, we can estimate the probable sort of properties of the material based on what's commonly available. My own bias is towards aerospace; a marine engineer or sports equipment designer might well have different estimates.

You used metric units so I'll do so too. A woven carbon/epoxy is likely to have a strength of maybe 900 MPa in tension undamaged, maybe 680 MPa in compression undamaged. These will be about 240 MPa for a quasi-isotropic layup (as many ±45° plies as 0/90s) with an open hole present in compression. If compression after impact allowables are needed they will be less.

The likely stiffness (Young's modulus or E) for a woven carbon 0/90 is 55000 MPa. A quasi-isotropic layup will be have an E of maybe 42000 MPa. Shear stiffness of a quasi-isotropic layup is likely to be maybe 15000 MPa.

Emphasis: These are guesses. You need to find out this stuff for your material.

For thicknesses the only numbers you give are a deflection, for an undefined loading direction but a magnitude of 2900 N, of 0.1 mm for a plate 220 mm square.

If the loading is simple tension:
[tt] ___________
/| | ->
/| | ->
/| | -> total load 2900 N.
/| | ->
/|___________| ->
220 mm
[/tt]
Deflection is given by d = strain * 220
Strain is stress/E = stress/42000
Stress = load/A = 2900/(220*t)

So 0.1 <= 2900/(220*t) / 42000 * 220. Thus required t is a minimum of 2900/0.1/42000 * 220/220 = 0.7 mm or maybe 4 plies (perhaps a layup of 0/45/45/0) if the assumptions above are valid.

The direction of the 2900 and whether it is some sort of point load or disributed and whether it puts the plate in tension/compression or shear or bending needs defining.

You are presumably doing this as some sort of home job. If it matters more than simple inconvenience then you need to get a structural/stress engineer involved. A mech eng graduate or someone who has studied engineering at a technical college level should be ok.

While we're usually willing to help out a raw beginner we're not here to size your structure for you or indeed to teach you how to do so in detail. If you want to learn how to size structures for yourself I'm sure someone can advise on a suitable course if you share your location.

 

[stars25]

Hello,

Thank you very much for your explanation. It really helps.

I am an industrial designer and exploring composites, your explanation really helps, but it is still a little too difficult.

I am based in Rotterdam, the Netherlands. If there is anyone willing to help to solve my problem or if someone knows a good company. Then I am open for suggestions.

Thanks

 

[berkshire]

Since you are in holland you could try contacting Delft university they have an aerospace department with a great deal of knowlege in advanced composites.
B.E.

http://lr.home.tudelft.nl/en/

The good engineer does not need to memorize every formula; he just needs to know where he can find them when he needs them. Old professor

 

[RPstress]

Stressing (for composites or otherwise) is largely a matter of common sense and simple formulas but can be oddly baffling to those who haven't dealt numerately with structures. While getting a bit of experience in an academic environment is no substitute for a few years in a stress office, it would probably be immensely useful for someone in Stars25's position (I don't know how 'industrial' the 'industrial design' mentioned might be). While TU Delft is an excellent university particularly for things like composites and innovative structures, it might be overkill for Stars25. A more local institution with a more focused short course might be more suitable (if one can be found).

Also, Stars25, for someone in your position who is not quite happy with the level of explanation I was able to give I'd have thought that it would pay to read Professor Gordon's books which as well as being readily available, affordable and short, are immensely readable and entertaining. They have helped people who are unused to quantifying things about structures to understand the process. You should be able to pick up a secondhand paperback for a handful of euros.

The New Science of Strong Materials: Or Why You Don't Fall Through the Floor (Penguin Science).

http://www.amazon.co.uk/New-Science..._1_1_title_2_mm?ie=UTF8&qid=1321438095&sr=1-1

http://www.penguin.co.uk/nf/Book/Bo..._New_Science_of_Strong_Materials_J._E._Gordon

ISBN 0691023808

Structures: Or Why Things Don't Fall Down (Penguin Science).

http://www.amazon.co.uk/Structures-Things-Dont-Penguin-Science/dp/0140136282/ref=tmm_pap_title_0

http://www.penguin.co.uk/nf/Book/BookDisplay/0,,9780140136289,00.html?/Structures_J._E._Gordon

ISBN 0306812835

The Science of Structures and Materials (Scientific American Library series).

http://www.amazon.co.uk/Science-Str...ic-American/dp/0716750228/ref=ntt_at_ep_dpi_3

ISBN 0716750228

The third one seems to sort of conflate the first two covering much the same ground. I seem to remember that in one of them he says of composites that "...when you stick a lot of thin, stiff fibres together with plastic the properties are pretty much what you expect would them to be," or words to that effect. Also of interest, I'd have particularly thought for an industrial designer in the Netherlands, is 'Lightness: the inevitable renaissance of minimum energy structures' by Adriaan Beukers and Ed van Hinte. This won an award from TU Delft. Stars25 may already know of it.

http://books.google.co.uk/books?id=...ce=gbs_ge_summary_r&cad=0#v=onepage&q&f=false