Composites

[fencer]

Hi,

I m a newbie in composites and am looking for information on how to calculate the properties of laminates. Working in a project of designing a superstructure of composite material, I am thinking of using E glass with a polyester resin system, but I am not able to figure out how to calculate the modulus of elastiticy etc for the laminate. I can use the information given in the books, but it will be better to know how the whole thing works out from scratch.

Thanks
Fencer.

[Tim B]

Have a look in "Principles of Yacht Design" By Larrsson and Eliasson. There is a fairly good explanation in there.

Tim B.

[Kreso]

Check Germanischer Lloyd or Bureau Veritas Rules&Guidelines, there is detail laminate calculation, you can design laminate by yourself, choose desired mat or rowing laminate...

Good luck

Kreso

[Franklin]

An important principle to remember is, with no change in skin thickness, simply doubling the core thickness of a sandwich composite panel makes it eight times more rigid.

[fede]

Although I'm reading more and more articles around (magazines websites) where I see a lot of delaminated and problematic cored structures...I'm becoming a bit sceptical about cored structures (in the hull)

[Eric Sponberg]

Gentlemen,

You have to realize that engineering composite structures is really very complicated, and you cannot necessarily get the answers out of charts in boat design books. To be as accurate as possible, composite structures involve a lot of matrix algebra on some pretty large matrices. A first-pass estimate using mixture-of-modulus type of algebra is OK, but for final assessment, matrix algebra is necessary.

One of the key parameters in the matrix algebra is the shear modulus of the laminate. Shear modulus is rarely calculated or tested for in boat structures, but it is used with exact precision in aircraft structures. This is one of the drawbacks of most boat composite engineering.

Another factor that is troubling to naval architects in composite boat structures is that classical composites engineering with matrix algebra gives results in strains, not strength or modulus. And the determination of failure is usually related to "first ply failure strain". So how does that relate back to the actual strength, in PSI or MPa, of the laminate? Classical matrix engineering does not give you that answer. It will only tell you what the load per unit width of the laminate is at failure, and you have to make an estimate of what the failure strength and stiffness are based on unit cross-sectional area.

The alternative, to be really accurate in boat structures, is to use Finite Element Analysis, for FEA, such as is used for the boats in the America's Cup, or major offshore designs such as the Volvo 70s or open class 60s. However, using FEA requires very sophisticated computer programs and the appropriate engineering talent and background to run them. The hard work is in creating the complex hull and deck models for the structure, and this takes a lot of time and is quite expensive. That is why you see FEA done only in the largest boats--where the owners or syndicates have the money to pay for it.

In my experience, most common failures occur because the proper thought and engineering along with the good practical boatbuilding knowledge has not gone into the design. That is, you can be as theoretical as you want, but if you build it without taking into account certain aspects of boatbuilding that you generally learn on the shop floor, you are not really going to be doing a complete job. So the theoretical aspect has to be there, but so does the practical aspect. You need both to make a sound composite laminate design.

Eric

[yokebutt]

Eric- Good one, I've always felt that whenever you're doing a larger project, you need to occasionally step backwards a few paces, look it over, crack open a beer, and ask yourself, "does this make sense?"

Yoke.

[fede]

Gentlemen,

You have to realize that engineering composite structures is really very complicated, and you cannot necessarily get the answers out of charts in boat design books. To be as accurate as possible, composite structures involve a lot of matrix algebra on some pretty large matrices. A first-pass estimate using mixture-of-modulus type of algebra is OK, but for final assessment, matrix algebra is necessary.


Eric
Eric, I guess that we all have to realize that a very few builders,unless a really big project is involved,would put the money to have a laminate engineered with matrices and FEA.
Real life tells that many builders do not even hire somebody to have scantlings calculated.
I have a friend that has been making very succesful boats (including racing offshores popular all over the world) for 35 years and he can't even tell what's the square root of 4 ...he decides how the laminate should be,never had a problem with structures in 35 years.

I guess that as in all things there might be different approaches for different situations.
I guess that for a 21 kn 30 ft lobster FEA and Matrices are a bit too much,the bounch of rules taken from a design book might be ok...am I wrong?

[Eric Sponberg]

Yokebutt,

I think one of the best things I learned from Everett Pearson at TPI where I was Chief Engineer many years ago was that your work has to feel right in the seat of your pants. That is, you should have some expectations of what the answer is before you calculate the result. If it does not feel right, or appear right, then something may be wrong, and it is worth checking your calculations again. Occasionally, you do learn something new, but it is always wise to double-check, and record the fact that you have double-checked.

Fede, you always have to consider what the project will bear. If your friend has made a success in building his boats his way, that is fine. To each his own. Whatever works is fine. Certainly, guidebooks are usually quite conservative, and you may be perfectly safe in using them in many or most applications. You may not be optimized for weight and strength, but it will likely get the job done. And the same applies about it feeling right in the seat of your pants--it has to give an expected result. If it doesn't, something might be wrong and needs more checking. The theoretical practice has to mate well with practical practice.

Eric

[Karsten]

Fencer,
The mixing rule works sort of well for stiffness. First you can multiply the fibre E modulus by the fibre volume fraction of the laminate. Say 40% is fibre and 60% is resin you multiply by 0.4. That is basically the stiffness if all the fibres would run in one direction. For a 0/90 material you would have to multiply the stiffness again by 0.5 because half of the fibres run in the 'wrong' direction. This works well for 0/90 unidirectional laminates but as Eric mentioned things get pretty tricky for all other angles. This completely neglects the resin and is therefore on the conservative side although woven fibres are again less stiff because they go over and under the transverse fibres all the time.

To calculate the strength of the laminate you first have to think about what sort of load case you have. If it is a 'working load' you probably don't want any damage to the structure at that load. Therefore the strain has to be less than the strain where the resin starts to crack and debond from the fibres. This type of damage would not be catastrophic straight away but would weaken the structure over time. Resin cracking and debonding strongly depends on the resin type and even brand used and is therefore quite difficult to determine unless you test the actual laminate.

If your load is an ultimate load like 'hitting a rock at 8kn' and you don't want the keel to fall off but can accept cracking damage to the structure you have to look at the strain where fibre failure occurs. If you know the strength of the fibre you can carry out the same calculation as for the stiffness and come up with a rough estimate.

Cheers,
Karsten

[water addict]

Hi,

I m a newbie in composites and am looking for information on how to calculate the properties of laminates. Working in a project of designing a superstructure of composite material, I am thinking of using E glass with a polyester resin system, but I am not able to figure out how to calculate the modulus of elastiticy etc for the laminate. I can use the information given in the books, but it will be better to know how the whole thing works out from scratch.

Thanks
Fencer.

Get marine composites second edition by Eric Greene Associates. www.marinecomposites.com
This is the most definitive, applicable, all-in-one-place treatise of marine composites I've seen so far. The web site has it it downloadable pdf files.