Carbon Fiber Values for Structural Design 1

[Jim508]

Hello,

I'm working on an alternative design for a structural box beam and I'm trying to find design values to use in my analysis.

There isn't a lot of literature regarding carbon fiber in structural applications. I was wondering which tensile values should be used for flexure calculations. I've seen values ranging from 275ksi to 512ksi. I'm assuming the larger values are derived strictly from tensile testing and the lower values are derived from application testing. Can these values be use in traditional beam design calcs or are there other considerations that need to be taken into account?

Are there any limitations to how many plys can be used? Most of the structural shapes that I've seen are thin-walled, perhaps 4-6 plys. Is it possible to fabricate box beams with 1 inch webs? Can these shapes be fabricated by hand with wet lay up or do they require more advanced technique?

All information is greatly appreciated.

Best Regards,

Jim

 

[ESPcomposites]

Jim,

What is your application? For composites, you may want to consider the effects of holes, impact damage, joints, etc. which will dramatically reduce the capability.

Some other items to consider, beyond "typical" properties are:
- Statistical distribution (composites exhibit more scatter)
- Environmental conditions (hot/wet and cold/dry are typical scenarios)

Tension and compression allowables are significantly different, especially for holes and after impact. You may to design for impact damage since it can occur without subsequent indication (i.e. robust design).

Sorry the answer is not as simple as you would like, but these are all necessary considerations that can have dramatic affects to the actual structure's capability.

Brian

http://www.espcomposites.com

 

[Jim508]

Hi Brian,

Thanks for the info, it all helps. Composite design is relatively new to me but like anything else, I can learn.

I'm designing a 15ft wind turbine blade. I was considering using a wood structure + E-glass but now I'm also looking at other alternatives.

Also, the connection, how do you make a positive connection at the blade root? How do you design for a fiberglass to steel connection. I've seen the compression ring method but have no detailed information regarding this approach. Also, when fabricating the two halves for an airfoil, how do you actually combine them? What is the technique used to actually attach one shell to the other?

Can you reccomend any resources for composite design and manufacturing techniques?

Any and all help is greatly appreciated!

Best Regards,

Jim

 

[rstupplebeen]

If you switch to carbon make sure you isolate it from the steel otherwise you can have galvanic corrosion. A couple of layers of glass is usually all that is needed. I hope this helps.

Rob Stupplebeen

 

[cpinz]

Jim,

about the method for the construction of the blade:
-upper and lower shells made in sandwich, using pvc foam or balsa wood as core;
- shells are usually jointed at the leading edge by a splice and structural adhesive, at the trailing edge with adhesive;
- unless the shells are very thick, a spar is used for taking the bending moment;
- the attachement at the hub: in this area the sections are very thick since one method of attachment is by bolts inserted in the root of the blade ( where the section is circular) as "carrot connector".

Now, the description above must , of course, take account of loads acting on the turbine and , as you know, the lower the class of wind, the lower the loads are (Beaufort scale). Nevertheless this imposes a review of the material strenght, that cannot be simply the max. tensile/compressive strenght derived from test on unnotched specimen.
The list and comments made by Brian are "with composite airplanes" in mind, but they are accepted by the wind turbine community since it is a safe way to design primary composite strucures (like a wind turbine blade is ). To apply knockdown factors to the results obtained at coupon level is the first stage to derive the material values that you are searching for;and you will never find values like 275ksi for designing with carbon fiber

See at NREL and Sandia Lab websites: you will find there the literature and some of the "numbers" that you need.

Corrado

 

[Composites_FEA_Guru]

Hi Jim,

The following link may help you out in your design process (particularly page 118 regarding your root connection question):

http://ns7.freeheberg.com/~mach085/Guidelines_for_Design_of_Wind_Turbines.pdf

If you are looking for some software to help out in your design, I would recommend looking into Helius:CompositePro. It has some simple structural analysis tools that allow you to specify a composite layup on a particular beam cross-section to determine the beam response to a chosen loading scenario. There is a large material library that would allow you to switch out materials quickly and compare different commercially available products.

Hope this helps,
-Dan

Dan Milligan
Firehole Composites (

http://www.firehole.com)

FHC provides software and consulting for analysis of composite structures.

 

[bluecomp]

Hi JIM508, how goes your work, have you found a solution?

 

[Jim508]

Hi guys,

Thanks for the great information, it is appreciated. I'm still in the early stages of design, just working on the foundations and gaining an understanding of each system.

I have not settled on one particular design, I'm exploring several different methods as we speak.


Can anyone recommend a good book regarding composite analysis & design procedures?

Best Regards,

Jim

 

[ESPcomposites]

My picks would be:

- Niu, "Composite Airframe Structures"

- Mil-Hdbk-17 - free. (now CMH-17 but not free)

- ASM Volume 21

- For more academic treatment: (Jones, Daniel & Ishai, and Barbero)

Some like Gay and Hoa's book.

Those are mostly analysis books, but have some design mixed in as well.

Brian

http://www.espcomposites.com

 

[Composites_FEA_Guru]

Hi Jim,

I have found "Mechanics of Fibrous Composites" by Herakovich very useful.

-Dan

Dan Milligan
Firehole Composites (

http://www.firehole.com)

FHC provides software and consulting for analysis of composite structures.

 

[RPstress]

Re the original post,

"...values ranging from 275ksi to 512ksi"

These are almost certainly values for the fiber alone. A real carbon/polymer laminate will have values a lot lower, especially if it has to be damage tolerant. More typical values might be in the range of 25 to 50 ksi, perhaps as much as 100 ksi if you're lucky. Be very wary of joints.

"...limitations to how many plies...Is it possible to fabricate box beams with 1 inch webs?"

Laminates of almost unlimited thickness can be made, and certainly 1" is doable. Thicker laminates are harder to make and will have more problems than thinner, but only in degree.

In addition to the references above good practical advice is available free at

http://www.marinecomposites.com/

.

 

[ESPcomposites]

RPstress,

Thanks for the link to

http://www.marinecomposites.com.

Brian

http://www.espcomposites.com