Talk Composites - The Forum for Advanced Composites

Strength of carbon composite—sound logic?

http://www.talkcomposites.com/Topic20485.aspx

By njl - 2/4/2016 7:36:05 PM

Hi all,

I'm new to this forum and new to carbon fiber composites in general.

My current project is a mast for a sailboat dinghy.

As my first question on this forum, I'd like to ask if my logic is sound regarding the strength of a carbon fiber component.

I have a hollow wooden mast 12 feet (3.6 meters) high and would like to replace it with a carbon fiber mast of the same diameter.
The wooden mast is 3 inches (75 mm) in diameter with a wall thickness of 0.75 inches (20 mm).

(The carbon cloth I plan to use will be bi-axial twill sleeve, along with West System epoxy.)

When I compare the material properties of the wood to the carbon fiber, I find:
in compression, carbon fiber composite is between 12—16 times stronger
and in tension, carbon fiber composite is between 9—12 times stronger.

As I understand it, I can build the dinghy mast with the same height and diameter.
If I divide the wall thickness of the wooden mast by 9, I get 0.08 inches (2 mm) for the corresponding carbon fiber mast.

So a carbon fiber mast with a wall thickness of 0.08 inches (2 mm) would be similar in tension and much stronger in compression than the corresponding wooden mast.

Is my logic sound?

Thanks for your help!
By Chris Scott - 2/5/2016 4:07:50 PM

Having the angle less than 45* will give additional torsional stiffness, but will reduce the stiffness along the length of the mast (compared to a unidirectional fiber).

As a disclosure, my structural design experience is related to aircraft, not boats.  So I'm not sure exactly what the design criteria would be for a boat mast.  I imagine you have to balance between stiffness and toughness... a mast too stiff might fracture, so I'm sure you want some flexibility to go into it.  This is why they taper towards the end.

I would agree that you would likely need a combination of fabrics and orientation to achieve the overall strength and stiffness... some unidirection fibers to help with the bending of the mast and some biaxial to help retain the cross sectional shape and torsional loads.  This will also help with the overall durability.

I suggest checking out some forums that deal directly with boat/mast construction possibly?