Spar shear stress 1

[Chickenhawk]

I have constructed an experimental aircraft spar of wood and pultruded carbon fibre. The wood core is 1/2" by 1" and two flat rods (0.092"*0.220&quot are epoxied flat side down, side by side, on both top and bottom wood surfaces. The spar is lightly loaded in vertical shear which is handled by the wood core. The spar caps of pultruded fiber have been loaded succesfully to twice the max. bending moment. Axial load between compression struts is the most critical loading factor and the buckling tendency is taken care of by additional epoxied carbon fibre strips in the critical areas on the sides of the spar. FEA analysis indicates all is well. I would like to ensure against delamination of the fibre rods from the wood by wrapping an epoxied fibre tow around the spar but not to the extent that the fibre takes the shear and not completely enclosing the spar. Like wrapping at 45° diag. one way with a matching 45° the opposite. Or just vertical wraps at 3" centers or so. Any suggestions as to how to accomplish this without shear problems in the fibre would be appreciated.

 

[ishvaaag]

It seems you want cable-like behaviour for the around tying strip. This would mean the strip be nonattached to the wood sides. In any case rarely it should be more critical than corners, since I assume there you will be having quite a sharp bend, and there you distroy the fiber continuity, then having of need to rely in the bindng agents to pass the forces. There are also the alternatives of making the 2 contrary strips coincicing at corners or not. Not coinciding probably will lead to a cleaner aspect of the thing, and probably will preserve a bit the confining strips of the (I presume vibrating) shears you don't want at all by them taken.

In my view, an all bound strips solution will behave better because the binding agent will retard of the apparition of the effects of some particular fibers being broken, also contributing to alternative loadpaths when required.

 

[Chickenhawk]

I appreciate the reply Ishvaaag..
I was thinking of maybe multiple wraps of carbon fibre tow along the same path (repetitive wraps)for depth, with epoxy resin matrix and bonded directly to the wood surface. In my mind if these wraps are near vertical and spaced 2-3" apart they may not compete with the wood for the shear stresses in the beam and serve mainly as binding forces or essentially additional bonding surface for the cap strips. The added pultruded fibre strips on the sides for anti-buckling create essentially an I beam in those areas making it difficult to completely wrap the entire surface. (It would also cover the wood completely..which looks pretty neat sandwiched between the fibre strips and would add some weight) I'm out of my league here some in determining the possible problems.

 

[butelja]

Good luck chickenhawk. For liability reasons, I wouldn't touch this one with a 10 ft (or 3 m) pole.

 

[ishvaaag]

In account of the cautionary entry by butelja and your further explanations, consider if out of construction and boundary constrictions your thing can in some way lose its general shape, and if yes, if by some stiffener or so you can prevent it.

 

[Chickenhawk]

Butela..you have succeeded in making me anxious. Try a 20' pole (6.1 meters) or just enough to let me know your concerns. No liability in expressing a concern.
Dave

 

[RiBeneke]

I am not an expert here, and will just comment :
1) Wrappings at 45 deg to the spar will probably 'absorb' a large amount of any shear strain that is present, and to avoid this, wrappings at 90 deg to the axis of the spar would be better.
2) If delamination of the reinforced edges is a concern, then what about the shear stresses that must exist at the glue interface between the wood and the reinforcement. These must exist to transfer load to the reinforcing edge strips. Have you modelled these in your FEA ?