Spar shear stress
Spar shear stress
(OP)
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") 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.






RE: Spar shear stress
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.
RE: Spar shear stress
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
RE: Spar shear stress
RE: Spar shear stress
RE: Spar shear stress
Dave
RE: Spar shear stress
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 ?