Something isn't right here. What kind of glue are you using for the scarf joints? Are they at least 8:1 scarfs? I'd suspect that an 8:1 scarf glued and clamped with carpenters glue would be stronger than the wood once the glue is fully hardened. What kind of wood are you using? Does it take glue well? How much curve is there in the sheer and do the joints have to be at the maximum curve? (Which I suspect is near either end.)