The problem (engineering p.e. exam prep question):
a shaft, composed of an outer (aluminum) shaft fitted into an inner (brass) shaft; the questions asks what the maximum shear stress for each shaft would be;
after finding the shear modulus of each, determining the stronger material and putting it (via shear modulus ratio) in terms of the weaker material and adding the total equivalent polar moments together, it then solves for the maximum torsional shear of each material in the fitted shaft.
My confusion comes in solving for the max shear (shear = toque x radius/polar moment). For the outer aluminum shaft, the outer radius was used in finding shear, because the torque is highest there; but on the inner, brass shaft, the inside radius of the (hollow) brass shaft was used--- why? It seems to me that the outer radius of the brass shaft, even though it is snug against the inner radius of the aluminum shaft, should be used to find max shear.