Dealing with Hollow and Solid shafts....

[pressurised]

When dealing with a bar (fixed on one end), if it is hollow for a certain length from the edge and then turns solid up until the fixed point, can the condition of static equilibrium be used to find the reaction torque, just like you would of a bar with 2 different diameters.
(Assuming all this is of the same material and a torque is provided only on the non fixed end right at the edge)

To me it seems like the same method can be used for this, dividing the bar into segments to use pure torsion formulas and find the variation of shear stress and angle of twist.

 

[Mapes]

Sure, you can split up any object and make free-body diagrams of the components. From the free-body diagrams, you can find (for example) the torque on a uniform part of the composite beam and thus the shear stress in the thin wall.