Position and orientation choices on large, rotating power generation components (generators, turbines, etc.) are usually based on a worst case for depth and orientation, relative to the large forging, plate and/or bar that the component is made from, as well as a consideration of principal stresses. For example, a large generator rotor might have radial specimens taken at some depth into the rough machined rotor, in order to meet requirements that are based on a worst-case orientation and depth combination. Keep in mind that these could be rotors with a main body diameter of 70 inches, among other components.
With turbomachinery, that worst case location/orientation tends to be carried through to all other critical metallic components. By the way, "longitudinal" is quite often a best-case location; i.e., optimistic.
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