This part of the crack is created by shear stresses that cause atoms to slide past each other. The broken carrot shows what is interesting about torsional loading. The magnitude of the normal and shear stresses are equal to each other. So it is common for both brittle and ductile cracks to form in the same part.
The longitudinal cracks are less frequent unless, as in the carrot, there is a naturally weak plane in a direction corresponding with the maximum shear stress. The exception might be a thin disk that is somehow loaded in torsion so that the radial planes have less cross section than the transverse planes, which have less cross section in a more typical long shaft.
We will conclude our discussion of torsion next time.