Torsion is the name for the loading geometry experienced by the shaft of a screwdriver as the tip is held in the slot of a screw and the operator twists the handle (the important part here) while pushing down to drive the screw (Fig. 1).
The white arrows of the photo of the broken carrot (Fig. 2) show the classical “helical” fracture shape of a macroscale brittle crack due to torsional loading. A piece of chalk twisted will also give a very nice helical crack. These helical cracks are due to normal stresses, or those stresses that cause material to pull away from its adjacent atoms. But notice also that the crack in the carrot has a flat portion, running lengthwise – short purple arrows.
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.
Sometimes, parts that are made from virtually identical material and loaded in very similar ways will break in what appear to be totally different configurations (Fig. 3). These images show broken-off stubs from fancy large torque wrenches. The stub on the left has mostly helical features, while that on the right has a very flat fracture surface. The swirl marks show us that it broke in torsion. Both wrenches were HRC 46. Brittle and ductile crack mechanisms “compete” in torsion. Between the carrot and the ductile hex stub we see that there are two types of ductile or sliding cracks created by torsional loading.
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.