What it shows:

A spiral fracture is incurred when a torque is directed along the axis of a limb or shaft. Planes perpendicular to the axis are unaffected, but those parallel are twisted, which causes pure tensile forces in one part of the limb, pure compressive forces in another. Fracture occurs when either the compressive or tensile limit of the material is exceeded. This demo shows a spiral fracture in a simulated skiing accident.

How it works:

An old ski boot has a wooden plug placed snugly inside it acting as a foot. A 3 × 4cm square hole accommodates a 0.5m wooden rod of the same cross section which acts as a leg. The boot is attached to a ski. A torque is applied to the leg using a wooden 'key' (see figure 1.) while the ski and boot are fixed in place. Without too much effort, the wooden shaft cracks and splinters in a spiral pattern.

figure 1. The ski boot and torque key

Setting it up:

Mount the ski on the front edge of a lecture bench and use a C-clamp to hold it securely in place. If possible put it towards one end of the bench, so the demonstrator can apply the torque without obscuring the view.

Comments:

Below is a table of breaking points of human limbs. This is a very clear (and graphic) demo of twisting stresses, best saved for the beginning of the winter season.
table 1. Breaking torque and breaking angle for human limbs ¹

1 Kane & Sternheim Physics 3rd ed., Wiley, 1988