Torsional Failure Modes
When using a circular tube, we must ensure the wall thickness is sufficient to prevent wrinkling or buckling of the tube wall. A radius to thickness ratio, such as routside / t < 10 or 20, may be specified.
FSAE half-shaft tests
longitudinal vs. cross-sectional shear failure
Equal shear stresses act on both cross-sectional and longitudinal planes. If a material is weaker in one of these directions (e.g. wood), then it will begin to crack in that direction first.
tensile vs. shear failure in torsion
Pure shear on the surface of the bar is equivalent to equal tensile and compressive stresses acting on an element oriented at 45o.
If the material is weaker in tension than in shear, then failure will occur in tension along a helix inclined at 45o to the longitudinal axis. (e.g. chalk, high strength steel)
spiral fracture in bone