Torsional Snap-Fit

A lever pivoting on a torsion bar — the classic press-to-release catch. The limit here is a shear strain, which for plastics is about (1 + ν) ≈ 1.35× the permissible tensile strain. Travel comes from the bar being long and thin, not from bending a beam, which is why a torsional snap can be released with very little force and survives far more cycles.

Solid circular torsion bar, small-angle assumption. The shear-strain limit γ ≈ (1 + ν)·ε is the standard plastics approximation — about 1.35× the tensile strain at ν = 0.35. Two bars double the torque for the same twist. Travel scales with bar length and inversely with radius, so a long slender bar is what buys you a soft, long-travel catch; the fillets where the bar meets the lever and the frame are the usual failure sites.