US Patent 7214108 Shearable fastener

A shearable fastener has a threaded shank adapted for engagement with a threaded bore and a head portion formed for engagement with a drive tool, the shank being formed with a series of axially separated weakenings defining a series of shear planes. The weakenings are formed such that the applied torque necessary to cause the shank to shear increases progressively from the shear plane furthest from the head portion to the shear plane nearest the head portion. The weakenings are holes, grooves or slits formed in the shank and the radius at the bottom of the holes, grooves or slits is varied to produce or contribute to the increase in shear torque.