The development of torque in biaxial rotation fatigue testing of fibres

Abstract

In fatigue testing by rotation of a bent fibre, which leads to rupture by multiple splitting in a way similar to failure in use, a high level of twist develops. This is real twist when the drive is from one end, and false twist in opposite directions in the two halves of the specimen when the drive is from both ends. When rotation is over a pin, surface friction will be a source of torque, but the twist levels are surprisingly high and they also occur in freely bent monofilaments without any frictional forces. The torque is attributed to the need to overcome the energy dissipated through bending hysteresis, which leads to a change in the direction of the moments. A rough calculation shows that the internal friction will dominate except in fibres with very low loss. It appears to be the combination of cyclic bending with torsional shear stresses which causes the rapid breakdown of fibres.