Shear fatigue of CFRP under low frequency torsion

Abstract

The behaviour of unidirectional CFRP under shear fatigue has been studied by means of low frequency, large amplitude, cyclic torsion experiments. Under both constant shear stress and shear strain amplitude conditions there is a clearly-definable fatigue life at the end of which failure occurs. The fatigue life is related approximately logarithmically to the stress or strain amplitudes. Prior to failure, the compliance increases due to debonding between fibres and matrix, failure occurring when a debonded zone of critical size has developed. The rate of change of compliance increases with stress or strain amplitude. No effect has been observed due to varying resin toughness, nor has any difference between unidirectional and reversed cycling been observed. There is no definite evidence of a fatigue limit as there may be a change in the fatigue mechanism at low strain amplitudes.