The shear fatigue of unidirectional fibre composites

Abstract

The shear fatigue of unidirectional composites of carbon, glass and Kevlar 49 fibre, has been studied by low frequency torsion cycling. During either constant shear stress or shear strain amplitude cycling there is initially a slow decrease in shear modulus, strain amplitude increasing and stress amplitude decreasing approximately linearly with log (number of cycles), due to fibre/matrix debonding, until eventually macroscopic cracks initiate and propagate at a clearly definable fatigue life. The rate of change of shear modulus prior to cracking increases, and the fatigue life decreases, with increasing stress or strain amplitude. Fatigue in this mode of testing is considerably more severe than in short beam interlaminar shear fatigue, with fatigue lives of 10 3–10 4 cycles at 50% of ultimate stress or strain.