Fatigue behavior of angle-ply AS4/PEEK composite

The fatigue behavior of thermoplastic composite AS4/PEEK was investigated. Tensile-tensile fatigue tests were conducted at three frequencies on straight sided angle-ply specimens. The cyclic stress-strain data and temperature variation during fatigue tests were recorded. The variations of viscoelastic properties during fatigue were used for the study of the fatigue damage mechanisms in the material. The results showed that AS4/PEEK composite may display cyclic softening, hardening or combined hardening/softening behavior depending on the loading condition. Cyclic hardening or hardening/softening occurs at low stress levels and frequencies whereas softening becomes dominant at high stress levels and frequencies. Analysis shows that the transition between the two types of behavior corresponds to a specific temperature rise value, defined as the cyclic thermal softening limit.