Response of notched carbon/PEEK and carbon/epoxy laminates subjected to tension fatigue loading

In this study a comparison is made between the tensile static and fatigue b ehaviours of quasi-isotropic carbon/PEEK and carbon/epoxy notched laminates , selected as separate representatives of both tough and brittle matrix com posites. Damage progression was monitored by various non-destructive (ultra sonic scanning and x-radiography) and destructive (deply and microscopic ex aminations) techniques, and by continuously measuring the change in stiffne ss, in order to identify the effect of damage on mechanical properties. The experimental observations indicated that fatigue damage in carbon/epoxy laminates consists of a combination of matrix cracks, longitudinal splitti ng and delaminations which attenuate the stress concentration and suppress fibre fracture at the notch; as a consequence, fatigue failure can be reach ed only after very high numbers of cycles while tensile residual strengths continuously increase over the range of lives investigated (10(3)-10(6) cyc les). Due to the superior matrix toughness and the high fibre-matrix adhesi on, the nature of fatigue damage in carbon/PEEK laminates strongly depends on the stress level. At high stresses the absence of early splitting and de laminations promotes the propagation of fibre fracture therefore resulting in poor fatigue performances and significant strength reductions; while at low stress levels damage modes are matrix controlled and this again transla tes into very long fatigue lives. These results indicate a strong influence of the major damage mechanisms typical of the two material systems on the behaviour of the laminates, with the nature, more than the amount, of damag e appearing as the controlling parameter of the material response up to fai lure.