Fibre failure assessment in carbon fibre reinforced polymers under fatigue loading by synchrotron X-ray computed tomography

In situ fatigue experiments using synchrotron X-ray computed tomography are used to assess the underpinning micromechanisms of fibre failure in double edge notch carbon/epoxy coupons. Observations showed fibre breaks along the 0° ply splits, associated with the presence and failure of bridging fibres, as well as fibres failed in the bulk composite within the 0° plies. A tendency for cluster formation, with multiple adjacent breaks in the bulk composite was observed when higher peak loads were applied, exceeding 70% of the ultimate tensile stress. Ex situ fatigue tests were used to assess the accumulation and distribution of fibre breaks for different loading conditions, varying peak load and number of cycles. A direct comparison with the quasi-static case for an equivalent peak load, considering the same material system and geometry, has shown that fatigue produces a significantly higher number of fibre breaks. This supports the hypothesis that fibre breaks are indeed caused by the load cycling.