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 (SRCT) are used to assess the underpinning micromechanisms of fibre failure in double notch carbon/epoxy coupons. Observations showed fibre breaks along the 0º ply splits, associated with the presence and failure of bridging fibres, as well as fibres failed in the bulk composite within the 0º 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 strength. 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