Mapping fibre failure in situ in carbon fibre reinforced polymers by fast synchrotron X-ray computed tomography

Fast, in situ synchrotron X-ray computed tomography (CT) has been used to capture damage evolution, particularly fibre failures, before final fracture (within 99.9% of the ultimate tensile stress) in cross-ply carbon fibre/epoxy coupons under continuous monotonic tensile loading for the first time. It is noteworthy that fewer than 8% of the fibres in the 0° plies have fractured at 99.9% of the failure load. The majority of fibre breaks appear as isolated events, although some instances of multiple adjacent breaks (clusters) do occur at intermediate and high stress levels. Contrary to conventional wisdom, a cluster of failed fibres always occurred in a burst as a singular failure event: clusters were never seen to accumulate additional broken fibres as load increased suggesting low-level stress concentration local to fibre breaks. Several instances of multiple fractures along individual fibres were observed, providing an estimation of the critical stress transfer length between the fibre and matrix. The factors affecting fibre failure appear to be complex, with distinct sample-to-sample variability being identified for the length-scale tested. This highlights the need for improved understanding of the mechanisms that contribute to final failure, particularly criteria controlling the arrest or otherwise of clustered fracture events.