Stress mapping reveals extrinsic toughening of brittle carbon fiber in polymer matrix

We conducted an in situ study on CFRP fracturing process using atomic-force-microscopy-based stress-sensitive indentation. Tensile stress distribution during fracture initiation and propagation was directly observed quantitatively. It led to a discovery that previously believed catastrophic fracture of individual carbon fiber develops in a controllable manner in the polymer matrix, exhibiting 10 times increase of fracture toughness. Plastic deformation in crack-bridging polymer matrix was accounted for the toughening mechanism. The model was applied to explain low temperature strength weakening of CFRP bulk material when matrix plasticity was intentionally 'shut down' by cryogenic cooling.