Effect of span length on micromechanics of fracture under flexure load in CFRP composites

Unidirectional (UD) and bidirectional (BD) woven carbon fiber reinforced plastic (CFRP) composites were tested under three-point flexure at different span lengths ranging from 20 to 60 mm. Expectedly, flexure load decreases with increasing span length. A sharp change in flexure load is observed at a span length of 30 mm with a transition in failure mechanism. Analysis suggests that crack initiation occurs at compression side and tension side, respectively, in UD and BD woven composites. In case of UD composites, at a span length of 20 mm, the failure was governed by kink band initiation (two fractures per fiber) followed by tension side delaminations, and at other span lengths, microbuckling (multiple fiber fractures) together with compression side delaminations led to failure. On the other hand, the failure in BD composites at a span length of 20 mm is controlled predominantly by thickness crack propagation with tensile fiber pullouts, whereas for other span cases the failure occurs due to the development of delaminations at interlace regions. The stress analysis by finite element modeling (FEM) also supports the experimental observations in an attempt to correlate the failure mechanisms under flexure loads in CFRP composites.