A micrographic study of bending failure in five thermoplastic-carbon fibre composite laminates

The local deformation and failure sequences of five thermoplastic matrix composites were microscopically observed while bending the samples in a small fixture attached to a microscope stage. The thermoplastics are polycarbonate, polysulphone, polyphenylenesulphide, polyethersulphone and polyetheretherketone. The composites made from these plastics contain a variety of carbon fibres, though all with similar properties, and have fibre volume fractions ranging from 32 to 66%. Comparison is made to an epoxy matrix composite, 5208-T-300. Laminates tested are (0/90)_2S, with outer ply fibres parallel to the beam axis. Four-point bending is used at a typical span-to-thickness ratio of 39:1. A shallow notch is put in the samples at mid-span to avoid failure under the loading pins. It was found that all the thermoplastic composites failed by abrupt longitudinal compression buckling of the outer ply. Very little precursory damage was observed. Micrographs reveal typical fibre kinking associated with longitudinal compression failure. Curved fracture surfaces on the fibres suggest they failed in bending rather than direct compression. Delamination was suppressed in the thermoplastic composites, and the delamination that did occur was found to be the result of compression buckling, rather than vice versa. Microbuckling also caused other subsequent damage such as ply splitting, transverse ply shear failure, fibre tensile failure, and transverse ply cracking.