Failure mechanisms in thick composites under compressive loading

Failure mechanisms were studied in a unidirectional carbon/epoxy composite under uniform and linearly varying longitudinal compression. The first failure mechanism is shear yielding or shear failure in the matrix precipitated by initial fiber misalignment. It was shown how an initial fiber misalignment of 1.5° can produce the measured compressive strength of 1725MPa (250 ksi). Matrix failure is followed by fiber buckling and fracture, resulting in the formation of a kink band. The kink band orientation is constant in the range of β = 20-30°, whereas the kink angle α varies from a small initial value to a maximum value of 2β. Kink band widths varied between 4 and 20 fiber diameters. Kink bands can occur on different planes which can rotate along the band. Kink band multiplication or broadening with increasing stress was observed at points where the maximum kink angle α was reached.