Splitting-related kinking failure mode in unidirectional composites under compressive loading

The longitudinal compressive strength of unidirectional composites is empirically found to depend on many factors including the composite fabrication and testing techniques [1,2], the strength and modulus of the matrix or fibre [3, 4], the shear properties of the composite [5], poor alignment of the fibres [6] and composite defects [7]. However, an analytical understanding of what factors affect the compressive strength of fibre composites has been slow to develop. In reality, a typical compression failure mode in carbon fibre-reinforced plastics (FRP) and organic FRP is so-called kinking, which is the formation of reoriented material bands on a plane at some angle to the direction of loading [8, 9]. The kinking was observed in unidirectional glass and boron FRP [8, 10], and in multidirectional fibrous [11] and cloth [29] laminates under compression or bending [12]. There are a variety of opinions about the kinking failure mode. Some scientists have presented an idea that kinking was initiated by microbuckling of fibres in small volumes of a composite near defects [7], free edges [11, 13] or voids [14]. Budiansky [15] treated kinking as one of the specific forms of elastic fibre buckling when the direction of buckling was not perpendicular to the reinforcement axis. The compressive strength, cry-, in this model is given by [15]