Fracture mechanics concepts applied to a debonded fibre in composites

Fibre reinforced composites (FRCs) are complex multiphase materials, typically with high mechanical properties. The loss of bearing capacity of such a class of materials is frequently related to the so-called debonding phenomenon, i.e. a matrix-fibre detachment, fibre breaking, matrix cracking, etc. In the present paper, the fibre-matrix debonding is examined by using a fracture mechanics approach. The stress singularity arising at the extremities of a perfectly bonded cylindrical fibre is obtained, and the related Stress-Intensity Factors (SIFs) employed to evaluate the detachment initiation at the fibre extremities are determined. Once the detachments have occurred, the circular shape boundary line, representing the edge of the debonded region, is analysed as a 3D crack front located between two different materials, and the Stress-Intensity Factors are evaluated for different remote loadings and composite characteristics. The proposed approach can usefully be adopted for studying the mechanical behaviour of fibre reinforced composite materials with a dispersion of short fibres variously arranged in the bulk material (unidirectional or randomly distributed fibres can be taken into account). Both static and cyclic load can be considered, and the fibre debonding can easily be quantified by the knowledge of the fibre-matrix interface fracture toughness.