Microbuckle initiation from a hole and from the free edge of a fibre composite

Microbuckle initiation from an open hole, from a ®lled hole, and from the free surface of a unidirectional ®bre composite is examined by a ®nite element method. The role of ®bre bending resistance is included in the formulation so that a hole size e€ect is predicted. It is found that the open hole knocks-down the compressive strength to the order of 20tY, while the ®lled hole reduces the strength to about 40tY, where tY is the in-plane shear modulus of the composite. Pre-existing ®bre waviness adjacent to the hole gives a further reduction in the compressive strength. The role of in-plane shear stress and transverse stress in reducing the open hole compressive strength is examined: the compressive strength is reduced by the presence of shear and transverse tension, but the e€ect is less pronounced than that observed previously for the case of a circular patch of waviness. The study concludes with an investigation of the in¯uence of the free surface on microbuckle initiation from a band of waviness and from a semi-circular patch of waviness: a negligible e€ect of the free surface on the compressive strength is predicted. #