AN ANALYSIS OF THE INTERFACE SHEAR STRENGTH IN DAMAGED UNIDIRECTIONAL COMPOSITE MATERIALS

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Shear resistance along the interface between matrix and fiber in composite materials shears a large part of the fracture energy, although there is a variety of microscopic damage propagation process. This shear resistance has been investigated by using a single fiber model or interlamina shear strength test indirectly. The final goal of this research is to measure the shear resistance as a function of shear damage propagation n a microscopic sense. Recent related results are as follows.(1) A basic failure model being composed of a microscopic matrix crack perpendicular to the fiber and a debonding crack starting from the microcrack and propagating parallel to the interface shows that the debonding tries to close after small amount of its propagation less than a fiber diameter. This suggests the strong shear resistance against the debonding propagation due to the friction.(2) An exact solution of the stress distribution in the above model is presented with various parameters of material constants.(3) A joint for the kevlar rope using a metal socket is shown to have the similar characteristics as the microscopic damage propagation in composites. It supports the load with the following damage propagation, that is, shear fracture of the interface > slip > normal stress at the interface > friction > increase of rope axial force. There is no strong stress concentration due to slip. The effects of material constants and geometrical parameters on the normal stress and rope axial force need further studies.(4) An new method to handle the caustics of an interface crack is proposed to obtain the shear mode stress intensity factor. The previously proposed method needs the location of points, which do not converge practically.