19 - Failure models for composite joints: An approach based on singular stress states

Abstract

In the present chapter, the failure prediction of composite joints is addressed in the framework of failure induced by the presence of singular stress states at the multi-material corners appearing at the end of the overlap region. A comprehensive description of the singular stress characterization at multi-material corners of composite materials is presented. The asymptotic stress field is defined by the stress singularities (characteristic exponents), the characteristic angular shape functions, and the generalized stress intensity factors (GSIFs). The singular stress field at these multi-material corners is used to predict failure initiation, using a fracture mechanics approach in which the GSIFs are compared with their allowable values, namely the generalized fracture toughness values. An experimental procedure based on the Brazilian disk specimen is proposed to determine the generalized fracture toughness of the corner, and a failure envelope and a failure criterion are proposed based on the singular parameters of the corner. Experimental tests on double-lap joints are in good agreement with the proposed failure envelope. Finally, and to facilitate the application of this proposal to real design processes, a parametric analysis was carried out to calculate the GSIFs in a wide range of geometrical and mechanical combinations of the single- and double-lap joint configurations. The results have been presented in a graphical form, allowing the GSIFs to be calculated by means of a simple structural analysis of the joint.