Competition between deflection and penetration at an interface in the vicinity of a main crack

The mechanisms which govern crack deflection and crack penetration at interfaces must be understood in order to design composites and layered materials. Experimental observations have shown that a realistic description of crack deflection must take into account the initiation of fracture mechanisms by the stress field of an approaching matrix crack. Fracture mechanisms which include interfacial debonding and penetration are thus analysed in the vicinity of a main crack. For this purpose, a unit cell consisting of a single fibre surrounded by a cylindrical tube of matrix is studied with the help of a finite element model. Initiation stress and nucleation length are determined for both mechanisms by using an initiation criterion which requires to fulfil an energy and a stress condition. Investigating the competition between the initiation of the two mechanisms provides decohesion/penetration maps which depend on the strength and toughness of interface and fibre. It is shown that the debonding or penetration condition can be reduced to an energy or a stress condition depending on the relative value of some characteristic fracture lengths of interface and fibre. Finally it is noted that a low toughness interface is not systematically a sufficient condition to promote the initiation of deflection.