This paper presents a numerical model of the interface between a quasi-brittle substrate and a thin elastic adherend subjected to mixed-mode loading. The interface is modeled by zero-thickness contact elements, which describe both debonding and contact within a unified framework using the node-to-segment contact strategy. Uncoupled cohesive interface constitutive laws are adopted in the normal and tangential directions. The formulation is implemented and tested using the finite element code FEAP. The model is able to predict the response of the bonded joint as a function of the main parameters, which are identified through dimensional analysis. The main objective is to compute the debonding load and the effective bond length of the adherend, i.e. the value of bond length beyond which a further increase of bond length has no effect on the debonding load, as functions of the peel angle
