INTERFACE MIXED MODE MODEL R. Walter, J. F. Olesen and H. Stang Department of Civil Engineering, Technical University of Denmark, Kgs. Lyngby, Denmark ABSTRACT A mixed mode model, based on fracture mechanics is presented. The model takes into account gradual softening of shear and normal stresses for normal and sliding crack propagation. The model is introduced and implemented in a commercial finite element package using user-supplied subroutines. Finally, an example using the model is illustrated on debonding of a cement-based overlay bonded to a steel plate. 1 INTRODUCTION The present study has its origin in an ongoing research project concerned with the strengthening of steel bridge decks using a cement-based overlay [1]. An interface mixed mode model is presented and applied to study the debonding mechanism between a cement-based overlay and a steel plate. A debonding process is often considered a discrete process and modelled as taking place at the bi- material interface. An appropriate model should take into account softening behaviour and the mixed mode nature of the problem involving both shear and normal stresses at the interface. Much work has been done in the field of softening models for quasi-brittle materials and the present model has its origin in the fictitious model, FCM, by Hillerborg [2]. His concept of a tension softening stress-crack opening relationship in concrete has reached vast acceptance. Less work has been carried out on constitutive modelling of concrete under mixed mode loading. Since the most severe load case for many interfaces is mode 1 loading, emphasis has consequently been placed on making a material description for this case. In the following analysis emphasis is put on opening of an interface in a combined opening mode of shear and tension. The present work is carried out in the framework of Wernersson [3]. 2 MODEL Consider two stress-crack opening relationships: IJ(δ t ) and ı(δ n ), cf. Figure 1. The curves are described in two parts: The first ascending part is from zero stress to peak stress and is characterized by a very large stiffness, D t and D n , to model initial continuous geometry of the interfacial zone. The post peak behaviour is described by a descending, softening part, which relates the stress acting across the crack to the normal opening (δ n ) or sliding (δ t ). Figure 1: Uniaxial stress-crack opening relationships in (a) pure mode I and (b) pure mode II. The curves are described by a stiff, linear, ascending part until peak stress and a multi linear post peak softening part.