Mechanical Behaviour of Adhesive Layers: Methods to Extract Peel and Mixed Mode Properties

Mechanical Behaviour of Adhesive Layers Methods to Extract Peel and Mixed Mode Properties TOBIAS ANDERSSON Department of Applied Mechanics Chalmers University of Technology ABSTRACT This thesis is concerned with methods to extract material properties of thin adhesive layers loaded in peel and in mixed mode. The first part of the thesis is devoted to an experimental method to determine the complete stress-elongation relation (or cohesive law) for an adhesive layer loaded in peel using the DCB-specimen. The method is based on the concept of equilibrium of the energetic forces acting on the specimen. Two sources of energetic forces are identified: the start of the adhesive layer and the positions of the two acting loads. By use of the concept of equilibrium of energetic forces, it is possible to measure the energy release rate of the adhesive layer instantaneously during an experiment. The complete stress-elongation relation is found to be the derivative of the energy release rate with respect to the elongation of the adhesive layer at its start. By this procedure, an effective property of the adhesive layer is measured. The validity of the approach is investigated by experiments where the adherends deform 1) elastically and 2) plastically. It is found that a unique stress-elongation is obtained when the adherends deform elastically. The same relation cannot be used to predict the experiments where the adherends deform plastically indicating that the approach has limited applicability. The second part of the thesis is concerned with the development of a mesomechanical finite element model of a thin adhesive layer loaded in mixed mode. The model is calibrated to previously performed experiments. In these, the adhesive layer is loaded in monotonically increasing peel or shear. An in situ SEM-study is also performed and used to guide the modelling and calibration. The purpose of the mesomechanical finite element model is to facilitate the development of constitutive laws for adhesive layers. The modelling is based on Xu and Needleman’s method where all continuum finite-elements are surrounded by interface elements that allow for the development of micro cracks. Thus, this enables the modelling of the entire process of degradation and fracture of the adhesive layer. A genetic algorithm is developed for the calibration. The simulations are shown to be in reasonably good agreement with the experiments. Keywords: adhesive layer, stress-elongation relation, J-integral, energetic force, experimental method, RVE, interface elements, genetic algorithm