System for measurement of cohesive laws

Abstract: In this thesis an experimental method to calculate cohesive characteristics for an adhesive layer in a End Notched Flexure (ENF) specimen is presented and evaluated. The method is based on the path independent J-integral where the energy release rate (ERR) for the adhesive is derived as a function of the applied forces and the rotational displacements at the loading point and at the supports of the specimen. The major advantage with the method in comparison with existing theory known by the job initiator is that it is still applicable with ENF specimens that are subjected to yielding of the adherends. The structure of this thesis is disposed so that the theory behind the J-integral method is shortly described and then an evaluation of the method is performed by aid of finite element simulations using beam and cohesive elements. The finite element simulations indicates that the ERR can be determined with good accuracy for an ENF specimen where a small scale yielding of the adherends has occurred. However when a fully cross sectional yielding of the adherends is reached the ERR starts diverging from the exact value and generates a too high ERR according to input data in the simulations, i.e. the exact values. The importance in length of the adhesive process zone is also shown to be irrelevant to the ERR measured according to the J-integral method. Simulation performed with continuum elements indicates that a more reality based FE- simulation implies a higher value of the applied load in order to create crack propagation. This is an effect of that the specimen is allowed to roll on the supports which makes the effective length between the supports shorter than the initial value when the specimen is deformed. This results in a stiffer specimen and thus a higher applied force is needed to create crack propagation in the adhesive layer. An experimental set up of an ENF specimen is created and the sample data from the experiments are evaluated with the J-integral method. For measuring the rotational displacements of the specimen which are needed for the J-integral equation an image system is developed by the author and validated by use of linear elastic beam theory. The system calculates the three rotational displacements of the specimen by aid of images taken by a high resolution SLR camera and the system for measuring the rotations may also be used in other applications than for a specific ENF geometry. The validation of the image system shows that the rotations calculated by the image system diverge from beam theory with less than 2.2 % which is a quite good accuracy in comparison with the accuracies for the rest of the used surveying equipment. The results from the experiment indicates that the used, about 0.36 mm thick SikaPower 498, adhesive has an maximum shear strength of 37.3 MPa and a critical shear deformation of 482 µm. The fracture energy is for this thickness of the adhesive is determined as 12.9 kJ/m2. This report ends with a conclusion- and a suggested future work- chapter.