Doubly-Curved Interfaces for Adhesion Strength Testing

Abstract

It is worthwhile to have a uniform stress distribution within the interface in order to properly determine adhesion strength between materials. In the case of elastic material properties, most test geometries cause stress singularities at the edges of the sample as well as non-uniform stress distributions along the interface. To avoid this, a curved interface test was proposed in a previous publication by the author that provided the maximum stress concentration at the pole and no stress singularities at the sample edges in the plane of the curvature. However, in the thickness direction, where the sample shows ninety degree corner angles, stress singularities again appear. This paper extends the idea of singly-curved interfaces to doubly-curved interfaces that eliminates this problem. Depending on the ratio of the Young's moduli of the two materials, there exist certain radii of interface curvature for which stress singularities disappear. After the determination of the appropriate radii of curvature and associated wedge angles at the edges of the sample with a 2Dfinite-element model, a 3D modelling of doubly-curved interfaces was carried out for stress calculation. The calculation of stress distribution for elastic materials provides the maximum radial stress at the pole. For non-linear material behaviour of one of the components, this maximum slightly shifts away from the pole. The experiment provides the critical applied load for debonding initiation. Inserting this value into the calculated non-linear relation between the maximum normal stress at the interface and the applied load provides the adhesion strength.