Micromechanical influence of near-surface adherend morphology and bondline fillers on the strain distribution in shear-loaded composite bonds

The near-surface morphology in the joining areas of adherends has a significant impact on the fatigue behavior of adhesively bonded composites. In the production process of composites, numerous opportunities exist for dictating surface morphology through the utilization of different top-layer semi-finished products. For instance, release films and peel ply fabrics can be employed to modulate the thickness and topography of the upper resin layer. These materials are subsequently removed prior to the further processing and assembly of the composites, leaving their negative imprint on the material surface. The resultant structures typically exhibit dimensions on the scale of a few micrometers. Another component in adhesive bonds are fillers, which have diameters in the same size range. Both adherend morphology and fillers influence the macroscopic behavior of the bonded joint. By in situ examining bonded end-notched flexure specimens inside a scanning electron microscope, the adhesive layer and the near-surface region of the adherends can be observed in detail under load. Evaluating these scanning electron microscope images using digital image correlation enables quantitative analysis of the strain distribution within the micrometer range. This allows the identification of the effects the adherend's near-surface morphology and the composition of the bondline (e.g. fillers, meshes) have on the shear strain distribution along bondlines. Consequently, conclusions can be drawn regarding their influence on the service life of adhesive bonds.