Mechanical shear properties of adhesives

Abstract

A glued joint is a suitable connection between a glass pane and a framework. The mechanical properties of adhesives are not given in specifications. The most important property of adhesives is the shear - strain relation. Tests are carried out to determine this relation under prescribed conditions. Curing conditions, short-lived increase of temperature, storing time and testing protocol are the same for each adhesive. Two adhesives, polyurethane and epoxy, are tested at a constant deformation rate and at three different temperatures. Polyurethane has a low stiffness which slightly increases with larger strains. The stiffness and shear strength decrease with increasing temperature. Epoxy shows a stiff and toughened shear - strain relation at room temperature. Under frozen conditions, the epoxy has a larger stiffness and shear strength, but fails brittle. The stiffness significantly decrease at high temperature accompanying with large strains. Both\nadhesives are subjected to a short-lived increase of temperature three days after making the specimens. This had favourable effect on epoxy (post curing), leading to a more toughened behaviour. Polyurethane shows smaller values for shear stress and larger values for strain. After longer storing time each adhesive shows a slightly increase of the shear strength. The shear modulus of the adhesives has been determined. The shear modulus of polyurethane can be simplified by a bi- or tri- linear diagram and for epoxy by a linear diagram. Finally, the found shear strength is not the ultimate shear strength, because all test specimens fail adhesively instead of cohesively.