Shear strength measurements of ductile polymer films with controlled normal to shear plane angles

Shear strength of the adhesives and polymer films was usually measured by lap shear test method. However, their fracture often associates with the combination of interfacial, interfacial–cohesive and cohesive failures. To confine the shear failure exclusively in cohesion so that the reported shear strength can be regarded as an intrinsic property of the polymer films, in this study we designed a unique test fixture that is capable of controlling the normal to shear plane angle in a confined testing space. Two poly(methyl acrylate-co-methyl methacrylate) film specimens were loaded in series in the test fixture such that as one fractured the other was in a state just prior to fracture and could be used for the investigation of shear bands by scanning electron microscopy (SEM). As the normal to shear plane angle was increased from 30 to 60°, the shear bands were more concentrated on the center region owing to the higher normal compressive stress. For the fractured specimen, the fracture surface investigated by SEM showed the striations which can be related to the shear bands. As the normal to shear plane angle was increased from 30 to 45°, the concentrated shear bands caused less striations perpendicular to the fracture direction. Interestingly, it also leaded to the appearance of the striations parallel to the fracture direction. As the angle was further increased to 60°, small islands formed by the cross-over of parallel and perpendicular striations appeared in the fracture surface. A possible mechanism for the formation of this unique fracture surface was proposed.