Measuring the shear strength of structural adhesives with bonded beams under antisymmetric bending

Standard tests for adhesive bond characterization suffer from several deficiencies. The simplest specimens to make and test are lap joint geometries (e.g. single, double, symmetric, etc.) that generate complex stress distributions with irregularities and even singularities of the stress state. Those with the stress state closer to pure shear (e.g. napkin ring or Arcan) are difficult to make and require special test fixtures. This paper examines the stress state in the adhesive of a simple beam specimen obtained by bonding two flat plates upon one another and loading the final sandwich in four-point antisymmetric bending. This test improves over a previously investigated sandwich test performed in three-point bending. A simple analytical model is used to optimize the specimen for piece-wise constant shear stresses along the adhesive. The accuracy of the model is validated by finite element analyses, showing good agreement for thin layers of relatively stiff adhesives. A Digital Image Correlation technique is exploited to verify the analytical and the finite element models and the first results confirm the applicability of the method for assessing the shear strength of glassy adhesives.