Novel Torsion Machine to Test Adhesive Joints

Adhesive joints are increasingly being used in structural applications due to their improved mechanical performance when compared to other classical mechanical joining methods. In order to predict the mechanical performance of adhesive joints, it is first necessary to determine the mechanical properties of the materials being used (e.g. adhesives and adherends). These properties can be characterized under three basic loading modes: tensile, compression and shear. Tensile and shear tests are commonly used to determine the Young’s modulus, tensile strength, and the strain to failure of materials. However, shear tests under torsion loading have been shown to provide significantly higher accuracy in the measurement of the strain to failure since no stress concentrations occur in the specimen. To explore the advantages associated with shear testing, a novel torsional testing machine was developed. It allows for accurate determination of the mechanical properties of structural adhesives by ensuring correct alignment of the specimens and avoiding any spurious bending moments, compression or tension loads during the tests. Different structural adhesives were selected and characterised under shear loads using this novel machine and the results obtained were compared with those obtained with thick adherend shear tests (TAST) results. This work showed that by performing torsion tests in the newly developed testing machine, the adhesive is properly characterised, especially for the determination of the strain to failure.