Measurement of epoxy film adhesive properties in torsion and tension using tubular butt joints

With the increasing amount of composite materials used for aircraft structures, structural bonding and bonded repairs are needed. However, a huge challenge is the availability of reliable material properties for the adhesives. A common approach for determining the material properties is to use adhesively bonded tubular butt joint specimens, tested in a biaxial testing system. In order to determine stress-strain curves of the adhesive systems local strain or deformation measurements needs to be performed. But because of the small bondline thickness of an e.g. epoxy film adhesive, the resulting deformations are very small. Within this study, two different strain measurement techniques are used. First a high resolution digital image correlation (DIC) system. Second capacitive sensors combined in a manner, which allows the decoupled measuring of axial and torsional movements. Both methods and results are compared and discussed. Both have their advantages and the ability to measure the small deformations.