Measurement of Epoxy Film Adhesive Properties in Torsion and Tension Using Tubular Butt Joints

Kurzfassung

With the increasing amount of composite materials used for civil aircraft structures, joining and repair methods suitable for composite structures are necessary. Possible methods are structural bonding and bonded repairs. However, a huge challenge is the availability of reliable material properties for the adhesives. A common ap-proach for determining the material properties is to use adhesively bonded tubular butt joint specimens (e.g. ISO 11003-1). Within the Presented work they are manufactured with a new joining device and tested in a biaxial servo hydraulic testing system where tensional and torsional loads can be applied individually but also simultaneously. Thus, the multiaxiality of the bondline stress can be selectively adjusted and the adhesives yielding and strength can be determined. In order to determine stress-strain curves of the adhesive systems to be investigated a local strain or deformation measurements needs to be performed. However, the small thickness (approximately 0.1mm) of an e.g. epoxy film adhesive is a huge challenge. Because of the thin bondline, resulting deformations are very small. Therefore, the measuring method is crucial for a reliable adhesive characteri-zation. Within this paper, two different strain measurement techniques are presented and compared. The first is a high resolution digital image correlation (DIC) system. DIC measuring has the advantage of measuring a complete strain field in each direction within the measurement area. The second method relies on capacitive sensors, which are com-bined in a manner, which allows the decoupled measuring of axial and torsional move-ments of the adhesively bonded adherends. This method is easy to use, online applicable also to fatigue testing but gives only a global displacement information. Both methods and results are compared and discussed.