Bonding of ceramics: An analysis of the torsion hourglass specimen

There is a recent and growing interest in joining ceramic parts due to their increased use in several fields such as next-generation nuclear plants, aeronautic engine parts and aerospace components. For high temperature applications, glass-ceramics are used as an “adhesive” for ceramic parts, this generates the need for test methods suitable to assess their bond strength. Unfortunately, the various test procedures currently used lead to different results.One recent test is based on torsion of hourglass shaped joined ceramics, originated from a modification of the ASTM F734-95 standard, with the aim of obtaining failure under a pure shear state in the bondline subjected to torsion.However, results obtained from different versions of the hourglass geometry show differences which are still difficult to compare. Moreover, due to the brittle nature of the materials and especially when the adhesive strength is comparable to that of the substrates, the failure is not confined in the bond and propagates also in the substrates. In this case, the results are still of arguable application for design purposes.The aim of this paper is to give an insight on torsion of hourglass-shaped joined ceramics and on the interpretation of the obtained results, by means of detailed analytical and numerical studies of the stress distribution in the specimen, and taking into account the brittle nature of the materials. The main findings are: i) the stress state in the bondline is not singular; ii) a non-negligible stress concentration arises out of the bondline.