# Torsion Test vs. Other Methods to Obtain the Shear Strength of Elastic-Plastic Adhesives
## 1 Introduction  
Assessing the shear properties, in particular the strength, of an adhesive is a goal of great interest to achieve, as witnessed by several contributions in the past and recent literature [1, 2, 3]. Indeed, before other more sophisticated parameters, such as the fracture toughness, the shear strength is the first design value of practical use, especially in an industrial scenario. This static parameter is very important because it can be taken as a starting point for more in-depth analyses, such as the cyclic response [4]. Additional problems arise in the case of brittle (or nearly brittle) adherends such as ceramics, due to the difficulties in manufacturing the specimens and applying the test load. For this class of materials, a specific standard is available [5] based on the 4-point asymmetric bending (A4 PB in the following) test. Its shortcomings, thoroughly discussed in [6], are briefly summarized here: the samples for A4 PB tests need precise alignment during preparation and testing (which can be difficult in the case of miniaturized samples); they should not be prepared one by one; A4 PB is not usable if the bond strength is greater than 50% of the adherent bending strength; and \"U\" or \"V\" notches are required in the joined region if the bond strength is greater than 25% of the adherent bending strength. Obtaining a reliable value of shear strength for joined samples, not affected by other stress components and stress concentrations present in the commonly used tests, is a well-known problem: in this view, simple lap shear tests are not suitable, as recognized also by the disclaimers present in some standards, e.g., ASTM D905, ASTM D1002 [7, 8]. Therefore, results from lap tests should, in general, be used only for comparative purposes and not to evaluate the shear strength for design purposes. A particular type of lap shear test is the single lap offset (SLO in the following) test in compression, a modification of the ASTM D905 (designed for wood substrates) in which, together with thick and stiff substrates, ad hoc fixtures are adopted to keep these substrates as parallel as possible during the test to minimize peel stress due to rotation and/or bending of the substrates [6]. Torsion represents a suitable alternative to other methods to provide reliable shear strength values. Recently, an hourglass shape of the specimen (THG in the following) was proposed, which allowed one to obtain a bond of a reduced circular section. Advantages of this choice, reported in references [9; 10; 11], are mainly represented by the pure (although not uniform) shear stress state for the joined section under torsion, and by the possibility to obtain failure in the bond in case of a high-strength joining material. Another popular configuration for adhesive testing is given by the Arcan fixture [12; 13], in which the joint specimen, formed by two (relatively thick) plates bonded by an adhesive layer, can be loaded under different adjustable directions, which are variable between pure normal (tension or even compression) and pure shear, with intermediate \"blended\" cases. Special modifications have been designed [14] to minimize edge effects in the bondline ends. However, in the scenario of the present work, the Arcan scheme would not behave differently from a lap shear scheme, therefore it was not adopted. A previous paper from our group [10] considered the features of the torsion test in the case of a brittle adhesive and THG specimen. In particular the advantage represented by the lack of stress singularity at the adhesive-adherent interface, and a moderate notch effect (\(K_{t}\) = 1.2-1.3), was due to the non-constancy of the cross section. Most of adhesives' technical datasheets just provide lap shear strength values for joined components, often without specifying the way it is measured and if the adhesive has plastic or brittle behavior. This paper tries to fill this gap by providing a procedure for evaluating the \"true\" shear strength for adhesively joined components.