# Torsion Test vs. Other Methods to Obtain the Shear Strength of Elastic-Plastic Adhesives
## 4 Conclusions  
On the basis of an experimental campaign and an analytical model, this work tried to shed some light on the assessment of the shear strength in the case of a ductile (elastic-plastic) joining material. Logically, different testing techniques, if correctly applied, should give the same result, with the same failure mode.  
The starting point accounted for the stress distribution, under an elastic regime, for the different specimens: (i) the torsion hourglass specimen exhibited a slightly non-linear distribution that increased from its centre to the periphery; (ii) the 4-point asymmetric specimen exhibited a parabolic distribution, which peaked at mid-height; (iii) the single lap offset specimen exhibited a nearly constant stress distribution (apart from peaks in the ends of the overlap) in the version adopted within this work, which created a nearly uniform longitudinal relative displacement of the adherends, thanks to their stiffness and to the particular fixtures used.  
When the joining material yielded, under a perfectly plastic regime the stress distributions in the related zones became approximately constant, making it easy to assess the shear strength. The same applied also to the single lap offset specimen, provided that the joining material was ductile enough to smooth local stress and concentrations in the overlap ends (as it occurred with the adhesive tested in this work). If this was not the case (e.g., different fixtures, brittle joining materials, etc.), the single lap offset test, or other lap tests, should be used for comparative purposes only and not for assessing the shear strength of the joining material.  
Thus, if the torsion hourglass test is adopted, the experimental strategy to deal with the effect of plasticity would be to use hollow specimens (TDHG) with increasing inner diameters until the obtained strength (simply evaluated as in the elastic regime) converged to a constant value. However, difficulties can arise from the practical viewpoint, as bonding the specimen on a thin ring-shaped area is challenging. Therefore, the simple model presented in this work can be used to evaluate the shear strength by fitting the measured moment-rotation curve obtained from a full specimen (THG) with the analytical one. It can be remarked that in this way the shear modulus can also be obtained by fitting the initial linear part of the curve.  
**Author Contributions:** Validation, M.S., V.C. and S.D.L.P.; formal analysis, L.G. and A.B.; data curation, L.G. and A.B.; writing--original draft preparation, M.F. and L.G.; writing--review and editing, A.B.; supervision, M.F. and M.S.; funding acquisition, M.F. All authors have read and agreed to the published version of the manuscript.  
**Funding:** The research leading to these results received funding from the European Community 7 th Framework Programme (FP7.2007-2013, call identifier FoF.NMP.2013-10) under grant agreement n\({}^{\circ}\) 609188, within the project \"Advanced MANufacturing routes for metal/COMposite components for aerospace\" (ADMACOM, [[https://cordis.europa.eu/project/id/609188/it](https://cordis.europa.eu/project/id/609188/it)]([https://cordis.europa.eu/project/id/609188/it](https://cordis.europa.eu/project/id/609188/it)) (accessed on 15 March 2022)).  
**Conflicts of Interest:** The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.