Contents lists available at ScienceDirect International Journal of Adhesion and Adhesives journal homepage: www.elsevier.com/locate/ijadhadh Experimental validation of a simple shear strength model for hybrid friction- bonded interfaces M. Ragni ⁎ , D. Castagnetti, E. Dragoni Univ. of Modena and Reggio Emilia, Via G. Amendola 2, 42122 Reggio Emilia, Italy ARTICLE INFO Keywords: A. anaerobic B. interfaces C. mechanical properties of adhesives constitutive model ABSTRACT The paper deals with the experimental measurement of the shear strength in hybrid interfaces, press fitted and bonded with anaerobic adhesives. The aim is to validate and improve the applicability of a constitutive model, which describes the interface behavior up to complete failure, by combining a cohesive law with a pure friction law. This paper presents an extensive experimental test plan, which deeply investigates the shear strength of two strong anaerobic adhesives, over four nominal contact pressure levels. The tests involve cylindrical specimens, butt bonded and pressure reinforced over an annular surface, and seven replications, giving 56 tests. The ex- perimental torque-rotation curves up to complete failure highlight a similar response between the two adhesives, and confirm that the strain energy up to complete failure sums up a cohesive term and a pure friction term. In addition, the main parameters of the model linearly depend from contact pressure through simple relationships. 1. Introduction The paper experimentally investigates hybrid interfaces, which are pressure-reinforced and bonded with anaerobic adhesive. Anaerobic adhesives are a simple way to remarkably improve the responses of friction joints, where the coupling forces are provided by mechanical clamping [1–6]. Despite the broadband application of hybrid interfaces in mechan- ical couplings, a constitutive model describing their elastic and post elastic response, up to complete failure, it is not available. Some re- searchers [7–11] suggest that the static shear strength of hybrid friction bonded joints sums up two contributions: the shear strength of the adhesive and the friction between the interfaces of the joint. Other experimental tests (Dragoni and Mauri [12]), show that the contact pressure promotes an increase of the shear strength in the hybrid joint, but, in addition, the type of adhesive affects the increase rate: the stronger the anaerobic adhesive, the higher the shear strength increase rate. In particular, above a given contact pressure, a weak anaerobic adhesive lowers the joint strength, compared to that of the dry inter- face, thus acting as a lubricant. By relying on these experimental results, Dragoni et al. [12–14] propose a simple micro-mechanical model, which assumes that a thin adhesive layer always separates the adherend surface protrusions. First, the model suggests that the adhesive significantly improves its shear strength when loaded by the high local pressure occurring between the adherends protrusions. Second, the curves of the experimental shear stress versus relative sliding of the hybrid joint up to complete failure are the combination of a cohesive fracture energy and a pure friction strain energy. The applicability of this simple model has been confirmed both by preliminary systematic experimental test plan [15,16] and by a mi- croscopic-scale finite element simulation plan [17,18]. These pre- liminary studies support the hypothesis that a thin layer of anaerobic adhesive always separates the roughness protrusions of the adherend surfaces. In particular, the tests on a weak and a strong anaerobic ad- hesive [16] clearly show that the curve of the shear strength of the hybrid joint as a function of the relative sliding linearly increases with the nominal contact pressure in the joint. Moreover, these tests confirm the different increase rate of the shear strength between different anaerobic adhesives, and highlight that all the parameters describing the curve, with exception of the elastic stiffness of the strong adhesive, linearly depend by the contact pressure. In addition, the strong anae- robic adhesive shows a higher scatter in the results, which prevents a reliable identification of the model parameters. Considering the large use of strong anaerobic adhesives, for example to increase the capability of power transmissions in industrial appli- cations, this work improves the preliminary investigation performed in [16] by focusing specifically on strong anaerobic adhesives. Two are the aims of the work: first, to definitely validate the proposed micro- mechanical model by performing an extensive systematic experimental test plan, involving a statistically significant number of replications. Second, to assess, clearly, the response of strong anaerobic adhesives, in https://doi.org/10.1016/j.ijadhadh.2018.02.026 ⁎ Corresponding author. E-mail address: marina.ragni@unimore.it (M. Ragni). International Journal of Adhesion and Adhesives xxx (xxxx) xxx–xxx 0143-7496/ © 2018 Elsevier Ltd. All rights reserved. Please cite this article as: Ragni, M., International Journal of Adhesion and Adhesives (2018), https://doi.org/10.1016/j.ijadhadh.2018.02.026