## 1 Introduction  
Steel reinforced elastomeric isolators (SREIs) are currently the most used bearings for seismic isolation purposes. They are made with steel reinforcing layers (2-3 mm thick) alternating with layers of elastomer (5-20 mm thick), which are bonded together by means of a bonding compound and a vulcanization process. These isolators are delimited at the top and at the bottom by two 25-30 mm thick steel endplates, which are anchored to the isolator steel end layers and anchor the isolator to the sub- and super-structure. In the process of producing the SREI core, i.e., the central part of the isolator without the thick steel end plates, the steel reinforcements are cut into the desired shape, sandblasted, cleaned with acid, and coated with bonding compound. The sandblasting roughens the steel surface to facilitate bonding with the elastomer. The bonding compound is made of two chemical substances: the primer and the mastic. The primer, which is applied directly to the steel, is meant to improve adhesion between the steel and mastic. The mastic is applied over the primer and glues the elastomer layer to the steel plates. The steel and the elastomer layers are then stacked in a mold and subjected to a pre-determined compression at a temperature and pressure that causes them to be glued together by means of vulcanization, so that they are held together and constitute a single body. Good adhesion is very important in all rubber composite devices for the correct functioning of materials bonded to the elastomer [1; 2; 3; 4; 5; 6; 7; 8; 9]. Moreover, it is important, because the theoretical calculus of rubber composite devices is based on the hypothesis of perfect bonding between the elastomer and the other materials [1; 2; 10]. The adhesion conditions of the steel and elastomer layers, obtained in SREIs as described above, become critical under two particular conditions:  
* the isolator is subjected to direct tensile stresses, i.e., a tensile action is present
* the isolator is subjected to large shear strains, which induce tensile stresses in the isolator extremities bounded by the white dotted line shown in Figure 1.  
The first condition may occur in tall buildings [11], which undergo large overturning moments that induce tensile stresses on the isolators and uplift of the building. However, this is not the only situation in which isolators undergo tensile stress. [PERSON] and [PERSON] [12] investigated the earthquake response characteristics of the isolators in a base-isolated high-rise building with a set-back and found that axial stresses in isolators at the corners below the low-rise part are higher than those in isolators under the high-rise part, with some strong motions inducing maximum tensile stresses exceeding 1 MPa. [PERSON] [13] performed an extended parametric study that revealed that bearing uplift due to tensile action may also occur in isolated bridges, and this effect is more critical for the bearings on shorter piers. This finding was also confirmed by [PERSON] et al. [14]. The standard UNI EN 15129 [15] takes into account the possibility that the isolators are able to undergo tensile stress; however, to avoid the cavitation of the rubber the value of this load is limited to 2G, where G is the isolator shear modulus measured at a 100% strain. The strain is calculated as the ratio between the maximum applied displacement and the isolator rubber total thickness. Since the G values provided by the isolator manufacturers generally range from 0.4 MPa to 1.4 MPa, the above limit ranges from 0.8 MPa to 2.8 MPa. The second condition is where tensile stress arises in the isolator due to large shear strains, even if the isolator is subjected to compression. [PERSON] et al.