# Experimental Investigation of a Device to Restrain the Horizontal Sliding of U-FREIs
## 8 Test Results and Observations
### Shear Test Results  
#### 8.2.1 Bond Test  
Figure 11 shows the load-displacement curves of all the performed bond tests. Each diagram reports the results relative to pairs of identical specimens according to the test setup (Figure 7), where the force has been divided by two to represent the single isolator behavior. Hence, on the diagrams, the letter Q is reported for specimens Q1 and Q2, and the letter B for specimens B1 and B2. It is noted that the Q specimens were subjected only to tests without constraint and with constraint on one surface, because during the latter test, damages occurred in the elastomer and in the metal hooks. Although these damages also occurred in the B specimens during the test with one constraint, tests with two constraints were also carried out on these specimens. For the B specimens, the tests were performed until the compression stress of 0.5 MPa, at which point bond failure occurred.  
The isolator sliding instability due to bond failure occurs when the force-displacement curve reaches a maximum force and then shows a softening branch. For the Q specimens, in the case without constraints, this occurred at a compression stress of 0.375 MPa (Figure 12a), while it occurred at 0.25 MPa in the case with one constraint (Figure 12b). Therefore, it can be said that the presence of the constraint improved the isolator bond behavior, moving bond failure to lower compression stresses.  
For the B specimens, bond failure occurred at the compression stress of 0.5 MPa in all cases (Figure 12c). The best performance was given by the case with one constraint, for which increases of 13% of the maximum force and 15% of the corresponding displacement were observed. The improvement given by two constraints was a little lower, probably due to the damage that occurred during the test with one constraint.  
In the specimens subjected to sliding due to bond failure, it was observed that the hooks with less stiffness and flexural strength in the direction parallel to the load application bent and ceased to resist (Figure 13a).  
Figure 10: Average K\({}_{V}\) values at third cycle and K\({}_{V}\) variations due to the presence of the constraints.  
Consequently, the isolator began to slide, and its external elastomer layer in contact with the Metaklett strip suffered a superficial incision (Figure 13b) exerted by the metal hooks orthogonal to the loading direction, which did not bend. This type of damage could be avoided if the hooks have the same stiffness and flexural strength in both directions. Regardless, for the B specimens, after having straightened the hooks bent during the test with one constraint (Figure 11b), and having rotated the isolators 90 degrees with respect to the loading direction, the test with two constraints was performed (Figure 11c). The observed behavior was similar to that obtained by the configuration with one constraint for compression stresses greater than 0.5 MPa. This means that the damage on the external elastomer layers did not greatly influence the isolator behavior under shear deformation.  
Figure 11: Force–displacement curves of bond test: (**a**) without constraint; (**b**) with constraint on one contact surface; (**c**) with constraints on both contact surfaces.