# Experimental Investigation of a Device to Restrain the Horizontal Sliding of U-FREIs
## 8 Test Results and Observations  
### Compression Test Results  
Figures 8 and 9 show the load-displacement curves of all the performed compression tests. In all cases, it can be observed that the loading path presents a low slope in the first part, which corresponds to high deformability, with a high slope (low deformability) in the second one. This is because the carbon fiber fabrics were not pre-tensioned during the isolator production process. Hence, the fabrics need the elastomer to be deformed to be put under tension and to exert their containing action to elastomer deformations. This occurs in the first part of the loading path. After the fibers were put under tension, the slope of the loading path became steep. At the end of every cycle, a residual displacement was observed due to the viscous-elastic properties of the elastomer. The residual displacement increased from the first to the second cycle, while it approximately stabilized at the third cycle, according to the Mullins effect [47]. \begin{table}
\begin{tabular}{c c c c} \hline \hline  & **Q1 and Q2** & & **B1 and B2** \\ \hline I & CT-0 & II & CT-0 \\ III & ST-0 & IV & ST-0 \\ V & CT-1 & VI & CT-1 \\ VII & ST-1 & VIII & ST-1 \\  & - & IX & CT-2 \\  & - & X & ST-2 \\ \hline \hline \end{tabular}
\end{table} 
Table 2: List of tests with their chronological order. Figure 8: _Cont_. Table 3 reports the vertical stiffness values, \(K_{V}\), calculated according to Equation (2) at each loading cycle for all of the tested specimens and configurations. In this table, the specimen's name is followed by a dash and a number, where the number has the same meaning as in Table 2. Table 3 also reports the average of the \(K_{V}\) values obtained from identical specimens at the third cycle, and these values' variations between the test configuration with one constraint and the configuration with no constraint (\(\Delta\)K\({}_{\text{V}}\)-0), and between two constraints and one constraint (\(\Delta\)K\({}_{\text{V}}\)-1). Figure 8: Force-displacement curves of compression tests for specimens Q1 and Q2: (**a**) without constraint; (**b**) with constraint on one contact surface. Figure 9: _Cont._  
Table 3 shows that the \(K_{V}\) values increased from the first cycle to the third. The explanation of this phenomenon lies in the fact that the containment action of the fibers is more effective starting from the second cycle. As with the presence of the residual displacement, the elastomer is already deformed; therefore, a shorter load path is sufficient to induce tension in the fibers. It can be said that the vertical stiffness of the isolator when the structure is progressively loaded by gravitational forces during its construction is that of the first cycle. When the structure is subjected to an earthquake, instead, the value more representative of the real isolator vertical stiffness is that of the third cycle, earthquakes being a cyclic action. From Table 3, it can be observed that, even if the isolators are identical in pairs, there are differences in the vertical stiffness. By considering the \(K_{V}\) values at the third cycle, the percentage difference between the identical specimens, calculated on the lower value, is between 3% and 12% when the configurations without constraints and with constraints on one contact surface are considered. These differences lie in a range that can be attributed to the isolator building process, because the fiber fabric pieces are cut and inserted into the mold by hand. Due to this, the isolators may exhibit little differences. Considering the average vertical stiffness values calculated at the third cycle, it can be observed that the specimens with quadri-directional fabrics (Q) were vertically stiffer than the specimens with bi-directional fabrics (B) due to their higher shape factor (Figure 10).