## 3 Results  
### Data Overview  
The changes in the load and deformation of the specimen, measured by the electrical LVDTs in the central part of each face of the specimen over a length of 250 mm, are represented by graphs in Figure 6. Some differences in the horizontal and vertical deformations were observed on the four lateral faces of the specimen. These can be explained by the heterogeneity of the materials, the geometric asymmetry of the specimen, and eccentric loading. It is difficult to avoid these factors in these types of tests. Figure 6: Specim behavior under load: (**a**) variation in compressive force, \(F\); (**b**) specimen vertical deformation, \(d_{yz}\); (**c**) specimen horizontal deformation, \(d_{h}\), with time, \(t\), as measured with electrical LVDTs at the center of each specimen lateral face—A, B, C, and D—during the test. All vertical and horizontal deformations of the specimen returned to zero in the first three cycles of the load after the vertical compressive force \(F\) was removed. This indicates an elastic behavior of the specimen under a vertical compressive force of \(F\leq 530\) kN. With \(F>530\) kN in the last load cycle, the vertical deformation increased non-linearly with force \(F\). Such a behavior was especially noticeable on faces C and D of the specimen, which showed the change from an elastic to a plastic deformation regime. It is worth noting that the electrical LVDT sensors did not detect the onset of cracks in the specimen, because they were installed in the center of the four faces of the specimen, while cracks formed on one of its corners. At the same time, the development of structural cracks led to the breakage of the optical fibers crossing the cracks, and accordingly, to a decrease in the output signals of the monitoring system. In some optical fibers, the breakage was observed visually due to the red light coming from the broken optical fibers (Figure 7). The plots in Figure 8 show the variation in the applied vertical compressive force on the stack, \(F\), and the respective dimensionless amplitude of the output signal, \(A\), of the twelve optical fibers with time, \(t\). The graphs of the dimensionless amplitude of the signal, \(A\), are displayed in groups of four for levels 1 and 2, 3 and 4, and 5 and 6 of the specimen. The sharp decrease in the signal amplitude observed in these diagrams was caused by optical fiber breaks, complete or partial, due to the occurrence of structural cracks. There are several characteristic points on the plots in Figure 8, denoted with the lowercase italic letters \(q\), \(r\), \(s\), \(t\), and \(u\). Some optical fibers (1a, 1b, 3a, 3b, 4a, and 4b) broke and consequently decreased their optical transmission at a load of about 400 kN in the fourth load cycle (interval \(q\)-\(r\)), and then they partially recovered it when the load returned to zero (interval \(r\)-\(s\)). Other optical fibers (2a, 2b, 5a, 5b, 6a, and 6b) sharply decreased their optical transmission at a larger load of about 620 kN in the fifth load cycle (point \(u\)). Figure 7: Broken optical fibers in locations where they intersect structural cracks: (**a**) completely broken optical fibers; (**b**) optical fibers with broken silica cores and claddings that stay in the intact but stretched elastic coatings. The observed differences in the behavior of some of the optical fibers can be attributed to the structural inhomogeneity of the specimen. This led to inhomogeneous cracking of the specimen under load, and the first structural cracks developed in the most strained parts of the specimen. The optical fibers that crossed the structural cracks were stretched, and their silica members were broken, while the elastic plastic coatings were stretched but not broken. The optical transmission decreased but was not completely interrupted (interval _q-r_). With the load dropped to zero, the structural cracks partially closed in the generally elastic specimen.