## 4 Discussion  
Silica-core silica-clad optical fibers can be installed in new and existing buildings and serve as artificial \"nerves\" that indicate structural failure, just as nerves indicate pain in the body, during the lifetime of the buildings. This technique is simple and inexpensive, for the following reasons:  
* The use of ordinary optical fibers of silica without any built-in special sensing elements, such as gratings and tapers. This contributes to the low cost of the optical fiber detector array. * The use of multi-mode optical fibers rather than single-mode optical fibers. Multi-mode optical fibers have a large core diameter, which allows the use of inexpensive and reliable LEDs instead of expensive and delicate semiconductor lasers needed to efficiently launch light into the small diameter core of single-mode optical fibers. * The use of optical fibers with their original polymer coating. The removal of the polymer coating used in the manufacture of FBGs with subsequent recoating compromises the strength of the optical fiber. Keeping the original coating intact contributes to the long service life and reliability of optical fibers. * The use of a sufficiently large number of optical fibers in the crack detection array. This makes it possible to reliably detect structural cracks of a certain width. * The use of a binary conversion and interpretation of the optical signals. * The use of time division multiplexing (TDM) to monitor the optical transmission of all fibers of the crack detection array. TDM is much simpler and cheaper than the optical spectral multiplexing used in distributed FBG and OFDR sensors. Many of the characteristics of this method, as well as the singularities of its application in buildings of different types, require further collaborative study by experts in civil engineering, and measurement and information technologies. It is preferable to install optical fiber arrays at locations of maximum stress and in the direction of the maximum equivalent strain. This ensures the predominantly perpendicular crossing of the optical fibers and the structural cracks. For a given crack, signal loss with a perpendicular crossing is less than that in other cases. More specifically, the signal loss depends on the angle between the crack trace and the fiber axis. Therefore, the expected geometry of the intersection of a fiber with a crack should be taken into account when placing optical fibers on structures. Typical crack patterns in structures under different loadings are generally well known. Moreover, proper stress and strain fields can be obtained by modeling the stress-strain state in the most loaded areas of the structure using numerical methods, such as the finite element method and artificial neural networks [67,68]. \begin{table}
\begin{tabular}{c c c c c c} \hline \hline
**Number of Optical Fibers of the Detection Array** & \multicolumn{4}{c}{**The Range of Crack Width in Which the Fiber Break Occurs, \(\Delta\delta_{c}\) (mm)**} \\ \hline \(n\) & 1–1.5 (\(p=0.167\)) & 1–2 (\(p=0.333\)) & 1–3 (\(p=0.583\)) & 1–4 (\(p=0.833\)) & 1–5 (\(p=1.0\)) \\ \hline \multicolumn{5}{c}{Probability that at least one fiber of an array of \(n\) optical fibers breaks, \(P_{1}\)} \\ \hline
1 & 0.167 & 0.333 & 0.583 & 0.833 & 1.0 \\ \hline
2 & 0.306 & 0.556 & 0.826 & 0.972 & 1.0 \\ \hline
4 & 0.518 & 0.802 & 0.970 & \textgreater{}0.999 & 1.0 \\ \hline
6 & 0.665 & 0.912 & 0.995 & \textgreater{}0.999 & 1.0 \\ \hline
8 & 0.767 & 0.961 & \textgreater{}0.999 & \textgreater{}0.999 & 1.0 \\ \hline
10 & 0.838 & 0.983 & \textgreater{}0.999 & \textgreater{}0.999 & 1.0 \\ \hline
12 & 0.888 & 0.992 & \textgreater{}0.999 & \textgreater{}0.999 & 1.0 \\ \hline \hline \end{tabular}
\end{table} 
Table 1: Probability that at least one fiber of an array of \(n\) optical fibers breaks, \(P_{1}\), calculated for several ranges of crack widths \(\Delta\delta_{c}\) and detection arrays of 1, 2, 4, 6, 8, 10, and 12 optical fibers. There are many options for informing users of the occurrence of structural risk.