## 3 Experimental Investigation
### Test Results
However, an in-depth discussion can be carried out in terms of characteristic mechanical properties that can be obtained from the test observations. Based on [28], the characteristic axial force at failure for each series of specimens was calculated as:  
\[F_{ax,char}=exp\left(\overline{y}-k_{s}s_{y}\right) \tag{1}\]  
with:  
\[\overline{y}=\frac{1}{n}\sum_{i=1}^{n}lnF_{xx,i} \tag{2}\]  
\[s_{y}=\sqrt{\frac{1}{n-1}\sum_{i=1}^{n}(lnF_{ax}-\overline{y})^{2}} \tag{3}\]  
where \(n\) denotes the number of test repetitions for each series, \(k_{s}\) is a coefficient adopted from [28]. Furthermore, the maximum axial force at failure (both in terms of mean and characteristic values) was correlated with the resisting surface of the bond-line of each specimen, \(A_{bond}\), given that:  
Figure 3:0 \begin{table}
\begin{tabular}{c c c c c c c} \hline \hline  & \multicolumn{6}{c}{**Bond**} \\ \cline{2-7}  & \multicolumn{6}{c}{**Two-Component Epoxy**} & \multicolumn{6}{c}{**Two-Component Polyurethane**} \\ \hline
**Moisture** & **Parameter** & **Avg.** & **CoV. [\%]** & **St.Dev.** & **Avg.** & **CoV. [\%]** & **St.Dev.** \\ \hline \multirow{3}{*}{9\%} & \(F_{ax}\) [kN] & 9.722 & 25.7 & 2.501 & 10.908 & 7.0 & 0.760 \\  & \(d_{max}\) [mm] & 1.352 & 26.9 & 0.364 & 1.325 & 4.8 & 0.064 \\  & \(K_{cor}\) [N/mm] & 7360.208 & 36.3 & 2668.764 & 9869.502 & 17.0 & 1674.434 \\ \hline \multirow{3}{*}{18\%} & \(F_{ax}\) [kN] & 11.687 & 3.5 & 0.408 & 11.801 & 6.7 & 0.792 \\  & \(d_{max}\) [mm] & 1.944 & 12.9 & 0.251 & 1.824 & 10.6 & 0.193 \\  & \(K_{cor}\) [N/mm] & 6943.509 & 15.9 & 1105.534 & 7887.684 & 3.6 & 282.079 \\ \hline \multirow{3}{*}{27\%} & \(F_{ax}\) [kN] & 3.149 & 25.8 & 0.814 & 8.914 & 6.6 & 0.587 \\  & \(d_{max}\) [mm] & 0.573 & 19.4 & 0.111 & 1.330 & 9.3 & 0.124 \\ \cline{1-1}  & \(K_{cor}\) [N/mm] & 5675.074 & 13.9 & 788.096 & 7476.200 & 6.1 & 456.498 \\ \hline \hline \end{tabular}
\end{table} 
Table 4: Maximum axial force, displacement and slip modulus for specimens with perpendicular rod-to-grain arrangement (mean experimental values). \begin{table}
\begin{tabular}{c c c c c c c c} \hline \hline  & \multicolumn{6}{c}{**Bond**} \\ \cline{2-7}  & \multicolumn{6}{c}{**Two-Component Epoxy**} & \multicolumn{6}{c}{**Two-Component Polyurethane**} \\ \hline
**Moisture** & **Parameter** & **Avg.** & **CoV. [\%]** & **St.Dev.** & **Avg.** & **CoV. [\%]** & **St.Dev.** \\ \hline \multirow{3}{*}{9\%} & \(F_{ax}\) [kN] & 15.216 & 9.1 & 1.390 & 15.545 & 3.3 & 0.519 \\  & \(d_{max}\) [mm] & 1.222 & 20.3 & 0.248 & 1.362 & 11.4 & 0.155 \\  & \(K_{cor}\) [N/mm] & 14,548.086 & 10.5 & 1528.301 & 12,475.290 & 9.6 & 1197.480 \\ \hline \multirow{3}{*}{18\%} & \(F_{ax}\) [kN] & 12.015 & 10.8 & 1.301 & 11.987 & 13.1 & 1.575 \\  & \(d_{max}\) [mm] & 1.233 & 6.4 & 0.079 & 1.227 & 27.1 & 0.333 \\  & \(K_{cor}\) [N/mm] & 6769.256 & 28.7 & 1942.825 & 10,362.893 & 15.6 & 1613.003 \\ \hline \multirow{3}{*}{27\%} & \(F_{ax}\) [kN] & 5.117 & 11.5 & 0.591 & 6.294 & 20.6 & 1.293 \\  & \(d_{max}\) [mm] & 0.607 & 17.1 & 0.104 & 0.625 & 20.9 & 0.131 \\ \cline{1-1}  & \(K_{cor}\) [N/mm] & 8341.809 & 22.1 & 1843.689 & 10,728.730 & 4.1 & 441.031 \\ \hline \hline \end{tabular}
\end{table} 
Table 3: Maximum axial force, displacement and slip modulus for specimens with parallel rod-to-grain arrangement (mean experimental values). where:  
\[A_{bond}=0.5\;\pi d_{h}L \tag{5}\]  
for the half-size specimens. The estimated results are shown in Figure 8, in terms of stress peak at failure for each set of specimens (under the assumption of uniform stress distribution for the bond-line as a whole). Mean and characteristic values of ultimate stress are grouped by adhesive type and rod arrangement, as a function of the service class/moisture content. As expected from Tables 3 and 4, a major scatter of mean and characteristic values was observed especially for the epoxy-bonded rods, rather than polyurethane samples. Besides, the global decrease of stress peak can be observed, both in mean and characteristic parameters, as far as the moisture level increases.