## 4 Results and Discussion
### Experimental Results  
The experimental results are discussed in terms of ultimate strength, displacement, shear stress (\(\tau\), the ratio between load and total adhesive area), shear strain (\(\gamma\), the ratio between displacement and thickness of the adhesive layer), and global stiffness (k, the ratio between load and displacement) of the joint. Failure modes are also analyzed. For the calculation of the stiffness, only the linear portion of each curve is used since the beginning of nonlinearity indicates the occurrence of damage in the adhesive joint. Figure 5 reports \begin{table}
\begin{tabular}{l c c c} \hline Longitudinal Young’s modulus & \(E_{n}\) & [MPa] & 4000 \\ Transversal Young’s moduli & \(E_{s}=E_{t}\) & [MPa] & 1350 \\ Nominal stress I mode & \(\sigma_{n}\) & [MPa] & 42 \\ Nominal stress II mode & \(\sigma_{s}\) & [MPa] & 15 \\ Nominal stress III mode & \(\sigma_{t}\) & [MPa] & 15 \\ Critical fracture energy, mode I & \(G_{n}^{C}\) & \(\mathrm{Jm}^{-2}\) & 380 \\ Critical fracture energy, mode II & \(G_{s}^{C}\) & \(\mathrm{Jm}^{-2}\) & 190 \\ Critical fracture energy, mode III & \(G_{t}^{C}\) & \(\mathrm{Jm}^{-2}\) & 190 \\ \hline \end{tabular}
\end{table} 
Table 6: Numerical characterization of adhesive: material properties of adhesive layer. Figure 4: Boundary conditions (orange symbols represent the boundary conditions). the load--displacement curves for all specimen series up to the failure point, namely until the last load increment is recorded. Figure 5: Load—displacement curves of all treated double-lap joints: (**a**) CTRL; (**b**) SIL-G; (**c**) 111G; (**d**) SIL+111G; (**e**) SIL-G_111T; (**f**) 111G_111T; (**g**) SIL+111G_111T; (**h**) SIL-G_94T; (**i**) 111G_94T; (**l**) SIL+111G_94T; (**m**) EPX; (**n**) A; (**o**) B series. A detailed description of the failure modes, specimen by specimen, is provided in Table 7, while Figure 6 depicts all the observed failure modes, using specimens CTRL_1, CTRL_3, and SIL-G_5 as an example. The results obtained are compared in terms of average values and summarized in Table 8, where the prevalent failure modes are also listed. The highest failure load is achieved by the series treated with the epoxy paint, while the two acrylic-painted series register the worst behavior; in particular, for paint A (Figure 5n), a drastic reduction in ultimate load is observed (70% lower than the CTRL series, Figure 5a), \begin{table}
\begin{tabular}{l c c c c c} \hline \hline
**Series** & **Decimen 1** & **Specimen 2** & **Specimen 3** & **Specimen 4** & **Specimen 5** \\ \hline CTRL & SB + TLC (G) & SB & SB + AF (G) + CF+ LTT & SB & SB + CF + LTT \\ SIL-G & SB & SB+ TT + TLC (T) & SB+ LTT + TLC (T) & SB+ LT & TT + LT + TLC (T) \\ IIIG & SB + TT + TLC (G) & SB + TTC (G) + LTT & SB + TT + TLC (G) & SB & SB + TT \\ SIL + 111G & SB + LTT + TLC (T) & SB & SB & SB + LTT + TLC (T) \\ SIL-G & SB & SB+ LTC (G) + LTT + AC (G) & CF+ LTT + TLC (T) & SB & SB +TT + LTT + TLC (T) \\ IIIG\_111T & SB+ TT & AF (G) + LTT + TLC (T) & SB + LT + TLC (T) & SB & SB + TLC (T) + LTT \\ SIL-G\_111T & TT + LTT & SB+ TT & SB+ TT & LTT & SB +TL (T) + LTT \\ SIL-G\_111T & SB & TL(C) + LTT & SB+ LT + TLC (T) & SB+ LTT + TLC (T) & SB +LT + TLC (T) \\ SIL-G\_111T & TD (G) + LTT & SB+ LTT + TLC (T) & SB+ LTT + TLC (T) & SB+ LTT + TLC (T) \\ SE & SB & SB & SB+ CFT + LTT + TLC (T) & SB + LTT + TLC (T) & SB + LTT + TLC (T) \\ A & AF (G) + TLC (G) & AF (G) & AF (G) & AF (G) & AF (G) \\ B & TLC (G) + AF (G) & TLC (G) + SB + AF (G) & TLC (G) + AF (G) & TLC (G) + AF (G) \\ \hline \hline \end{tabular}
\end{table} 
Table 7: Failure modes. The (G) and (T) mean that the failures happen at the glass and timber interfaces, respectively. Figure 6: Failure modes of specimens CTRL_1 (**left**), CTRL_3 (**middle**), and SIL-G_5 (**right**). probably due to the inhibition of the cohesive process between the adhesives caused by the superposition of the paint layer.