## 3 Experimental Investigation
### Test Results  
The analysis of experimental results was carried out at different levels, including (a) qualitative analysis of observed failure mechanisms, (b) measured load-displacement laws, (c) DIC measurement of displacements in the bonded region. For sake of clarity, the specimens are labelled to detect the type of glue (\"E\" or \"P\" for epoxy and polyurethane), the moisture content (9%, 18% or 27%), the rod-to-grain orientation (\"0\" or \"90\" for parallel or perpendicular arrangement) and the sample number \(n\) for each group. The full set of pull-out test results is presented in Appendix A. Three regimes can be distinguished from the collected load-displacement curves, as shown in the examples of Figures 4 and 5 (specimens under 9% moisture and load parallel or perpendicular to the grain, respectively). Firstly, a linear elastic stage can be noticed in the load-bearing response of all the specimens, from which the initial stiffness \(K_{sar}=F_{ax}/d\) can be estimated from a linear regression procedure. After the yield point, a progressive decrease of stiffness occurs followed by a sudden failure of the connection. Worth to be noted, in this regard, that the failure path of all tested connections was located in the wood substrate in the vicinity of the wood-adhesive interface, as also emphasized in Figure 6. Thus, the nonlinearity observed in the collected force-displacement responses can be rationally justified in the quasibrittle damage of wood. Figure 3: Pull-out test set-up: schematic view and details (nominal dimension in mm). Finally, the ultimate load measurement for the experimental curves as in Figures 4 and 5 allows estimating the overall shear strength of the examined connections. Following Figure 6 and that the failure mechanism of BiRs is dependent on the mechanical properties of solid wood (strength and stiffness), the typical collapse of BiRs can be assumed as quasi-brittle for general applications. More in detail, for rods bonded parallel to the grains, wood failure was observed to start in the area around the adhesive matrix, while for rods bonded perpendicularly to the grains, failure typically originated in line between adhesive matrix and wood (Figure 6). The qualitative observations at failure, for a selection of specimens, were further explored by DIC system as in Figure 7.  
Figure 4: Experimental force–displacement results for specimens with (**a**) epoxy or (**b**) polyurethane, under 9% moisture and parallel rod-to-grain arrangement. Figure 5: Experimental force–displacement results for specimens with (**a**) epoxy or (**b**) polyurethane, under 9% moisture and perpendicular rod-to-grain arrangement. Average values of maximum force (\(F_{max}\)) and displacement (\(d_{max}\)), as well as of slip modulus (shear stiffness \(K_{ser}\)) and their corresponding coefficient of variation (CoV.) and standard deviation (St.Dev.) are listed in Tables 3 and 4. Major scatter of grouped predictions is found for the \"99%\" set in Table 4, which was found characterized by 36% CoV. in terms of slip modulus. On the other side, such an experimental outcome was severely affected by few specimens (like specimen #2 in Figure 5a). Figure 6: Example of failure configuration for the tested samples: (**a**) epoxy and (**b**) polyurethane bonded rods for half-size specimens (18% moisture, parallel rod-to-grain arrangement) and (**c**) full-size specimens. Figure 7: Example of the typical shear strain distribution along the rod, as obtained by digital image correlation (DIC). Considering the specimens grouped by adhesive type, from Tables 3 and 4 it is possible to notice a less pronounced sensitivity and scatter of polyurethane bonded rods, compared to the epoxy bonded samples. This effect is even more pronounced for service classes 2 and 3, with higher moisture content. Mean experimental data can be helpful for the analysis of climate and operational conditions on bonded rods for timber applications.