## 2 Experimental Test
### Experimental Results
\begin{table}
\begin{tabular}{c c c c c c} \hline \hline \multicolumn{2}{c}{**SAMPLE TYPE**} & \multicolumn{5}{c}{**TEST TEMPERATURE (\({}^{\circ}\)C)**} \\ \cline{2-6}
**(THICKNESS OF THE ADHESIVE LAYER)** & **20** & **40** & **60** & **80** \\ \hline \hline \multicolumn{6}{c}{**Force**} \\ \cline{2-6}
**PS 1 mm Failure type (cp)** & \(\mu_{\text{F}_{max}}\) (kN) & 9.39 (1.00) & 7.61 (0.81) & 6.26 (0.67) & 5.76 (0.61) \\ \hline \multicolumn{6}{c}{**Displacement**} \\ \cline{2-6}
**PS 1 mm Failure type (cp)** & \(\mu_{\text{S}_{max}}\) (mm) & 11.4 (1.00) & 10.4 (0.92) & 9.3 (0.86) & 7.9 (0.69) \\ \hline \multicolumn{6}{c}{**Initial elastic stiffness**} \\ \cline{2-6}
**K (N/mm)** & 849.0 (5.6) & 748.4 (4.2) & 805.5 (11.2) & 795.0 (7.2) \\ \hline \multicolumn{6}{c}{**Force**} \\ \cline{2-6}
**PS 2 mm Failure type (cp)** & \(\mu_{\text{F}_{max}}\) (kN) & 9.34 (1.00) & 7.03 (0.75) & 5.84 (0.62) & 5.25 (0.56) \\ \hline \multicolumn{6}{c}{**Displacement**} \\ \cline{2-6}
**PS 2 mm Failure type (cp)** & \(\mu_{\text{S}_{max}}\) (mm) & 11.4 (1.00) & 9.9 (0.87) & 7.7 (0.67) & 7.7 (0.67) \\ \hline \multicolumn{6}{c}{**Initial elastic stiffness**} \\ \cline{2-6}
**K (N/mm)** & 780.7 (7.2) & 838.8 (6.7) & 824.7 (3.2) & 767.5 (4.6) \\ \hline \multicolumn{6}{c}{**Force**} \\ \cline{2-6}
**PS 4 mm Failure type (cp)** & \(\mu_{\text{F}_{max}}\) (kN) & 9.41 (1.00) & 7.80 (0.83) & 6.27 (0.67) & 5.50 (0.58) \\ \hline \multicolumn{6}{c}{**Displacement**} \\ \cline{2-6}
**PS 4 mm Failure type (cp)** & \(\mu_{\text{S}_{max}}\) (mm) & 13.7 (1.00) & 11.0 (0.80) & 9.7 (0.71) & 9.1 (0.66) \\ \hline \multicolumn{6}{c}{**Initial elastic stiffness**} \\ \cline{2-6}
**K (N/mm)** & 789.0 (5.9) & 783.1 (5.9) & 881.7 (4.1) & 786.5 (9.3) \\ \hline \hline \end{tabular}
\end{table} 
Table 2: Results of the bending test. Figure 2: Force-displacement graphs: adhesive layer thickness t = 1 mm (**a**); adhesive layer thickness t = 2 mm (**b**); adhesive layer thickness t = 4 mm (**c**). (Table 2). The effect of adhesive thickness did not seem so pronounced, and the differences between the mean values (F\({}_{\text{max}}\)) of the 1-, 2- and 4-mm thicknesses were within 1% at 20 \({}^{\circ}\)C. However, the corresponding displacements were significantly higher for adhesive thickness of t = 4 mm than for t = 1 and 2 mm (17% difference). The results clearly show that flexible bonding does not show a decrease in initial elastic stiffness K with increasing temperature up to 80 \({}^{\circ}\)C (the slope of the curve is almost constant)--Figure 2. This observation is very promising for design procedure, because the use of safety factors of around 0.5 gives stable properties in the whole range of the considered elevated temperatures (up to 80 \({}^{\circ}\)C). As can be seen from Table 2, the differences between the highest and lowest average values of initial elastic stiffness (from the range 0.1 F\({}_{\text{max}}\) to 0.4 F\({}_{\text{max}}\)) are respectively:  
* for t = 1 mm \(\rightarrow\) 100.6 N/mm;
* for t = 2 mm \(\rightarrow\) 71.5 N/mm;
* for t = 4 mm \(\rightarrow\) 98.6 N/mm. It can be concluded that flexible polyurethane adhesives are stable at high temperatures up to 80 \({}^{\circ}\)C. The plastic form of damage is clearly visible for laminated beams with an adhesive thickness of 4 mm. The failure of the composite beam was triggered by a crack formed in the adhesive layer. Immediately following this, in fact, a sudden delamination occurred in the shear span (the span between a point load and the nearest support). As the load increased, the cracking propagated into the lower layer, where the damage occurred in the wood plank due to the tensile stresses along the fiber being exceeded.