## 3 Results and Discussion
### Board Properties
#### 3.2.2 Board Properties
This is consistent with the earlier discussed decrease in WML  
Figure 9: Bending strength of control (circles) and TM (squares) boards tested directly and after weathering. Mean (\(f_{\rm m,mean}\)) (dash-dotted line) and 5-percentile (\(f_{\rm m,05}\)) (dashed line) values of bending strength are given for each sample set. Table 5 also shows failure type at maximum load (\(F_{\max}\)), defect at location of failure and type of fracture surface in percentage frequency of occurrence for each sample set tested in bending. After exposure, compression failure at \(F_{\max}\) was recorded for control and TM boards, while splinter tension (note: this failure mode is often combined with compression failure) became more common for  
Figure 10: Work-to-maximum load (WML) of control (left \(y\)-axis/circles) and TM boards (right \(y\)-axis/squares) tested directly and after weathering. \begin{table}
\begin{tabular}{c c c c c} \hline \hline
**Treatment** & \multicolumn{2}{c}{**Control**} & \multicolumn{2}{c}{**TM**} \\ \cline{2-5}
**Exposure** & **Direct** & **Exposed** & **Direct** & **Exposed** \\ \cline{2-5}
**Interval** & **–** & **After** & **–** & **After** \\ \hline \multicolumn{5}{c}{CURVE TYPE 1  \\ \hline
1 & 15 & 21 & 37 & 56 \\
2 & 47 & 40 & 63 & 43 \\
3 & 15 & 17 & 0 & 0 \\
4 & 23 & 23 & 0 & 1 \\ \hline \multicolumn{5}{c}{FAILURE TYPE} \\ \hline Simple tension & 69 & 68 & 80 & 81 \\ Cross-grain tension & 27 & 16 & 18 & 13 \\ Splinter tension & 4 & 11 & 1 & 0 \\ Compression & 0 & 5 & 0 & 2 \\ Horizontal shear & 0 & 0 & 1 & 4 \\ \hline \multicolumn{5}{c}{DEFECT AT LOCATION OF FAILURE} \\ \hline Around knots & 80 & 65 & 36 & 32 \\ Through knots & 13 & 30 & 56 & 61 \\ Reaction wood & 6 & 4 & 3.5 & 1 \\ Cross-grain & 1 & 0 & 3.5 & 0 \\ Top rupture & 0 & 1 & 0 & 2 \\ n/a 2  & 0 & 0 & 1 & 4 \\ \hline \multicolumn{5}{c}{FRACTURE SURFACE} \\ \hline Brash & 38 & 24 & 64 & 50 \\ Fibrous & 62 & 71 & 35 & 44 \\ n/a 3  & 0 & 5 & 1 & 6 \\ \hline \hline \end{tabular}
\end{table} 
Table 5: Type of load–deflection (\(F\)–\(w\)) curve, and characterisation of failure type, defect and fracture surface at maximum load of control and TM boards tested directly and after exposure (% frequency of occurrence). control boards and horizontal shear failure more common for TM boards. Figure 11 shows examples of these three failure modes. These observations may be related to the presence of checks in boards after weathering. This was especially clear for failure in the compressive zone, which appears different in boards tested directly and after weathering (Figure 11a,b). After weathering, buckling of wood fibres due to compressive failure is combined with tensile failure perpendicular to the grain in the radial-longitudinal plane initiated at checks. This type of failure was governing 5% of the control boards and 2% of the TM boards after exposure, whereas it did not occur when boards were tested directly (Table 5). Horizontal shear failure at \(F_{\text{max}}\) became more frequent for TM boards after weathering and is shown as example in Figure 11c. In line with previous studies, failure was still related to the presence of knots after weathering, and occurred more often through than around knots in TM than unmodified spruce timber [21; 81]. However, failure through knots became more common for control boards after weathering (Table 5). This may be related to the fact that knots in control boards checked upon weathering, whereas knots of TMT were already checked before weathering i.e., during thermal modification. The fracture surface on the tension side appeared more often brash for TM boards compared to control boards, before as well as after weathering (Table 5). Coefficients of determination (\(R^{2}\)) incl. lower and upper bound at the 95% confidence interval of relationships between NDT properties, and bending strength and stiffness of control and TM boards tested directly and after exposure are shown in Table 6. The results are in line with \(R^{2}\)-values typically found for unmodified and TM spruce timber [21; 22; 26; 72; 82].