## 3 Results and Discussion
### Board Properties
#### 3.2.2 Board Properties  
Bending strength of control and TM boards tested directly and after exposure is shown in Figure 9 in a cumulative percentage diagram. The 5 th percentile of bending strength (\(f_{\text{m,05}}\)), and mean value of bending strength (\(f_{\text{m,mean}}\)) are indicated in the diagram. Levels of \(f_{\text{m,05}}\) were 23.3 and 21.5 MPa for control boards and 10.8 and 10.2 MPa for TM boards tested directly and after exposure, respectively, and slightly lower after weathering. On average, bending strength (\(f_{\text{m}}\)) was 6% lower for control boards and 9% lower for TM boards after weathering (Table 4 and Figure 9), and bending stiffness (\(E_{\text{m,g}}\)) was 4% lower after weathering for both sample sets (Table 4). A \(t\)-test points out that the effect  
Figure 8: Moisture content (MC) and dynamic elastic moduli (\(E_{\text{a}}\)) of ‘exposed’ control and/or TM boards by exposure time: (**a**) MC (mean \(\pm\) standard deviation) and \(E_{\text{a}}\) (mean), (**b**) \(E_{\text{a}}\) (mean \(\pm\) standard deviation), and (**c**) \(E_{\text{a}}\) adjusted to 12% MC (mean \(\pm\) standard deviation) of control boards (legend in **a**). of weathering on mean \(f_{\rm m}\) and \(E_{\rm m,g}\) is not significant at the standard significance level of 0.05 (Table 4). Thus, 30 months of outdoor above-ground exposure has no significant effect on bending strength and stiffness of TM spruce timber. A similar conclusion was drawn by [PERSON] et al. (2019) who determined bending strength and stiffness of thermally modified terrace planking after 3 years of weathering. Exposed TM boards reached failure quicker than TM boards that were tested directly, whereas no such differences in time to failure (\(t\)) and deformation at maximum load (\(w_{\rm max}\)) were found for control boards (Table 4). In detail, \(t\) and \(w_{\rm max}\) and were on average 145 s and 25 mm for directly tested TM boards and 133 s and 23 mm for exposed TM boards, respectively. Figure 10 shows work-to-maximum load (WML) of control and TM boards tested directly and after weathering. WML is plotted on the \(y\)-axis: left for control and right for TM boards. Note that mean WML is approximately 70% lower for TM boards compared to control boards, as was shown earlier by others for specimens of small clear wood ([PERSON], 2018; [PERSON], 2018), and discussed in detail by [PERSON] et al. (2019) for the 100 control and 100 TM boards tested directly (Figure 1). Mean WML of TM boards was approximately 14% lower after weathering compared to TM boards tested directly, but this difference was not consistent over the full range of boards and not significant (Table 4 and Figure 10). This decrease could have been expected, since \(f_{\rm m}\) and \(w_{\rm max}\) were lower and \(E_{\rm m,g}\) remained unchanged after weathering (Table 4), and \(F\)-\(w\) curves of TMT are linear up to point of failure ([PERSON], 2018). In contrast, lower \(f_{\rm m,mean}\) of control boards after weathering with corresponding equal levels of mean WML can be explained by a (small) increase in \(w_{\rm max}\) and non-linearity in the bending behaviour that is typical for about 40% of Norway spruce timber coming from Sweden ([PERSON], 2018). In a preceding investigation, four different types of load-deflection (\(F\)-\(w\)) curves have been distinguished for unmodified and TM spruce timber: 1) sudden failure, 2) preliminary failure prior to failure, 3) non-linearity prior to failure, and 4) preliminary failures and non-linearity prior to failure ([PERSON], 2018). Table 5 shows how many control and TM boards tested directly and after weathering behaved according to each curve type in percentage frequency of occurrence. With the exception of one board, no non-linear behaviour was observed for TM boards, which is in line with previous results ([PERSON], 2018). After weathering, more boards failed suddenly without preliminary failure or non-linearity observed in \(F\)-\(w\) curves, especially for TM boards.