## 2 Materials and Methods
### Experimental
#### 2.2.2 Non-Destructive Tests (NDTs)  
All 190 board pairs were examined by non-destructive tests; the 190 control boards after kiln-drying and the 190 TM boards after thermal modification. The board pairs used for weathering were also evaluated during exposure at six-month intervals. Two non-destructive test (NDT) methods were used to determine axial dynamic stiffness: one based on the principle of time-of-flight (Tof) and one resonance-based method. Time-of-flight (\(\Delta t\)) was obtained by a Sylva-test Trio(r) test device (CBS-CBT, Switzerland). Two transducers, one at each board end, were connected to the board in a pre-drilled hole. The time-of-flight is the time required for an ultrasonic wave to travel through the board. In the resonance-based method, the frequency of the first mode of axial vibration (\(f_{a,1}\)) was obtained by a Timber Grader MTG handheld device (Brookhuis Micro-Electronics BV, The Netherlands). The Mechanical Timber Grader (MTG) was held against a board end and the frequency was measured by recording longitudinal vibrations from a built-in excitation hammer. A description of these methods and details of how they were implemented in this study can be found in [PERSON] et al. (2020).  
\begin{table}
\begin{tabular}{c c c c c} \hline \hline
**Climate Factor** & **Temperature 1** & **Precipitation 2** & **Solar radiation 1** & **Relative Humidity 1** \\
**Unit** & **(\({}^{\circ}\)C)** & **(mm)** & **(Wm\({}^{2}\))** & **(\%)** \\ \hline \hline  & \multicolumn{4}{c}{EXPOSURE PERIOD 3} \\ \hline Spring 2017–Autumn 2017 & 11.2 & 323 & 158 & 80 \\ Autumn 2017–Spring 2018 & 2.4 & 317 & 59 & 91 \\ Spring 2018–Autumn 2018 & 16.0 & 269 & 193 & 73 \\ Autumn 2018–Spring 2019 & 2.1 & 400 & 37 & 93 \\ Spring 2019–Autumn 2019 & 13.1 & 543 & 189 & 77 \\ \hline \hline  & \multicolumn{4}{c}{MEASUREMENT INTERVAL 4} \\ \hline Autumn 2017 (month 6) & 8.5 & 5.5 & 57 & 94 \\ Spring 2018 (month 12) & 17.2 & 0.0 & 271 & 61 \\ Autumn 2018 (month 18) & 8.6 & 0.1 & 45 & 91 \\ Spring 2019 (month 24) & 6.9 & 0.0 & 174 & 64 \\ Autumn 2019 (month 30) & 7.0 & 0.3 & 134 & 86 \\ \hline \hline \end{tabular}  
* Average of 1-minute values. 2 Sum of total per hour. 3 Given over a period of approximately 6 months. 4 Given over a period of 1 day.  
\end{table}
Table 1: Weather conditions for exposure periods and at measurement intervals.  
Figure 2: Control boards (light) and thermally modified (TM) boards (dark) placed on racks at the start of weathering: (**a**) overview, (**b**) detail of board positioned on its edge, and (**c**) detail of board positioned flat.  
Board mass (\(m\)) and volume (\(V\)) were obtained at the time of non-destructive testing. Before and during exposure, moisture content (MC) of 6 control and 6 TM boards, 2 board pairs from each group that were average in density, was determined by using the oven-dry method [64]. MC of a board was taken as the average MC of two board slices of approximately 20 mm thickness, one slice from each board end. These 6 boards used for determination of MC were excluded from all other tests, such that 84 board pairs (28 per group) after exposure were used in 4-point bending (4 PB) tests and to assess degree of checking. Weather conditions at the time of NDTs are shown in Table 1.