## 3 Results and Discussion
### Acoustic Emission along Grain Angle Direction in High/Low Tree Ring Density of Scots Pine
#### 3.2.4 Content of Publication  
Figure 6 reports a scheme that summarises the main information extracted by the selected reviewed documents. As a result, 27 over 31 documents explored softwoods behaviour, especially of pine, spruce, hemlock, fir and cedar; 10 of them also explored the behaviour of hardwoods such as alder, oak, ash, poplar, chinkapin, birch and lime wood; while one document considered wood products such as medium-density fibreboard (MDF), oriented strand board (OSB), plywood and particleboard. In most cases, the AE technique appeared to be used in laboratory experiments and in only one case among the reviewed documents, it was applied for an in-situ monitoring focused on movable objects only [11]. The tests conducted on the samples were of various type (mainly fracture tests, but also tensile, cutting, coating, compressive, drying and torsional tests) and they sometimes involved an acclimisation process (e.g., climate chamber [12; 13; 14] and laboratory chamber dryer [15]). \begin{table}
\begin{tabular}{c c c c} \hline \hline
**Journal** & **Subject Area** & **Quartile** & **N\({}^{\circ}\) of Doc** \\ \hline \multirow{4}{*}{Material Science and Engineering} & Physics & Q1 & \\  & Material Sciences & Q1 & 1 \\  & Engineering & Q1 & \\  & Nanoscience and Nanotechnology & Q1 & \\ \hline Forests & Forestry & Q1 & 1 \\ \hline Materials & Physics & Q2 & 2 \\  & Material Sciences & Q2 & 2 \\ \hline Physical Review Letters & Physics & Q1 & 1 \\ \hline Wood Research & Forestry & Q2 & 1 \\  & Material Sciences & Q3 & \\ \hline \multirow{4}{*}{Wood Science and Technology} & Forestry & Q1 & \\  & Engineering & Q1 & 4 \\ \cline{1-1}  & Material Sciences & Q2 & \\ \cline{1-1}  & Plant Sciences & Q2 & \\ \hline \hline \end{tabular}
\end{table} 
Table 3: Journal’s names, corresponding main subject areas, journal’s quartile and number of published documents for each journal. Figure 6: Information extracted from the selected documents. In these works, the Acoustic Emission technique was used alone or combined with other techniques. In brackets the number of documents. The acoustic emission instrumentation was often found coupled with the testing machine to perform mechanical tests and the digital image correlation often used to record images during the tests. Among the analysed documents, only 7 included microscopic observations of the surface by means of scanning photography technique, scanning electron microscopy, optical microscopy, stereomicroscopy and electron microscopy. Finally, among the selected publications, 12 documents had both an experimental and theoretical approach, also performing numerical modelling or simulation and in some cases suggesting general mathematical formulations. The investigation showed that the most monitored acoustic Eeission parameters were amplitude (A), energy (E) and counts (C), which were frequently studied against time, crack length and load (even in their cumulative form) with their distribution often highlighted. Among the parameters less frequently studied there are the frequency characteristics [11, 16, 17, 18, 19, 20, 21], other parameters such as the energy release rate (G) [22], the fracture toughness (K) [23, 24], root mean square of the signal voltage (RMS) [25, 26, 27], density surface energy value (DSV) [16, 23], stress (\(\sigma\)), strain (\(\epsilon\)) [20, 28, 29, 30, 31, 32] and modulus of elasticity (MoE) [32, 33]. In many cases, wood samples were subjected to pre- or ongoing experimental settings, such as acclimatisation at some temperature (T) and relative humidity (RH) conditions in a climate chamber or simply moistening them in water [19, 34, 35]. In more than half of these documents, the time information related to the preconditioning period was missing even when the acclimatisation setup was specified, causing a problem in the reproducibility of the experiments. The AE parameters were often related to the moisture content (MC) of the tested sample, as in the case of [PERSON] et al.