## 5 Structural Investigations and Twist Performance
### WHT Model  
As already done for the WT model and for the 3-bay model realized for the WhT facility, the lateral views for the preload, external loads, and external loads (aerodynamic + centrifugal) + SMA configurations are compared (Figure 13). As done for the WT model, \begin{table}
\begin{tabular}{c c c c c c} \hline
**Load Condition** & **Total Twist Over 4** & **Delta Twist with Respect** & **Minimum** & **Sensor Plate** & **Delta Plate** \\
**(with Reference to** & **Bayes (Deg)** & **to the Pre-Load, Referred to the Entire Blade (Deg)** & **Safety Factor** & \(\mu\varepsilon\) & **Microstrain** \\ \hline pre-load & 4.58 & & & 2.1 & 126.5 & \\ aerodynamic & 4.40 & \(-1.01\) & 2.0 & 115.9 & \(-10.6\) \\ aerodynamic+SMA & 5.69 & 6.27 & 1.8 (*) & 159.3 & 32.8 \\ \hline \end{tabular} (*) this value, as explained in the text, was recomputed since it was due to an isolated, unrealistic spike; assuming an average stress in the zone of 650 MPa, a more realistic safety factor of 2.0 was estimated.  
\end{table} 
Table 10: WT model twist, minimum safety factor, and strain level on sensor plate vs load condition. Figure 11: Stress level of the skin of the WT model: stress level in the pre-load (**a**), aerodynamic loads (**b**), and aerodynamic loads + SMA fully on (**c**). Figure 12: Von Mises stress level within the SMA rods in the WT model for the fully activated condition (**a**) and detail at the neck of the connection (**b**). the XFOIL software was used to compute the aerodynamic loads. The maximum angular speed of 120 rpm prescribed for the tests was used to compute the linear velocity at each span station of the blade segment. The centrifugal loads were also generated using the RFORCE card of the MSC/Nastran solver. Differently from the WT model, it is not possible to identify a unique rotational pivot since the model is not pinned along a span axis, but the tip-free edge can flap under the action of the aerodynamic loads. In the plots in Figure 14, the stress generated in the main structure is illustrated in the preload condition, in the presence of aerodynamic and external loads, and finally, in the presence of these actions with the SMA rods fully activated. The von Mises stress produced in the configuration of the preload achieves a maximum level of 118 MPa. The application of aerodynamic and centrifugal forces leads to a slight mitigation of the twist with a consequent reduction of the stress level down to 110 MPa. Finally, the full activation of the SMA determines a new maximum of 152 MPa. The stress level produced in the ring transmission element is shown in Figure 15. Also, in this case, the slight increase in stress from the pre-load condition to the application of the aerodynamic and centrifugal forces indicates that the largest contribution to the structural solicitation is due to twist action (preload); the further increase in the stress level occurring for the full activation of the SMA rods again highlights the predominance of the twist actions. The same trend is noticed for the skin element (Figure 16). The rotation of the blade segment at the tip, the minimum safety factor, and the strain estimated on the sensor plate are summarized in Table 11. The resulting safety factors comply with the requirements, with a minimum value of 2.3 in the condition of full actuation. Finally, even in this case, the strain estimated on the sensor plates (8.8 and 70.4 \(\mu\varepsilon\), in the presence of external forces, with SMAs off and  
Figure 13: WhT model lateral view: preload configuration (blue line, (a)), aerodynamic loads (green line, (b)), external loads + SMA (orange line (c)) configurations. on, respectively) are compliant with the capability of the acquisition systems. Finally, a situation similar to the WT model was observed for the SMA stress level, with a generally uniform distribution, apart from the neck zone, where a maximum level of 614 MPa was achieved (see Figure 17).