## 5 Structural Investigations and Twist Performance
### WT Model  
The lateral views of the WT blade model are reported in Figure 8. In the scheme (a), the model in pre-load configuration is depicted. The black line, the track of the chord at the root, represents the reference, while the green line represents the chord of the fourth bay (tip) at the completion of the pre-load operation. In scheme (b), the effect of the aerodynamic actions is illustrated; the fourth blade appears to be upward twisted with respect to the pre-load configuration, as shown by the blue line. Finally, the effect of the full activation of the SMA rods is illustrated in scheme (c); the orange line highlights the pitch ward inclination achieved by the tip blade. It is worth noting that the pivot of rotation (intersection of the different tracks of the chord) does not coincide with the center of the SMA rod. In fact, to meet the tight requirement prescribing that the center of gravity falls within the 1 st quarter of the chord, the SMA actuators and, consequently, the spars and the heating coils were placed ahead of the pivot of rotation. The aerodynamic loads used for sizing the model and predicting its deformed configurations were generated using the XFOIL software [57]. A wind speed of 25 m/s and angles of attack within the range of \(+/-12\) deg were considered, and the most severe load condition was selected for the structural verification. Finally, a temperature of 180 \({}^{\circ}\)C was applied to all the SMA rods. The stress levels achieved in the different subparts of the model were reported in the following figures. In the plots of Figure 9, the von Mises stress generated in the three configurations is compared for the main structure. The preload causes maximum stress of 144 MPa on the aluminum alloy structure; this level is slightly reduced to 124 MPa by the upward twist contribution of the aerodynamic loads. Finally, a net increase, up to 159 MPa, was estimated for the SMA switched-on configuration. Figure 8: WT model lateral view: preload configuration (blue line, (**a**)), aerodynamic loads (green line, (**b**)), aerodynamic loads + SMA (orange line (**c**)) configurations. Figure 9: Stress level of the main structure of the WT model: stress level in pre-load (**a**), aerodynamic loads (**b**), and aerodynamic loads + SMA fully on (**c**). Figure 10 illustrates the stress level achieved on the most solicited ring. Here, the level ranges between 637 and 711 MPa, the latter one achieved for the full activation configuration. Finally, the skin situation is shown in Figure 11; also, in this case, the most critical conditions are achieved when all the SMA rods are fully activated, arriving at a maximum level of 322 MPa against an initial value of 320 MPa in the preload configuration and 305 MPa under the aerodynamic loads. For all the configurations, the diagonal solicitation of the skin caused by the twist (preload or SMA switched on) is evident. In addition, per each configuration, a strain level was observed on the plates with sensors. Finally, Table 10 provides a summary of the twist angles estimated for all configurations, together with the lowest safety factor achieved among the subparts (main structure, connection rings, skin) and the strain level estimated on the sensor plates. The safety factor of 1.8 was computed for the stress level of 711 MPa of the ring element when aerodynamic loads are applied, and the SMA rods are fully activated. This value was achieved at one edge of the rectangular hole (ring inner view in Figure 10c). Since the refinement of the mesh in the zone is not able to represent a continuous contact transmission between the SMA rod and the ring itself and also considering the local merging of the nodes of the two elements, stress fictitious spikes are expected. A more realistic stress level of 650 MPa averaged in the close zone can be assumed, giving a safety factor of 2. Finally, in Figure 12a, the stress level produced in the SMA rods for the most severe condition, that is to say, in a fully actuated configuration, is illustrated.