## 3 Working Principle of the Concept and Main Features
The external surface of each SMA rod, except the zones clamped within the main block (180 mm over a total length of 220 mm), was covered by the heating coil, as shown in Figure 1. A total of 16 turns were used per SMA, spaced between each other to guarantee a uniform distribution of the temperature along the rod axis and compensating conduction losses occurring at the clamped edges. A 5 \(\times\) 3 mm rectangular cross-section was selected for the coil curb to guarantee an effective flat contact with the SMA surface. The process identified for the preload can be summarized in the following steps:  
Figure 3: Comparison plot between the blade equivalent torsion elastic line and the load-unload torsional cycles of the SMA at different temperatures. With reference to Figure 1, the main structure is clamped on the left side; this operation is performed through a bench vise. 2. The heating coil is placed between the two spars and centered with respect to the lateral holes present on the two sides of the main structure. 3. The SMA rod is inserted into the main structure, passing through the circular hole illustrated in the bottom right detail of Figure 1 and, then, through the heating coil; the operation ends with the embedding of the square milled left edge of the SMA rod into the shaped hole present in the left side of the main structure. As a result, the SMA rod is clamped on the left edge of the main structure. 4. The crown component is integrated on the SMA-free edge on the right and hosted within the circular hole. Note that the crown component presents a shaped hole suited to embed the edge of the SMA rod. The lateral interface is now completely inserted in the main structure. 5. A torsion couple is applied through a tailored tool, fitting the lateral interface of the crown illustrated in the bottom right detail of Figure 1. This operation lasts to achieve the alignment of the radial holes of the crown with the corresponding ones on the main structure. 6. Pins are inserted in the holes of the main structure to match the corresponding radial holes of the crown; as a result, the SMA-crown assembly is definitely constrained to the main structure. 7. The torsion couple is removed, letting the main structure and the SMA achieve the equilibrium. The process described above is monitored through a dedicated sensor network to avoid exceeding the stress threshold and acquire the pre-load twist obtained. To this scope, the above-mentioned strain sensors were used. The integration process of these sensors was addressed after having joined all the cells along the span direction and before the installation of the skin. For both demonstrators, a unique interface through which driving cabling was identified. This is represented by the lateral cross-section suited for the connection to the whirl tower hub and to the actuator used to change the AoA during the tests in the wind tunnel. The cables relevant to the sensors of a certain cell are separated by the bundle and redirected to specific parts of the cell, while the remaining ones pass through the subsequent ribs to enter the next cell. A bonding process through an epoxy resin curing at RT was adopted for the integration of the strain sensors. Following this approach, a specific layout was defined on a structural part, an aluminum alloy plate, clamped at one rib of the bay and sliding into a guideway within the opposite rib. Figure 4 illustrates the concept at the basis of the sensing network. Pure twists and pure bending between two consecutive ribs can be detected, as well as a combination of them. When, as during the pre-load process, pure bending is transferred on the plate (scheme (a)), bending and twisting are generated. The bending is detected by the sensors at the edges of the plate, placed in a collocated configuration, while only the twist is detected in the middle due to the inversion of the bending moment towards the plate span. When flapping solicitation occurs (scheme (b)), bending is detected both on the edges of the plate and in the middle.