# Design, Manufacture and Wind Tunnel Test of a Modular FishBAC Wing with Novel 3D Printed Skins
## 5 Structural Test: Skin and Stringers Panels  
One of the riskier aspects of this FishBAC wing design is producing the skins and stringers by additive manufacturing, using two different materials in a single print. To defisk this design feature, a feasibility study was performed by 3D printing a series of skin/stringer test coupons and testing them under tension. This experiment allows testing the interface between the stringers (Armadillo) and the skin (Ninjaflex), in a configuration akin to the FishBAC's skin/stringers structure. A series of 50 mm \(\times\) 87 mm skin/stringer test coupons were used in this experiment (Figure 15). With a total of three stringers with a spacing of 7 mm between them, these coupons resemble the FishBAC skin dimensions. Each one of the specimens was tested under tension using a Shimadzu tensile machine fitted with a 1 kN load cell.  
An Intertum Video Gauge point tracking optical system was placed perpendicular to the test specimens to measure in-plane strains. In this test, strains were measured in two  
Figure 14: Stress–strain curves of the Ninjaflex material-axial strain (**left**) and transverse strain (**right**).  
\begin{table}
\begin{tabular}{c c c c} \hline \hline
**Material** & **8552/IM7** & **Armadillo** & **Ninjaflex** \\ \hline \(E_{11}\) & 168.91 GPa & & \\ \(E_{22}\) & 8.00 GPa & 275 MPa & 15.3 MPa \\ \(E_{33}\) & 8.00 GPa & & \\ \hline \(G_{12}\) & 4.79 GPa & & \\ \(G_{13}\) & 4.79 GPa & 97.0 MPa & 5.50 MPa \\ \(G_{23}\) & 2.76 GPa & & \\ \hline \(v_{12}\) & 0.32 & & \\ \(v_{13}\) & 0.32 & 0.42 & 0.39 \\ \(v_{23}\) & 0.45 & & \\ \hline \hline \end{tabular}
\end{table}
Table 2: Summary of material properties of the composite FishBAC wind tunnel wing model. Note that \(v_{23}\) is obtained from a typical assumption for CFRP UD laminates, where \(G_{23}=E_{22}/2(1+\
u_{23})\)[31].  
Figure 13: CFRP tensile specimens (0\({}^{\circ}\) fibre direction): axial stress vs axial strain (**left**) and axial stress vs transverse strain (**right**).  
different regions: _(i)_ between the fixed end of the specimen and the first stringer, and _(ii)_ between the loaded end and the third stringer.  
Figure 16 shows strain-load curves for these specimens. The results show that these skin/stringer panels can be strained up to 400% before ultimate failure is reached. Furthermore, it was observed during the test (Figure 17) that these structures fail between stringers. However, it is important to note that the damage propagates very slowly and that it takes a significant amount of energy from the point where damage starts to be observed to the point where the structure catastrophically fail. These results are very positive due to two main reasons: _(a)_ the FishBAC's skin is pre-stretched by 15% and will see roughly another 10% strain during operation, and so the required strains are very far from failure and _(b)_ any cracks or surface damage on the skin should not lead to catastrophic failure at these strain levels. Therefore, the two-phase skin/stringer panels were deemed safe to use in this wind tunnel experiment.  
Figure 16: Stress-strain curves of the simplified skin/stringer 3D-printed panels. The colour code is as follows: dark blue (specimen 1), light blue (specimen 2), red (specimen 3), yellow (specimen 4), maroon (specimen 5).  
Figure 17: Skin/stringers tensile test: specimen clamped to test machine (**left**) and specimen near ultimate failure (**right**).  
Figure 15: Skin/stringer tensile test specimens.