## 2 Advanced Joint Configuration  
A flat sample configuration was chosen for rapid execution of the related testing activities even though the real joints were positioned on a cylindrical surface, as in Figure 2. Such a real mounting configuration was tested during the full-scale bending-shear test of the HELIPLAT prototype, as in [13], confirming the validity of the selected experimental campaign as described in the following sections. Two specific experimental configurations were proposed: a sandwich joint configuration with a 9 mm core-thickness of standard Rohacell 51 rigid foam reinforced with glass microspheres, and a solid sample with the same lay-up. The sample dimensions were 240-mm-long, 130-mm-wide with an active zone of damage around the hole of 170-mm-long and 130-mm-wide. The preliminary design of a composite joint typically starts with the definition of the correct laminate lay-up and material selection. According to [2], all of the evaluated samples tested in the experimental campaign were manufactured with the same Uni-Directional (UD) M40J/epoxy material and the same laminate lay-up [+20/\(-\)20/90/45/\(-\)45/90/45/\(-\)45]s for every single skin, which maximizes laminate bearing strength (Figure 3), avoiding net-tension/shear-out failure. The nominal thickness of a single composite laminate is 2.16 mm, and the approximate thickness of a single lamina is around 0.135 mm. The mechanical properties of the adopted materials are reported in Tables 1-3. The advanced composite joint is reinforced with two steel plates (yellow part in Figure 3) with an external diameter of 80 mm and a thickness of 1.5 mm. The internal steel cylindrical support connected to the flat external steel plates has an internal diameter of 20 mm and a thickness of 3.95 mm. The reinforcement is bonded to the composite laminate by means of Araldite adhesive, as detailed in Table 3 and as shown in Figure 4 (green layer). The internal cylinder provides the loading transmission between the joint structure and the bolt during the pin-loaded test. The relative positions of the Strain-Gauge (SG) are reported in Figure 3 according to [2]. Experimental static properties for the proposed flat configurations are summarized in Table 4[2]. The load was applied up to sample failure to understand the modes of fracture and the improvement obtained with the advanced configuration. The experimental results show a reduced capacity of the simple solid and sandwich configuration to overcome the typical 20 kN pin bearing load. The average failure load for the solid composite joint was about 20,481 N and 39,481 N for the sandwich configuration. \begin{table}
\begin{tabular}{l l l l l} \hline \hline \multicolumn{1}{c}{**Property**} & \multicolumn{1}{c}{**Units**} & \multicolumn{1}{c}{**Symbol**} & \multicolumn{1}{c}{\
\begin{tabular}{c} **LS-DYNA** \\ **Parameters** \\ \end{tabular} } & \multicolumn{1}{c}{
\begin{tabular}{c} **Experimental** \\ **Value** \\ \end{tabular} } \\ \hline Density & [kg/mm\({}^{3}\)] & \(\rho\) & RHO & \(1.6\times 10^{-6}\) * \\ Modulus _1-direction_ & [GPa] & \(E_{1}\) & EA & 215.0 * \\ Modulus _2-direction_ & [GPa] & \(E_{2}\) & EB & 66.7 * \\ Shear modulus _12-direction_ & [GPa] & \(G_{12}\) & GAB & 4.3 * \\ Shear modulus _23-direction_ & [GPa] & \(G_{23}\) & GBC & 3.6 * \\ Shear modulus _31-direction_ & [GPa] & \(G_{31}\) & GCA & 3.6 * \\ Major Poisson’s ratio & [-] & \(\
u_{12}\) & - & 0.27 * \\ Minor Poisson’s ratio & [-] & \(\
u_{12}\) & PRBA & 0.0837 \\ Strength _1-direction tension_ & [GPa] & \(\sigma_{11}\)\({}^{T}\) & XT & 2.365 \\ Strength _2-direction tension_ & [GPa] & \(\sigma_{22}\)\({}^{T}\) & YT & 0.072 \\ Strength _1-direction compression_ & [GPa] & \(\sigma_{11}\)\({}^{C}\) & XC & 1.270 \\ Strength _2-direction compression_ & [GPa] & \(\sigma_{22}\)\({}^{C}\) & YC & 0.140 \\ Shear strength _12-direction_ & [GPa] & \(\tau_{12}\) & SC & 0.063 \\ \hline \hline \end{tabular}
\end{table} 
Table 1: Stiffness material properties of M40J/epoxy as published in [PERSON] et al.