## 4 Discussions  
In the calibration phase that was initially performed for the skin coupons, we varied several parameters and evaluated how this variation influenced the spring-in displacement:  
* Fiber volume fraction through-thickness variation from the initial 52% to 56% brought a reduction of 0.15 mm in the displacement value;
* A constant fiber volume fraction through the thickness of the laminate brought an immediate response in the laminate distortion behavior, decreasing the max displacement to 0.39 mm;
* A further improvement through model calibration was obtained by applying a variable friction coefficient between the part and the tool. The parameter values were chosen as 0.01 before the gel point and as 0.1 past the gel point threshold. This step did not influence the deflection value;
* A trial that further decreased the computed spring-in displacement value modified the initial degree of cure for the resin and applied a constant friction coefficient (high value) = 1. The resulting calibration decreased the max displacement to 0.37 mm,  
Figure 37: Strain variations days after demolding for two spar2 coupons: (**a**) day 1 after demolding; (**b**) day 5 after demolding. Figure 38: (**a**) Degree of cure in the 5 mm C-spar2 coupon. (**b**) Maximum total strain at the end of the curing process for the 5 mm C-spar2 coupon. which was considered acceptable since no other calibration methods showed significant improvement compared to the test trial measurements;
* It has been shown in previous research [35] that friction coefficient values can be assigned at reasonable lower values (0.2-0.5) together with the implementation of orthotropic frictional behavior between the part and the tool. Given the quasi-isotropic lay-up of the laminates we studied, we considered this option to have a limited impact on distortion during our experimental and numerical trials [36]. Having learned these lessons, a different geometry was simulated--the C-shape specimens. These were extremely valuable in the overall frame of the ELADINE project, especially for providing geometrical tolling compensation data on the spar flange to web angle distortion. The C-shape specimens were manufactured, monitored, and measured following the same procedure that was used for the skin coupons--see Figure 23. Evaluating spring-in angles, we encountered some remarkable results:
* Both in the experiment and the simulations, the spring-in angle increased for larger corner radii. The mean value for a 3 mm thick specimen was a 1.34\({}^{\circ}\) (1.78%) spring-in angle increase for a 12 mm radius corner. For the same coupon, the 5 mm radius corner exhibited only a 1.03\({}^{\circ}\) (1.34%) increase in spring-in angle. In conclusion, the experimentally measured distortion shows that the spring-in has a greater magnitude for larger corner radii, thus needing more careful compensation measures. * The C-shape specimen web exhibited warping at measurement and simulation. This behavior was initially observed for the simulation of skin coupons and the experimental trials. It is critical to mention that the axis of curvature will differ for parts that are inherently stiffer due to their design geometry. The simulation strategy must consider the inherited geometry stiffness for simulating more complex geometries than flat or slightly curved panels. * Since the coupons were monitored over two weeks after the closing of the manufacturing cycle (no post-curing was performed), the specimens exhibited slight variations in internal residual stress, which led, in turn, to minimal distortion variation. The research team noted this observation, but as the value of the measured displacement change throughout the monitoring interval was considerably low, we chose not to include the post-curing behavior of the laminate in the simulation strategy. * The fact that the simulation results were very close to the experimental values, as summarized in Tables 10-12, is remarkable and encouraging for the research effort.