## 5 Numerical-Experimental Correlation  
In this section, the numerical results, obtained by means of the previously described numerical model, are compared to the data recorded during the experimental test. In particular, the strains, obtained from the experimental test, are compared to the strains predicted at strain gauges locations for both the Tie and the Fastener numerical models.  
Figures 12-18 report the numerical-experimental comparisons at strain gauge locations on the stringers (SG-1, SG-2, SG-5, and SG-6), on the composite skin (SG-3, SG-4, SG-7, SG-8, and SG-9), and on the aluminium skin (SG-10, SG-11, SG-12, SG-13, and SG-14). It is worth noting that, for this kind of test, a maximum deviation equal to \(\pm 5\%\) can be observed in the experimental data [(60; 61)].  
Figure 16: Numerical–experimental comparisons for SG-9 and SG-10: numerical TIE and Fasteners models.  
Figure 17: Numerical–experimental comparisons for SG-11 and SG-12: numerical TIE and Fasteners models.  
Figure 18: Numerical–experimental comparisons for SG-13 and SG-14: numerical TIE and Fasteners models.  
Figure 15: Numerical–experimental comparisons for SG-7 and SG-8: numerical TIE and Fasteners models.  
According to Figures 13-18, good agreement is found between the experimentally measured strains and those predicted by the numerical TIE model; while an excellent agreement is found between the experiment and the numerical Fasteners model which uses fastener connectors to couple the structure components. Indeed, the fastener model is able to provide a more realistic deformation field of the structure resulting in a less stiff global behaviour when compared to the TIE model. This behaviour is confirmed by Figure 19, which introduces the comparison between the stiffness of the investigated numerical models. Finally, Figure 20 shows details of the displacements of the Tie and Fastener numerical models. From this figure, the capability of the Fastener-based numerical model to allow the relative displacement of all the structure subcomponents can be appreciated.  
As already remarked, [PERSON]'s failure criteria were used to investigate the damage behaviour of the composite sub-components. Indeed, no damage occurred in composite sub-components if a tensile  
Figure 19: TIE and Fastener numerical models’ stiffness comparisons.  
Figure 20: Out of plane displacements at 300 kN (10\(\times\) deformation factor in the z-direction–units in mm).  
load of 300 kN was applied. This is confirmed by Figures 21 and 22, which introduce the values of the Hashin's failure criteria, respectively, for the fibres and for the matrix phases resulting from the Fasteners-based numerical model. Moreover, in Figures 21 and 22 the values of the Hashin's failure criteria induced by the stress concentration due to the fastener can be appreciated. Indeed, the different material characteristics of the CFRP and metal regions result in a complex stress field at their interface. The faster formulation adopted in this work allows the transfer of concentrated loads between the metal and CFRP regions. Despite the fact that this approach could lead to different predicted results at the interface, the results from a macroscopic point of view can be considered acceptable in terms of stress transfer between the CFRP and metal regions.  
Finally, the predicted plastic deformations in the metallic region, when a tensile load of 300 kN is applied, are introduced in Figure 23. Indeed, the metallic plate experiences plastic deformations; however, as expected, an increase of the metallic skin thickness would restrain plasticity, as confirmed by the fact that the metal portion of the metal skin coupled with the doubler (see Figure 10) does not experience any plastic deformation.  
Figure 21: [PERSON]’s failure criteria: fibre.  
Figure 22: [PERSON]’s failure criteria: matrix.