# Effect of Cohesive Properties on Low-Velocity Impact Simulations of Woven Composite Shells
## 3 Results
### Effect of the Interface Strength  
This section analyses the impact the response curves, delamination, and computing cost of the solutions as a function of the normal peak contact stress \(\tau_{n}^{0}\) and shear peak contact stresses, \(\tau_{s}^{0}=\tau_{t}^{0}\). For that purpose, simulations were carried out in two separate sets, assuming a constant interface stiffness of \(10^{6}\) N/mm\({}^{3}\), and in accordance with the values shown in Table 2: (i) with a normal peak contact stress \(\tau_{n}^{0}=15\) MPa and shear peak contact stresses \(\tau_{s}^{0}=\tau_{t}^{0}=30,45,60\) MPa; (ii) with a normal peak contact stress \(\tau_{n}^{0}=30\) MPa and shear peak contact stresses \(\tau_{n}^{0}=\tau_{t}^{0}=30,60\) MPa. The force-time, energy-time, and force-displacement curves are shown in Figures 9-11 for the first set, and in Figures 12-14 for the second set. The results demonstrate that adjusting the value of the shear interface strength has a minor impact on the force-time, energy-time, and force-displacement curves for the studied normal interface strengths (\(\tau_{n}^{0}=15\) MPa and \(\tau_{n}^{0}=30\) MPa). Additionally, all of the scenarios that were taken into consideration had comparable predictions for the maximum impact force, maximum displacement, and absorbed energy. On the other hand, it is possible to appreciate how the force-time and force-displacement curves are impacted by changes in the normal contact strength. When the normal contact strength increases from 15 MPa to 30 MPa, the maximum impact force as predicted by the numerical model increases by around 8%. However, as previously indicated, the variation in the normal interface strength has no impact on the maximum displacement and the absorbed energy because the velocity profile was integrated into the FE models according to the experimental results. This interface property, then, has an impact on the agreement between the numerical and experimental findings. A greater numerical-experimental correlation was found for \(\tau_{n}^{0}=15\) MPa than for \(\tau_{n}^{0}=30\) MPa, regardless of the shear interface strength. This discrepancy suggests that the chosen normal strength value of 30 MPa does not accurately capture the true behaviour of the material under investigation. Based on these results, it is evident that caution must be exercised when  
The delamination predictions are also affected by the variation in the normal interface strength. Figures 15 and 16 show, for \(\tau_{n}^{0}=15\) MPa and \(\tau_{n}^{0}=30\) MPa, respectively, the percentage of nodes in the composite shell with the output variables CSQUADSCRT = 1 and CSDMG > 0.8. Moreover, Figure 17 illustrates the extent and location of the delamination predictions in the FE models. Figures 15a and 16a show that neither the variation in the normal interface strength nor the variation in the shear interface strengths affect the percentage of nodes that achieve the damage initiation threshold. For every case that was looked at, its value is essentially constant and around 80%. It can also be observed in Figures 15b and 16b that, although the  
Figure 12: Effect of the interface strength on the force–time results with \(\tau_{n}^{0}=30\) MPa and \(k_{n}=k_{s}=k_{t}=10^{6}\,\mathrm{N/mm^{3}}\). Figure 13: Effect of the interface strength on the energy–time results with \(\tau_{n}^{0}=30\) MPa and \(k_{n}=k_{s}=k_{t}=10^{6}\,\mathrm{N/mm^{3}}\). amount of fully delaminated nodes decreases as the normal interface strength increases, the effect of changing the shear interface strengths on delamination predictions is less pronounced. In fact, the simulation results show distinct behaviours for the alteration of the shear interface strengths for \(\tau_{n}^{0}=15\) MPa and \(\tau_{n}^{0}=30\) MPa. With an increase in shear interface strengths, the percentage of totally delaminated nodes grows marginally for \(\tau_{n}^{0}=15\) MPa, but it falls for \(\tau_{n}^{0}=30\) MPa. However, the variation is not significant in any situation.