# Effect of Cohesive Properties on Low-Velocity Impact Simulations of Woven Composite Shells
## 4 Conclusions  
In this study, the influence of the interface stiffness and strength on the low-velocity impact response of woven fabric-reinforced composite shells was investigated. For that purpose, FE models that incorporated continuum shell elements and cohesive surfaces to account for the intra- and interlaminar damage, respectively, were generated in ABAQUS/Explicit. Figure 16: Effect of the interface stiffness with \(\tau_{n}^{0}=30\) MPa and \(k_{n}=k_{s}=k_{t}=10^{6}\) N/mm\({}^{3}\): (a) CSQUADSCRT output variable. (b) CSDMG output variable. Figure 17: Effect of interface strength on the delamination predictions with \(k_{n}=k_{s}=k_{t}=\)10\({}^{6}\) N/mm\({}^{3}\), represented throughout the FE model. The simulations were performed using interface stiffness values ranging from \(10^{1}\) N/mm\({}^{3}\) to 2.43\(\times 10^{6}\) N/mm\({}^{3}\) and interface strength values ranging from 15 MPa to 60 MPa. To be able to analyse the results, the force-time, energy-time, and force-displacement curves were presented. Additionally, in order to assess the extent of cohesive degradation and delamination within the FE models, the variation in the percentage of nodes initiating cohesive degradation and the percentage of nodes that become fully delaminated were plotted. It was observed that, for values between \(10^{3}\) N/mm\({}^{3}\) and \(2.43\times 10^{6}\) N/mm\({}^{3}\), the interface stiffness has a negligible effect on the force and energy histories, and, within this range, there is a satisfactory numerical-experimental correlation. The use of stiffness values outside this range resulted in convergence issues (high values of interface stiffness) or no numerical-experimental correlation (low values of interface stiffness). Although the interface stiffness has a minimal impact on the response curves' accuracy, it has a significant impact on the delamination predictions. Only for an interface stiffness value of \(10^{5}\) N/mm\({}^{3}\) do completely delaminated nodes become clearly visible. Furthermore, it is worth noting that a significant increase in computational cost was observed with the increase in interface stiffness. The results show that the shear interface strength values (peak contact stresses) have a negligible impact on the force and energy histories. On the other hand, the normal interface strength affects the numerical predictions of the maximum impact force. Increasing its value from 15 MPa to 30 MPa resulted in an 8% increase in the maximum impact force, and thus a deviation from the experimental data. Only the variation in the normal interface strength was found to have an effect on the delamination predictions. It was observed that the percentage of fully delaminated nodes decreases with the increase of the normal interface strength; however, the percentage of nodes that initiate cohesive degradation remains almost unchanged. In addition, it was observed that varying the interface strength had minimal impact on the computational cost of the solutions. The results obtained from this study provide valuable insights and recommendations for accurately and efficiently simulating delamination phenomena. By successfully validating the FE models used in this research, a strong basis is established for conducting future numerical investigations. It also presents an opportunity to reduce the reliance on costly and time-consuming experimental testing. Furthermore, the utilisation of numerical simulations enables researchers to investigate delamination in a more detailed and comprehensive manner. The FE models can capture intricate delamination mechanisms, predict failure modes, and provide detailed information about stress distribution and energy absorption during the process. This information can be utilised to enhance design guidelines, develop mitigation strategies, and optimise the performance and reliability of laminated shell structures. Conceptualisation, L.M.F. and P.N.B.R. ; methodology, L.M.F., P.N.B.R. and C.A.C.P.C. ; software, L.M.F. ; validation, L.M.F., P.N.B.R. and C.A.C.P.C.