Cohesive Zone Modelling (CZM) for Mesoscale Models of Fiber-Composite Laminates for the Numerical Calculation of Impact Damage

Kurzfassung

In order to calculate the impact damage and the deformed state of the composite laminates after the impact, especially the surface indentations, particular constitutive laws of the damage models are required. These constitutive laws describe the phenomenon of delaminations which cannot close completely after unloading. This is caused by matrix debris within inter-fiber fracture and ply debris within delamination and results in permanent indentation. Therefore, the common bi-linear traction separation law for describing delamination is modified and the feature of counter stresses is added. These counter stresses act during the unloading and open the blocked delaminations afterwards. The implementation of these constitutive laws is done in the Abaqus user-subroutine VUINTERACTION and determines the developed user-defined cohesive contact model. For the purpose of verification, the user-defined cohesive contact model is tested for several different FE models and compared to the built-in cohesive contact model of Abaqus capturing delamination damage. Further, an impact analysis of the user-defined cohesive contact model has revealed that mode II delaminations dominate mode I delaminations in the case of impact simulations. This is confirmed by a sensitivity analysis of the user-defined cohesive contact model in which the corresponding properties of the counter stresses in tangential directions are more crucial than the properties in normal direction. Additionally, to incorporate the phenomena of debris which penetrate existing mode II delamination and occurring roughnesses of delaminated interfaces resulting in permanent indentation as well, the user-defined cohesive contact model is extended by a coupling approach. This approach couples the counter stresses in tangential directions with the counter stresses in normal direction. Furthermore, an approach based on microscope images is presented to determine the parameters of the user-defined cohesive contact model, in particular those of the counter stresses. A final validation of the user-defined cohesive contact model in the case of a quasi-isotropic CAI specimen has shown that the model is suitable for describing permanent indentation. However, the simulative results of the deformed state are different compared to the experimental results. The reason is stated to be the phenomenon of fiber fractures with fracture surfaces that prise open and consequently, contribute to permanent indentation. This phenomenon is not included in the user-defined cohesive contact model but has to be part of the Abaqus user-subroutine VUMAT as a user-defined material model.