| Carbon fiber reinforced composites have been increasingly used in the fields of aerospace, aircraft and so on due to high strength/stiffness-to-weight ratios and designability. Yet, complicated failure mechanisms, especially delamination failure pose a large challenge to the light design and application of composites, and affects the stiffness, strength and integrity of composite structures largely. Currently, the cohesive theory which based on the elastic-plastic fracture mechanics has been used widely in the research of delamination failure for composite materials, as it can predict crack initiation and propagation simultaneously.There are already a lot of cohesive zone models (CZM), such as bilinear cohesive models, exponential cohesive models, trapezoidal models et. al. Until now there is no unified CZM that can represent all true fracture process zones during the delamination of composites under various loads and environments. Simultaneously, the influence of different cohesive law shapes and strength on numerical accuracy, robustness and computational efficiency of implicit finite element analysis (FEA) has not been fully elucidated, because of the softening curve of CZM.This paper developed zero-thickness cohesive numerical calculation technique to implement triangle cohesive model and improved Xu and Needleman exponential CZM based on the cohesive theory and Finite Element Analysis, explored the delamination failure mechanism of composites laminates under compression and bending loads, analyzed some numerical problems in FEA of CZMs such as convergence and mesh-dependence. The main research work as follows:firstly,2D and 3D cohesive finite technique was developed using ABAQUS-UEL and applied to the study of delamination failure mechanisms of composite laminates. The effects of cohesive shape, cohesive strength and mesh size on numerical convergence and mesh sensitivity is revealed, and proved the robustness of zero thick interface elements. Secondly, the delamination of composite laminates under compression and bending loads was studied based on the developed cohesive finite technique, the effects of structure size, layup et. al on progressive failure of the single or mixed mode crack and the load responses was studied compared with the experiments. It was decided by the fracture toughness of single fracture mode and the mode ratio of mixed I/II mode, and it reflected the universality in the study of failure mechanism when using this numerical technique.Finally, because the mode-II delamination failure of composites is accompanied by complex frictional contact, a cohesive/frictional contact coupled model based on a cohesive model and Mohr-Coulomb frictional contact law was proposed originally, the FEA code was developed, and the effects of the friction coefficient, cohesive strength, layup et. al on the delamination with friction and the load response was predicted compared with the analytical results.There are three innovations in this paper:firstly, a 3D cohesive finite technique was developed, and some numerical problems such as mesh size effect, mumerical convergence, virtual displacement jumping was better solved to some extent in the FEA based on cohesive model. Secondly, the finite technique was applied to the analysis of delamination failure under compression and bending, predicted the failure mechanism under the corresponding load accurately. Finally, a cohesive/frictional contact coupled model was proposed originally, and applied to the analysis of delamination failure with friction.This research provided theoretical and technical support for choosing cohesive law parameters appropriately, predicting composite laminates delamination failure mechanisms and stiffness degradation accurately and efficiently. The research results are expected to be applied in the composite structure of aerospace, composite cylinder, wind turbine blade, automobile and watercraft to predict their delamination failure mechanism in application. |
PDF Full Text Request