Failure Prediction And Enhancement Design Of Composite Laminate Bearing Behavior

Bearing failure of laminates is a critical factor limiting the mechanical performance of mechanically fastened joints in composite structures and the load-carrying efficiency of the whole structural system.However,due to its complex failure mechanisms and high sensitivity to design parameters,it is extremely difficult to develop a corresponding reliable and accurate failure analysis method.This thesis aims at systematically developing and validating a virtual testing model for the bearing failure of composite mechanically fastened joints with high reliability and fidelity,taking the failure mechanism as the center to conduct research from three aspects: physical observation,development and validation of failure predicting model,and enhancement design.Firstly,combing multiple measurement and inspection methods(digital image correlation,two dimensional(2D)X-ray radiography and section microscopy),systematical and multi-dimensional characterization of the pin bearing failure mechanism and failure process of composite laminates with different ply proportions,stacking sequences and ply thicknesses was conducted.It was found that the bearing failure of composite laminates is a considerably complex process with the initiation and interactive propagation of multiple types of damage,including longitudinal compressive failure under three-dimensional(3D)stress states,mixed-mode transverse failure with generic fracture plane angles,mixed-mode interlaminar failure,multiple kink band phenomenon under constraints of adjacent plies,macroscopic shear band crossing multiple plies due to the interaction of multiple damage modes,and accompanying in-plane and out-of-plane large deformation and large sliding on fracture surfaces.The experiment investigation also concluded that a relatively stable load plateau exists after the load drop of multi-directional laminates under pin bearing;different ply proportion has a significant impact on the proportion of different damage modes and the overall damage morphology;increasing the ply thickness results in lower bearing strengths.Secondly,based on the experimental observations,a physically-based failure predicting model of fiber-reinforced composite laminates was established.The material model considers all types of intralaminar and interlaminar mesoscale damage modes,the in situ effect in multi-directional laminates,pre-peak shear nonlinearity,and,for the first time,the notable cohesive-frictional behavior in and between plies of laminates under lateral confinement in bolted joints.It adopts invariant-based 3D phenomenological failure criteria and mechanism-based continuum damage models(longitudinal bilinear damage model and transverse 3D smeared crack model)for simulation of intralaminar damage,and discrete cohesive zone model for simulation of interlaminar damage.The numerical model is based on the finite element method and adopts the crack band model and a fiber-aligned mesh for alleviating the dependence of local continuum damage model on the mesh size and orientation when solving strainsoftening problems.The model was validated using experimental data of pin bearing in this thesis and bolted joints with different configurations and geometries from the literature.Good correlations between the simulations and tests were found in the macroscopic mechanical response as well as the mesoscle failure mechanism,proving the accuracy and versatility of the model.Finally,based on the experimental and numerical study of the bearing failure mechanism,from the viewpoint of suppression on damage onset and propagation,enhancement design schemes using local ply-level hybridization to single-and double-shear composite bolted joints were proposed.After evaluation of the schemes using the proposed numerical model,it was found the knee point load and ultimate load of the structures could be improved up to 59.21% and 50.77%,and the change of the mesoscale failure mechanism agreed with the design expectation,demonstrating the rationality and effectiveness of the design concept.