Investigation On Damage Evolution And Failure Mechanism Of 3D Woven Composites Under Out-of-plane Loads

Compared with traditional laminates,binder yarns enhance the integrity and impact resistance of 3D woven composites.The development of advanced weaving technology makes the fabrication of preforms faster and cheaper.The liquid moulding technology can realize the net size moulding of complex and large-size structural parts.Due to the above advantages,3D woven composites are gradually accepted among academia and industry and have been widely used in aircraft,aerospace and transportation industry.The author consulted the literature on damage evolution of composite materials,and summarized the state-of-art of damage mechanism investigation and mechanical properties prediction methods of woven composites from experiment and simulation.It is found that the research on damage evolution of 3D woven composites has achieved fruitful results.However,due to the complicated failure mechnism and the limitation of experimental methods,the research on the damage evolution mechanism and mechanical properties prediction method s under out-of-plane loads is far from perfect.In this paper,the damage evolution process,failure mechanism and mechanical properties prediction method of 3D woven composites under out-of-plane loads are studied.The main work is as follows:An in situ experimental method for damage evolution of 3D woven composites under out-of-plane loads is proposed.The damage evolution process inside the material is recorded continuously and in situ by synchrotron radiation computed tomography(SRCT).Using this method,the damage mechanism of 3D woven composites under out-of-plane tension and shear loads was studied.Through the analysis of the results,a "three-stage" damage law and the main failure modes of 3D woven composites under out-of-plane tension were found.Typical failure modes of 3D woven composites under out-of-plane shear loads were found through in situ shear tests.Based on the above results,the influence of braiding angle of binder yarns on the out-of-plane strengths of 3D woven composites is discussed.Expanding the application of SRCT into fatigue damage research of 3D woven composites.By combining SRCT and high-speed camera,a new method is proposed to study the damage evolution mechanism of 3D woven composites under out-ofplane tension-tension fatigue loads.The macro and micro damage mechanism of 3D woven composites under tension-tension fatigue loads is studied using this method.A "three-stage" stiffness degradation law of woven composites under out-of-plane tension-tension fatigue load is found.Based on these results,an fatigue life expression requiring a small amount of experimental data is proposed.Accurate elastic parameters of fiber bundles are the key to predict the elastic mechanical properties of 3D woven composites.Here,the author compares the predicted results of Chamis equation with those of finite element method based on random fiber distribution model.Introducing the modified index of fiber volume fraction into the Chamis equation,a new method is proposed to predict the elastic parameters of unidirectional composites.At last the elastic properties of carbon fibers and glass fibers unidirectional composites are predicted to verify this method.In order to overcome the problem of the traditional random fiber distribution algorithms which require lots of iterations and are difficult to generate random fiber distribution models with high fiber volume fraction,Honeycomb-based algorithm for generating random fiber distribution with high fiber volume fraction is proposed.The algorithm can generate random fiber distribution models with high fiber volume fraction with limited iterations.Then,a progressive damage method is used to verify the accuracy of the model generated by the presented method in predicting the transverse modulus and strength of unidirectional composites.In addition,the load direction dependence of the predicted transverse mechanical properties of differe nt fiber distribution models is discussed.Based on meso-structure reconstruction using CT images,a progressive damage evolution model of 3D woven composites under out-of-plane tension is proposed.In this model,3D Hashin and Von Mises criterion are used as damage initiation criterion of fiber yarns and the matrix,respectively.The exponential damage evolution law is used to describe damage evolution of materials.The cohesive zone model is used to simulate the damage and evolution process of the interface.The equivalent elastic parameters of the fiber yarns are calculated using the elastic parameters prediction method of unidirectional composites proposed in this paper.Based on the above research results,the prediction of out-of-plane tensile properties of 3D woven composites are carried out.The results show that this model can accurately predict the damage and failure behavior of 3D woven composites under out-of-plane tension loads.In addition,the influence of interface parameters on the out-of-plane tensile strength of 3D woven composites is discussed.Based on the above research work,the experimental and simulation method of mechanical properties of 3D woven composites under out-of-plane loads are systematically proposed in this paper.The failure mechanism of 3D woven composites subjected to out-of-plane loads is studied using the above method.The proposed method and related research findings provide the necessary research basis for the damage evolution research and simulation of 3D woven composites.