| Continuous carbon fiber reinforced polymer composites(CFRP)and their honeycomb sandwich composites are commonly used in aircraft structure design,the weak points of their mechanical properties are at the interface.Under external load,the place with large interface stress is easy to cause interlaminar delamination or adhesive layer debonding,resulting in crack propagation and early failure of the structure.In this work,the influence of fiber direction adjacent to the ply and the fiber bridging behavior at the crack tip during delamination propagation are fully considered.The meso interface debonding of carbon fiber/polymer system,the interface fracture and fatigue crack propagation behavior of two-phase composite materials are systematically studied.With the test methods and fracture mechanics methods,the interface failure behavior of fiber/polymer interface,laminate and its sandwich structure are deeply evaluated.The results can provide test and numerical technical basis for damage tolerance design and analysis of composite structures.Firstly,the success rate of micro-droplet debonding test is low,and the stress at the interface in micron scale is difficult to measure.Aiming at the above difficult problems,this work develops the micro-droplet virtual test method,analyzes the influence of component phase material parameters,geometric parameters and curing temperature on the interface strength,and finds the main controlling factors of debonding failure mode.In the numerical model,the flexibility of the free section of the fiber in the test process and the residual stress produced by high-temperature curing are considered.The stress transfer mechanism and failure mechanism of the interface between the reinforced fiber and the resin matrix in the process of droplet debonding are analyzed from three typical stages: damage initiation,damage propagation and friction slip.The damage criterion of micro-droplet test is discussed,which lays a foundation for further studying the influence of fiber laying direction and fiber bridging behavior on interlaminar fracture toughness.Secondly,based on the difficulty that the mechanical properties of the 0° ply interface are used to evaluate the load carrying capacity is too conservative,considering the influence of local adjacent ply fiber direction,the characterization method of fiber laying direction related delamination fracture toughness is developed,and the fiber bridging behavior produced in delamination propagation and its contribution to fracture toughness are discussed.The mode I and mode II delamination specimens with six different ply interfaces(four kinds of linear ply and two kinds of curve ply)were designed.The specimens were prepared by automated fiber placement process and hot pressing curing technology,and the equivalent stiffness of the specimens with different ply interfaces was compared.The modified beam theory(MBT)data reduction scheme is used to characterize the mode I fracture toughness of different ply interfaces.Based on the "Jump" phenomenon in the fracture process,the scattered points of R curve are filtered.The filtered R curve is fitted by Foote model and least square method,and a set of fracture parameters for different ply interfaces are obtained.According to the comparison of characteristic parameters,fiber bridging state and fracture surface analysis,the delamination failure mechanism of different local ply interfaces is deeply analyzed.On the other hand,the mode II fracture toughness of different ply interfaces was characterized and compared by compliance method by NPC and PC experiments.Thirdly,based on the Cohesive zone model(CZM)and finite element method,the mode I fracture behavior of different ply interfaces is predicted by trilinear traction-separation criterion,and the effective interlaminar interface parameters are characterized.Combined with the experimental results,the effectiveness of the finite element model is verified.At the same time,according to the obtained mode II fracture toughness of different ply interfaces,the tractionseparation relationship of mode II delamination interface is characterized by bilinear CZM,and the mode II delamination fracture process of different ply interfaces is reproduced in ABAQUS.At the same time,the axial compression finite element model of rectangular laminates with circular embedded delamination is established.The deformation law of the specimen surface during loading is measured and the displacement field is reconstructed by using 3D-DIC,and the effectiveness of the proposed interface model is proved by comparing with the finite element results.Finally,the influence law of the ply interface with different fiber directions on the axial compression bearing capacity of laminates is compared and analyzed.Fourthly,based on DCB-UBM test method,the characterization method of fracture toughness parameters of CFRP panel honeycomb core sandwich structure is established.Combined moment ratio(MR)vs mixed mode phase angle(φ)curve,the MR in an appropriate range is selected,and the static debonding experiments in two debonding directions under three MRs are completed.Based on the laminated beam theory and J-integral method,the strengthened sandwich beam is analyzed,and the expressions of energy release rate and mixed mode phase angle of rigid layer reinforced five layer DCB-UBM sandwich specimens are derived.The numerical model of crack propagation of sandwich specimens is established,and the initiation and propagation of debonding crack are predicted.Based on the fracture surface method,the basic shape and initial parameters of the traction-separation relationship of CZM model are obtained by using the direct method,and the basic interface fracture parameters are obtained by using the inverse method.This chapter can lay a foundation for further study of fatigue debonding behavior of honeycomb core sandwich structure.Finally,the evaluation method of fatigue debonding performance of CFRP panel honeycomb sandwich specimens was developed by using DCB-UBM test technology,and the fatigue crack propagation behavior of honeycomb sandwich panels was studied.The fatigue crack growth rate was characterized by modified Paris-Erdogan law and normalized Paris law.The mixed mode phase angle φ at crack tip is controlled by the moment ratio MR.The quasistatic debonding fracture toughness at different phase angles was measured.The fatigue test is carried out in the load control mode,select three different MRs,and the load ratio R = 0.1.During the fatigue test,the crack length is determined by fitting the compliance calibration curve,and the propagation mechanism of fatigue crack is analyzed. |
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