Study On Out-of-plane Mechanical Properties Of 3D Woven Composites Based On Mesoscale In Situ Method

Compared with laminated composites,3D woven composites have superior delamination and impact resistance due to their special spatial structure,and are widely used in aviation,aerospace and other fields.However,due to the special spatial structure,3D woven composites have very complex damage mechanism,which brings severe challenges to the accurate characterization and simulation of 3D woven composites.In this thesis,through the investigation of the literature related to composites,it is found that research on the out-of-plane properties of composites are still mainly focused on laminated composites.Test methods,damage mechanism research methods and simulation methods for out-of-plane properties of 3D woven composites are still incomplete.In addition,the key parameters of 3D woven composites under different failure modes lack reasonable access methods.These problems hinder the application of 3D woven composites in the engineering field.In order to solve the above problems,the key parameter characterization method,damage mechanism research method and simulation method of 3D woven composites under out-of-plane loading are studied in this thesis.The main research contents of this thesis are as follows:A mesoscale in situ(MIS)method is proposed to assess fracture toughness of intra-yarn and interface in 3D woven composites.Specimens used in experiments are cut from 3D woven composite plates,so that they contain typical in situ interface and yarn structures,and can reflect the real properties of the material.The specimen is selected in the form of compact tension.Through the special design of specimen thickness,cutting surface,cutting position and cutting tip position,relatively single failure mode and controllable crack propagation path are realized.The fracture toughness of interface and intra-yarn in 3D woven composites are calculated by the area method.The possible influence of specimen size and fracture toughness calculation method on the measured intra-yarn fracture toughness is discussed.In addition,the possible influence of matrix plasticity on the measured interface fracture toughness is discussed by the simulation.Based on the MIS method,an experiment-simulation method is proposed to obtain the interfacial normal strength of 3D woven composites.Due to the in situ cutting and special specimen design,the specimen contains typical in situ interface structure of 3D woven composites,and the failure mode with interface debonding as the core appears.The mesoscale DIC technique is used to obtain the strain distribution on the specimen surface.The accuracy of geometric modeling and elastic input parameters of the simulation model is verified by comparing the surface strain distribution predicted by the simulation with the measured results of mesoscale DIC.The interfacial normal strength of 3D woven composites can be determined by comparing the experimental and simulation results,and the effects of geometric modeling accuracy,mesh size and interface parameters on simulation results are discussed.In addition,interfacial shear strength of 3D woven composites is preliminarily determined based on the MIS method.An experimental method combining microscope observation,multiscale DIC and in situ test based on CT is proposed to reveal the complex damage evolution of3 D woven composites under short beam shear loading.The surface damage process and the final failure morphology of the short beam shear specimen are observed with a microscope,and the cause of fiber kinking is explained.The global and local strain distributions on the specimen surface are obtained by multiscale DIC,which explains the causes of interfacial debonding and transverse cracking of weft yarn.Finally,the in situ test based on CT is used to identify the reasons for nonlinear section of the load-displacement curve and the decline of load-bearing capacity in the later stage of the test.The information obtained by different observation methods is complementary,which is conducive to in-depth understanding of the complex damage mechanism of3 D woven composites under short beam shear loading.A progressive damage model considering the coupling of several damage modes such as interface debonding,fiber kinking,yarn transverse cracking and matrix plasticity is proposed.Based on micrographs of typical sections of 3D woven composites,a full-size geometric model of short beam shear specimens containing yarns,matrix and interface is established.The mechanical properties and damage evolution of 3D woven composites under short beam shear loading are predicted by using the material properties obtained by the MIS method as inputs.The nominal stiffness,maximum load,variation trend of the load-displacement curve and damage distribution at different stages predicted by the simulation model are in good agreement with the experimental results,which verifies the accuracy of the simulation model and the parameters obtained by the MIS method.Finally,the effect of interfacial shear fracture toughness on simulation results is discussed.