Progressive Damage And Failure Of Three-Dimensional Five-Directional Braided Composites With Yarn-Reduction Under Tension-Tension Fatigue Loading

Three-dimensional(3D)braided composites are widely used in key components of aerospace major equipment,such as missile cabins,engine skirts,engine nozzles and flames tubes,due to the advantages of excellent fatigue resistance,interlayer resistance and near-netshape.In general,these parts are conical rotary parts or variable cross-sections,which need to be realized by yarn-reduction or yarn-increment technology.Significantly,these parts are often used under fatigue loading.The introduction of yarn-reduction or yarn-increment will give rise to the structural deformation,crack initiation,and structural instability,threatening the safety.At present,relevant research at home and abroad mostly focuses on materials with “periodic cells”,while the research on special-shaped components is extremely lacking.Hence,it is difficult to match properties of materials and structures.In this paper,based on the conical rotary part,the three-dimensional five-directional braided(3D5D)composites without/with yarn-reduction are designed and prepared.The combination of experiment and numerical calculation is used to analyze the mechanical behavior and progressive damage of the materials under static tensile loading and tension-tension fatigue loading.This is helpful to clarify the effect of yarn-reduction to provide data and theoretical support for improving the reliability and load-bearing efficiency of components.The following are the particular research contents and conclusions:(1)The ultimate tensile strength(UTS)of 3D5 D composites without/with yarn-reduction are determined by static tensile tests performed on the Instron mechanical testing machine.The tension-tension fatigue tests with different stress levels(40%-75% UTS,R = 0.1)are conducted on the MTS fatigue testing machine.Based on X-ray computed tomography,a two-step damage classification method is proposed to analyze the internal damage mechanisms of tested specimens.The results show that the stiffness degradation of 3D5 D braided composites without/with yarn-reduction under tension-tension fatigue loading exhibits four-stage.Simultaneously,the internal damage distribution of 3D5 D braided composites with yarnreduction is wedge-shaped.Closer observation finds that the fracture mode of the carbon fibers in the misaligned yarns changes from mode I to mode II.The damage classification shows that the debonding damage between the yarns dominated the fatigue failure.The damage in the yarns is mainly distributed in the subsurface section of braided yarns and the deformed section of axial yarns.(2)Based on X-ray computed tomography,in-situ tension-tension fatigue tests are conducted to observe the full-field fatigue damage evolution in 3D5 D braided composites without/with yarn-reduction.The classified damage coordinate method and the pores’ health detection method are proposed to quantify the classified fatigue damage evolution in 3D5 D braided composites with yarn-reduction and the cracking process of voids in 3D5 D braided composites without yarn-reduction.The residual strength mechanism of 3D5 D braided composites without/with yarn-reduction is analyzed using acoustic emission and digital image correlation techniques.It is found that the initial fatigue damage originated near the yarnreduction point and the edge of the tested specimens.There is basically no damage occurred in the reduced yarns.The introduction of yarn-reduction point and voids does not disrupt the conventional fatigue damage sequence,but accelerates the fatigue failure process.Therefore,the voids located at the interface are considered to be the key voids that affect the fatigue life.Additionally,fatigue damage alters the stress redistribution in the materials during the residual strength tests,thereby weakening the residual strength.(3)A reconstruction method of statistical geometric model of 3D braided composites is proposed.The geometric parameters of reinforcement in the 3D braided composites are extracted by X-ray computed tomography and fitted by the Fourier function.Then CAD software is utilized to reconstruct the geometric model based on the statistical data to compare with the ideal model and the real structure.The results verify the validity of the statistical geometric model.Then,the method is utilized to reconstruct statistical geometric models of3D5 D braided composites without/with yarn-reduction,taking into account the twist of yarns,the compression of preforms,the deformation around the yarn-reduction point,and the geometric features of the full thickness.(4)An image-based multiscale model is established to predict the progressive damage of3D5 D braided composites without/with yarn-reduction under static loading.Meanwhile,the domain decomposition method is utilized to improve the numerical calculation efficiency.The continuous damage method describes the stiffness degradation of constituents.The model effectively captures the cracks evolution as well as damage variables after different increments.The viscous regularization is performed to improve the computational convergence.The results show that the predicted stress-strain curve,strain field distribution,and damage distribution are similar to the experimental results.Based on the discretization of the fatigue loading process,the residual stiffness model and residual strength model are used to describe the mechanical properties degradation of the constituents under fatigue loading.The damage morphology,stress redistribution,and damage variables after different fatigue cycles are predicted.The predicted results are in good agreement with the experiments.The damage types under fatigue loading and static loading are basically the same.The main difference is that the stress redistribution in the material under fatigue loading is wider,and the fatigue damage area is much larger.