Damage And Failure Analysis Of Braided Composites Special-shaped Structure Based On FFT Method

Carbon fiber reinforced polymer(CFRP)woven composites are widely used in load-bearing structural components in the aeronautic and astronautic fields due to their high specific stiffness,high specific strength,high damage tolerance,excellent impact resistance and design flexibility.Correctly analyzing and evaluating the mechanical properties of woven composite structures is the key to successfully designing relevant structural components.The mechanical behavior of the woven composite structure strongly depends on many factors,such as the complex internal microstructure and material properties.Especially,when the macroscopic woven structure presents a complex twisting variation,such as engine blade,the macroscopic nonlinearity is caused by the damage mechanism of the internal fabric structure.The experimental method based on phenomenology is considered to be a direct method to obtain the mechanical properties of materials.But the numerical simulation method is viewed as an efficient way to reveal the internal damage and failure process of the macrostructure subjected to external loads.In this dissertation,an efficient multiscale numerical simulation method that spans micro-meso-macro scales is established to analyze the macro-mechanical behavior,effective performance and micro and meso-scale damage and failure mechanisms of the woven composites and special-shaped structures.Firstly,the domestic and foreign researching development status of the mechanical properties analysis methods of braided composites,including the numerical methods based on unit cell model,multiscale methods,uncertainty quantification and transformation and researches on the mechanical properties of composite structures with special shapes have been discussed and analyzed.The general routine to evaluate the mechanical properties of woven composites with special-shaped structures by developing an efficient and accurate multiscale calculation framework has been provided.Secondly,a numerical simulation method based on Fast Fourier Transforms(FFT)combined with damage failure models is developed,which can quickly feed-back the mechanical response of periodic unit cell model,and correctly reveals the damage and failure mechanisms.Due to the obvious stress oscillations at the interface between different constituent materials in the FFT method,the reconstruction pixel technique combined with the laminate theory is further proposed to optimize the quality of the stress and strain field,which ensures the accuracy of the numerical method.Thirdly,the unidirectional composite periodic unit cell model with random fiber distribution is established in the microscale,where the exponential change of the interphase properties between fiber and matrix along the thickness direction and the longitudinal compression instability of the unidirectional fiber are considered.The established nonlinear FFT numerical method is utilized to accurately characterize the mechanical properties of composites at the microscale,which can provides reasonable inputs information for the mechanical properties of the mesoscale yarns within the woven composite.Then,the anisotropic damage evolution models are introduced into the FFT algorithm to analyze the damage and failure of the braided composite material under different loadings,in which the mixed failure modes are fully considered including the longitudinal tension and compression,transverse tension and compression,shear modes of the yarns and tension and compression modes of the matrix.Fourthly,the twisted specimens with different twist angles and the curved surface conforming fixture device are designed to conduct the cantilever beam experiment to study the failure mechanisms for woven composites twisted structure.Due to the distortion of the macro-structure,the internal fabric structure is randomly uncertain,the Micro-CT non-destructive testing technology is utilized to recognize and gather statistic the geometric feature information of the internal yarn.The multivariate Gaussian random field model is developed to quantify the uncertainty informations.Furthermore,the high-fidelity meso-RVC model can be established.Finally,the microscale FFT-based calculation model is further coupled with the macroscale FE model to establish FE-FFT concurrent multiscale method,where the boundary value problem of macro-structure is solved by FEM.The mechanical responses of the macro-element integration point are replaced by the analysis results of mesoscale periodic unit cells solved by FFT.The mechanical behaviors of the three-dimensional braided composites beam structure under three-point bending load is investigated by using the proposed multiscale method,the reliability and efficiency of the proposed multiscale numerical simulation are verified by experimental results of the literature and other numerical methods.Then,the uncertainty quantification model is further embedded into the FE-FFT multiscale calculation framework to reveal the damage and failure mechanism of the woven composites twisted variable structure under the cantilever load condition at different scales,where the uncertainties of internal fabric structure and material properties are quantifed within each scales and transferred between different scales.The results of the multiscale simulation are consistent with that of experiment.This research method is also applicable to other types of braided composite materials with special-shaped structures,and can provide a reliable theoretical basis for the application of braided composites in actual engineering components.