Tensile And Shear Damage And Failure Analysis Of 3D Woven Composites

As a new type of composite material,three-dimensional(3D)woven composites are widely used in aerospace vehicles,civil arcthecture,sports equipments and medical devices,owing to their mulifarious advantages such as excellent mechanical properties in the thickness direction,perfect impact damage resistance and good integrity.However,the inhomogeneity,anisotropy and unadvoidable cracks or voids within the 3D woven composites result in the complexity of the damage evolvement process of this type of composite.Thus to investigate the mechanical behaviors and damage evolvement mechanisms of 3D woven composites becomes a big chal enge.Aimming to fully understand the mechanical behaviors and progressive damdage mechanisms of this kind of material.This thesis focuses on the tensile and shear stiffness,strength and progressive damage progress as well as failure mechanism of 3D woven composites by using experimental and numerical simulation methods.Methods for studying the tensile and shear failure behavior of three-dimensional woven composites have been proposed.Firstly,the recent research progress of 3D woven compsites including the mesoscale geometric structure,quasi-static experiments and the damage evolvement models are summarized and analyzed.Focuses are on the research development of mechanical properties(stiffness and strength)of 3D woven composites experimenally and numerically under quasi-static uniaxial tensile,tension along ±45° degree fiber direction and shear loads.The purpose and significance of this thesis are clearified.Secondly,experimental investigation on quasi-static uniaxial tensile and along ±45° degree fiber directional tensile behaviors of 3D woven composites are conducted.The stiffness,strengths properties and mechanical response as well as the damage process of this kind of material are measured and tested by 3D Digital Image Correlation(3D DIC)technique.Macro and meso fractographic appearances of failured specimens are observed by a Optical Microscope.The damage modes and failure mechanism of 3D woven composites under quasi-static uniaxial and along ±45° degree fiber directional tensile loads are analyzed and determined in detail.Meanwhile,a test method for shear properties of 3D woven composites is proposed.A modified V-notched beam test fixture suitable for specimens with different thicknesses of 3D woven composites are proposed and designed based on standard apparatus as needed.V-notched beam shear test method and short-beam shear test method are applied respectively to inspect the in-plane and interlaminar shear properties of this material.Nonlinear shear responses and progressive damage process of 3D woven composites are obtained.Macroscopic and mesoscopic fractographic appearances of failured specimens are observed and analyzed at length to obtain the damage modes and failure mechanism of the 3D woven composites.Based on the structure of 3D woven composites,a mesoscale representative volume cell(RVC)model approximates to the real structure is determined.Combined with the multiphase finite element method,a tensile progressive failure model for 3D woven composites is established.The improved Puck criteria and parabolic criterion are adopted in this model with considering the fiber breakage,inter-fiber fracture and matrix crack in the level of the fiber yarn and the matrix.The progressive damage model is implemented into a user-defined material subroutine VUMAT in ABAQUS software to predict the stiffness,strength properties as well as the damage progression and failure process of 3D woven composites under uniaxial tension.Results of numerical simulation and experimental observations are compared and discussed.Finally,a shear nonlinear model for 3D woven composites is proposed to solve the large deformation problem under shear loads.This model fully considers the cause of material shear nonlinearity,and holds the view that the deformation of the fiber bundle generates together with the rigid body rotation under shear load,and the local coordinate system of the fiber bundle will change in real time with the deflection of the fiber bundle.Therefore,the Green-Naghdi objectivity is considered in the bundle damage model and the body-following coordinate system is introduced to "deduct" the effect of rigid body rotation.On the basis of the uniaxial tensile damage model,the Maximum Shear Stress criteria is introduced as the shear failure criteria of the faber yarns and the matrix.Combining with the multiphase finite element method,macroscopic finite element models of along ±45° degree fiber directional tension,Vnotched beam shear and short-beam shear are established with considering the damage progression of the fiber yarn and the matrix in mesoscale level.The stiffness,strengths properties as well as shear damage evolvement and failure progress of 3D woven composites are predicted and analyzed.Numerical and experimental are compared and discussed.Results are in good agreement and the assumption is correct.The shear nonlinear progressive failure model proposed in this thesis can effectively predict the shear nonlinear progressive damage law of 3D woven composites.