Investigation On Failure Behaviors And Progressive Damage Model Of 3D Woven Composites

Benefiting from the spatially interlaced reinforcement architecture,3D woven composites(3DWC)possess numerous outstanding mechanical properties,including exceptional structural integrity,designability,impact resistance,and fatigue resistance,making them widely used in aerospace,marine engineering,and automotive industry.However,owing to the complex braided structures,3DWC may simultaneously sustain multiple damage modes.Although the dominant damage modes can be observed based on the final fracture topography of specimens,the actual damage accumulation process is difficult to capture through macroscopic experiments,so the damage mechanisms are still not completely clear.In this dissertation,the mechanical properties and damage mechanism of 3DWC under quasi-static uniaxial tensile,uniaxial compressive and biaxial tensile loads are investigated by combining experimental and numerical approaches,so as to realize effective characterization and accurate prediction of the failure behaviors of 3DWC.This dissertation aims to establish a systematic basic research method for 3DWC to promote the application and development of 3DWC in the aerospace field.The main contents are included as follows:The domestic and foreign research statuses of quasi-static uniaxial damage mechanism of 3DWC,biaxial mechanical properties of composites,geometric construction of 3DWC and progressive damage model of composites are first reviewed and discussed.According to the reviews,the primary existing problems are summarized and analyzed,and then the main research contents of this dissertation are introduced.The quasi-static experiments of 3DWC,including uniaxial tension,uniaxial compression and biaxial tension,are separately carried out.With reference to relevant test standards and literature,the tensile,compressive and cruciform specimens of 3DWC are rationally designed.The stress-strain curves,failure strengths and dominant damage modes of 3DWC under different loading conditions are obtained from the experiments,which provides reliable support and experimental verification for the progressive damage models proposed in the later chapters.An innovative mesoscopic progressive damage model is proposed,which accurately predicts the stress-strain curves and damage accumulation process of3 DWC under quasi-static tensile loads in the warp and weft directions.The longitudinal fiber fracture,transverse inter-fiber crack and matrix failure are considered in the damage model.A mesoscopic representative volume cell(RVC)with high fidelity,accounting for the fiber yarn fluctuation,distortion and cross-section size,is constructed to numerically simulate the 3DWC.A set of fracture angle-dependent damage variables are introduced to characterize the multiple transverse cracks of fiber yarns,which efficiently eliminate the stress abnormal phenomenon.An exponential damage evolution model,based on equivalent displacement,characteristic element length and fracture toughness,is developed,which can be applicable to simulate the brittle fracture of composites and is easy to be numerically implemented.A progressive damage model considering fiber initial misalignment is proposed,which accurately predicts the failure behaviors and damage accumulation process of 3DWC under warp and weft compressive loads.The primary damage modes of 3DWC under compression,including longitudinal fiber kinking,transverse inter-fiber crack,matrix failure and interfacial debonding,are comprehensively considered.A mesoscopic RVC,consisting of fiber yarns,pure matrix and yarns/matrix interface,is constructed based on microscopic observations.Multiple stochastic distributions of fiber initial misalignment are generated through a developed user-defined subroutine.The influences of stochastic fiber initial misalignments and interfacial properties on the compressive performances and failure behaviors of 3DWC are estimated parametrically.For the special-shaped structures of 3DWC,a multiscale progressive damage model,combining microscopic,mesoscopic and macroscopic numerical computations,is proposed to accurately predict the strain distributions,failure strengths,and crack morphologies of 3DWC subjected to biaxial tensile loadings.A novel full-scale geometry modeling method,capable of characterizing the complex geometric features of fiber yarns,automatically defining the local material orientation,and appliable to special-shaped structures,is developed to reconstruct the biaxial specimens of 3DWC.Then,the multiscale modeling of 3DWC is established,including hexagonal microscopic RVC,full-thickness mesoscopic RVC,and full-scale model of cruciform specimens.Combined with the micromechanics of failure theory and 3D kinking model,the correlation between the mesoscopic stress of yarns and microscopic stress of constituents is established by em ploying a stress amplification factor.With the microscopic stresses,the fiber breakage and matrix failure can be separately judged at the microscale.