Refine Modeling Of Three-dimensional Braided Composites Based On Micro-CT And Analysising Of Mechanical Properties

Three-dimensional braided composite material with structure characteristics is one kind of textile composites,which can directly manufacture variety of complex shapes without secondary processing by using a special braid process.The braided composites with high specific strength,specific stiffness and no delamination etc.have potential to be applied in aerospace and civilian fields.Thus,it is essential to analyze and evaluate the mechanical properties of three-dimensional braided composites.The macroscopic mechanical properties of the braided composites are greatly associated with their mesoscopic structures.It is significant to establish a geometry model of braided composites including detailed meso-structure characteristics.In this paper,the mesoscopic geometry model of the braided composites is constructed by image processing and tomography(Micro-CT),and mechanical properties of the composites are analyzed.Firstly,an accurate and efficient method is introduced to reconstruct threedimensional geometry model of braided composite materials based on Micro-CT.The braided composites were scanned by Micro-CT.Some image processing techniques,such as filtering,swelling and contraction etc.,are used to deal with this tomography images.An automatic identification program is prepared to identify and extract the cross-section of the braid yarns in all slice images.The topology data of each braid yarn within the braided composites are obtained by using scattering,interpolating,reconstructing operations for braid yarn cross-sections.A realistic geometry model can be established.The proposed geometry modelling method can be extended to use geometry model of other braided composites in the future.Secondly,a non-uniform Voxel grid discretization method is used to mesh the obtained realistic geometry model.And the macroscopic effective properties of the braided composites are analyzed.The internal meshes of the braid yarns exhibit gradient distribution,which can describe the detailed geometry features between different phase interfaces within the braided composites and can reduce the calculation scale.A representative unit cell model and a small-size model extracted from the realistic geometry model can be used to calculate the effective properties of the braided composites by using volume homogenization method,which also compare with the results obtained from the representative unit model with ideal elliptical cross section of braid yarn.In addition,the effect of the surface fiber distortion of braid yarn on the effective properties of the braided composites is also taken into account.Finally,the damage progressive process of the braided composite under uniaxial tensile load is analyzed by using stiffness discount method.A stress averaging technique is used to average the stress concentration between different phase interfaces artificially introduced in voxel meshes.The damage of constituents within the braided composites can be judged by using the average stress,which can ensure the correct damage modes during computation.The effect of computation models(one representative cell model extracted from the refinement model,one small size models randomly extracted from the refinement model and an ideal representative cell model with an elliptical cross section),the twist degree of braid yarns and voxel grid size on the progressive damage analysis is discussed.