Research On Springback Deformation Of Weft-knitted Biaxial Composite Shel

Biaxial weft knitted fabric has been widely used as reinforcement of textile composites with complex curved structure because of its good formability and designability.Due to the non-synergistic nature of the components,residual stresses will inevitably appear because of the influence of material properties and external forming conditions.Residual stress will not only lead to cracks,delamination and other problems in the composite parts to reduce the structural strength,but also produce process-induced distortion and affect the dimensional accuracy of the parts.In this study,we propose a research plan for the typical problem of springback deformation of biaxial weft knitted composite helmet shell at three levels: material level,specimen level and component level,aiming to ensure structural stability through reasonable prediction of springback deformation and provide an important basis for improving product molding accuracy and process optimization.The main research contents are as follows:(1)In order to provide data support for the later simulation analysis,the thermalchemical behavior of R-806 vinyl ester resin during curing was studied based on nonisothermal differential scanning calorimetry test,and the curing kinetic equations and modulus evolution model were developed.Five kinds of pre-sheared specimens were prepared,and quasi-static tensile experiments in 0° and 90° directions showed that changes in shear angle significantly affect composite stiffness,strength and failure modes.(2)For the multi-field coupling analysis,the thermal-chemical-mechanical models of weft-knitted biaxial composites with five different pre-shear deformations were constructed by multi-scale analysis method based on the previous experiments.Among them: the effect of shear deformation of preform on the thermal-physical constants of biaxial weft knitted composites is revealed based on the representative volume element method.The elastic constants evolution model of weft braided biaxial composites was established by using the degree of curing stepping method,combined with the Cure Hardening Instantaneous Linear Elastic(CHILE)model;the curing shrinkage strain model of biaxial weft knitted composites was established by the equivalent temperature loading method.(3)For the simulation analysis based on the forming results of the preform.A binocular vision-based yarn orientation measurement system was built to construct the spatial trajectory of the yarn after preform forming process.The system only needs four times of collection to fully cover the rotating body samples,and the overall deviation is less than 0.2%.The measurement results can be applied to the definition of material direction and material attribute distribution in the numerical model.(4)A sequential thermal-force coupling method is used to predict the spring-back deformation of a sepcimen-level part,a biaxial weft knitted composite hemispherical shell,and introduce the distribution of material properties based on binocular visual measurements in real material directions and under five shear deformations.Finally,the springback deformation of the sample is measured by the laser scanning technology,and the simulation results are verified.The results show that the thermal-chemicalmechanical model established in this thesis is feasible and effective,and the springback deformation trend of the simulation results is exactly the same as the measured results,with the maximum deformation error within 7.5%,and by establishing an equal stiffness model for cross-sectional comparison,it proves that considering the shear deformation of yarn after preform forming is the key factor affecting the correctness of spring-back deformation prediction of textile composite shell.(5)The technical route verified by specimen level parts is applied to the springback deformation prediction of biaxial weft knitted composite helmet shell.When analyzing the spring-back deformation of point cloud data,aiming at the non convergence of the traditional registration algorithm,a "feature distance" method is creatively proposed to characterize the spring-back deformation of complex surface parts and verify the simulation results.The study shows that the relative error of key feature distance of simulation results is less than 7.3%,and the feature distance method can not only accurately describe the overall trend of spring-back deformation of complex surfaces,but also reflect the local detail information of spring-back deformation.