| Under the background of the national strategy of carbon peak and carbon neutral,continuous carbon fiber/nylon 6 thermoplastic composite(CF/PA6)has the advantages of high toughness,rapid prototyping,recycling and easy repair.As a green environmental protection material,CF/PA6 composites with high performance and low cost have a wide application prospect in aviation,aerospace,automobile and other fields.However,CF/PA6 composites in service environment face serious mechanical aging problems due to the coupling effect of temperature and humidity,resulting in mechanical property degradation and dimensional deformation of structural parts,which greatly reduces their service life.Therefore,the moisture absorption behavior and mechanical aging of CF/PA6 composites were studied to reveal the moisture absorption mechanism of the composites and performance evolution law under different loading conditions,aiming to providing a theoretical basis for the study of hygroscopic modification and aging resistance of CF/PA6 composites.The research work of this paper is mainly carried out from the following aspects:The moisture absorption behavior of CF/PA6 composites was studied by immersion experiment and numerical simulation.In order to describe the nonlinear moisture absorption behavior of composites,the advantages and disadvantages are analyzed between the Fick model,Langmuir model and two stages diffusion model.Moreover,the modified two-stage model considering orthogonal anisotropic of unidirectional laminates was further developed,which can accurately calculate the three-dimensional moisture absorption parameters of composite laminates.Aiming at the accelerating effects and barrier effects of interphase in the water absorption process,the interfacial parameters were introduced to modified the traditional Halpin-Tsai equation,and the critical value of interfacial diffusivity was proposed to reveal the mechanism of interfacial effects for the composites.Hygrothermal aging and freeze-thaw aging experiments were carried out to study the mechanical aging behavior of CF/PA6 composites.The effects of environmental factors such as aging temperature,water absorption content,temperature cycle and freeze-thaw cycle on the revolution of mechanical properties were studied.The aging mechanism was revealed by optical microscope,thermal expansion instrument,scanning electron microscopy,dynamic thermo-mechanical analysis,infrared scanning analysis,contact angle measurement and other characterization methods.The relation function between moisture absorption and mechanical parameters was established and applied to the constitutive equation of moisture-heat aging numerical simulation.The progressive damage process of CF/PA6 composites under transverse tensile,compressive and shear loading conditions with different porosity was analyzed based on the RVE mesoscopic model,and the different failure mechanisms before and after hygrothermal aging were revealed.The effect of moisture absorption on warping deformation of laminates with asymmetric layering was studied.The curing deformation and hygroscopic deformation of asymmetric laminated composite plates were studied by combining hygroscopic deformation experiment and numerical simulation.The thermal/moisture expansion coefficient was modified based on the full-field displacement comparison method,and the effectiveness of numerical simulation was verified.Established the included angle of the layer and lay-up mode,number of nonrepetitive layering angles,deformation shape,and water content with eight factors such as moisture absorption quantity of 2816 groups of lamina moisture absorption maximum deformation data sets,with R2 and MSE criterion,using the grid search and 5 fold crossvalidation method,choosing the optimal parameters and network structure,and training the BP neural network model is set up,which can quickly and effectively predict the maximum deformation of hygroscopic laminates. |
PDF Full Text Request