Study On Multiscale Mechanical Performance And Collaborative Process Optimization Of Composites Made By Automated Fiber Placement

Carbon Fiber Reinforced Composites(CFRPs)are widely used in aerospace,transportation,energy and national defense due to their lightweight,high strength and designable property.Among various manufacturing techniques of CFRPs,Automated Fiber Placement(AFP)is one of the fastest-growing and efficient automated manufacturing technologies in recent years because of its advantages of good surface adaptability,variable angle laying and high manufacturing efficiency.However,the tow structure and defect characteristics emerge multiscale effect.Also,the multiscale mechanical performance,defect characteristics and process parameters are coupled with each other,which is difficult to obtain accurate process optimization goals and their scope.Therefore,in order to fabricate the composites with low defect and high quality through process optimization,it is necessary to study the multiscale mechanical performance and defect characteristics of preform,multiscale energy communication mechanism,and multiscale collaborative process optimization during the AFP process.Firstly,the macro-and micro-mechanical performance of preform is studied.The composite meso-damage mechanics is used to bulid a model for constructing the relationship between defects and stress parameters,which verifies the feasibility of using stress parameters to evaluate the defect characteristics.Based on the viscoelastic mechanics,the nonlinear viscoelastic constitutive model is established.Furthermore,the Finite Element(FE)secondary development method is used to construct the random defect model for investigating the relationship between defects and stress wave characteristics.For exploring the effect of process loads on stress wave characteristics,macro-and meso-energy during the AFP process,a FE thermo-mechanical coupled dynamic model is established to analyze stress wave characteristics,all strain energy and meso-strain under different process parameters,in which the mechanism of the effect of process parameters on the macro-and micro-mechanical performance is revealed,which provides a theoretical basis for determining the testing position of stress wave signals and obtaining the boundary conditions for micro-interface analyses.To achieve multiscale energy communication,an anti-sequential hierarchical multiscale analysis method is proposed based on the sequential-parallel multiscale analysis method and Olson hierarchical relationship.Combining the meso-FE method of composite and a homogenization point,a multiscale energy transfer model is then constructed.Following that,the micro-interface molecular model is established using the Molecular Dynamics(MD)method.An annealing relaxation simulation is carried out to enhance the stability of the model.Furthermore,elasticity parameters are calculated to evaluate the accuracy of the model.In terms of MD simulations,the relationships between micro-interface viscosity,matrix fluidity along with the interface,interface energy structure,interface adsorption and process parameters are studied respectively.The correlations between micro-mechanical parameters and enthalpyentropy values within the micro-interface are evaluated using the thermodynamic enthalpy-entropy theory,which gives the reasonable range of the micro-mechanical parameters in the low-entropy region where the constraints of the process parameters optimization could be effectively provided,which provides constraints for process optimization of micro parameters.For verifying the accuracy of FE simulations and evaluating the relationship between defects and mechanical performance quantitatively,the AFP experiments are designed.The temperature fields and stress wave characteristics obtained by FE simulations and experiments are compared to verify the accuracy of FE models.Furthermore,the vacuum bag method is used to cure the preforms.According to the off-line detection after curing and the ultrasonic A-scan test before and after curing,the defect content under different process parameters are obtained,which reveals the relationship between defect content and process parameters,stress wave characteristics.The microstructures of different defects are then evaluated using the scanning electron microscopy.The interlaminar shear strength(ILSS)and flexural strength(FS)under different process parameters are tested by universal material testing machine,and their response characteristics to defect content are further explored,which can provide the exact scope of optimization goals of mechanical properties.Finally,combining the multiscale method and decoupling strategy,a multiscale collaborative process optimization method is proposed.To simplify the optimization process and evaluate the importance of the optimization objectives,the grey correlation analysis method is used to explore the correlations among process parameters,multiscale mechanical performance,and defects,in which the optimization objectives with weak correlation are eliminated.According to the analytic hierarchy,the multiscale hierarchical model and the multi-objective optimization hierarchical model are constructed respectively to evaluate the contribution of different parameters to the hierarchical structure and the relationships among optimization objectives.Through the hierarchical weight analysis,the importance of the objective parameters is obtained.Based on the multi-objective response surface method,the AFP process parameters are then optimized.The comprehensive performance of multiscale parameters under the optimal process parameters is verified by simulations and experiments.