Investigation Of Process-structure-performance Relation For Polymeric Composites By Fused Deposition Modeling

The resin matrix composites play a dominant role in national strategy of new materials due to its excellent properties,while fused deposition molding(FDM)is one of the most widely spread additively manufacturing technology.Therefore,resin matrix composites by FDM integrates dual advantages of process and material,bringing new motivation to achieve high performance structure.However,due to the style of layer by layer,FDM printed resin matrix composites possesses high process sensitivity,strong anisotropy,weak interfacial properties as well as the hierarchical and multi-scale characteristics.Therefore,the process-structure-performance relation is of great significance to the process optimization and structure design.This dissertation focuses on the relation among process,structure and mechanical properties,reveals the effect mechanism of process,micro-and meso-structure on the mechanical properties for FDM printed polymeric composites.On this basis,a FDM process-oriented integrated design method is proposed,helping to manufacturing high-performance parts.The main content of the dissertation is as follows:A mass conservation-thermal-sintering model is proposed for FDM printed polymers.Using Oblong shape to characterize the cross section of the beads,the mass conservation model is constructed to describe the extrusion of beads.Therefore,the single bead’s cross section can be predicted.Further,the thermal-sintering model is built to describe the coalescence of contacting beads.By the coupled model,the mesostructured of FDM polymers,including a single bead’s cross section,the coalescence of beads and the porosity,can be predicted according to process parameters such as printing speed,layer thickness,extrusion flow rate,extrusion temperature and et.al.The effectiveness of the model is verified by tests.In addition,the effects of process parameters on the mesostructure are comprehensively investigated.The relation between micro-and meso-structure and effective elastic modulus and yield strength is established for FDM printed polymers as well as polymeric composites.For polymers,the effective elastic modulus is calculated by elastic homogenization while the effective yield strength is estimated by Hill yielding criterion,for which the anisotropic parameters are determined by elastoplastic homogenization.Particularly,a modified method for the determination of anisotropic parameter is proposed,in which the off-axis yield strength of 45 degree angle is introduced.In this way,the accuracy and effectiveness of the estimated yield strength is higher for the proposed method(with relative error 4.9%)than the traditional method(with relative error 45.6%).Moreover,the computional efficiency is improved by nearly 10 times.For polymeric composites,considering the microstructure including porosity,fiber volume fraction,fiber aspect ratio and fiber orientation distribution,the analytical and numerical homogenization model is applied to examine the effective elastic modulus and yield strength.Particularly,a RVE generation algorithm considering fiber orientation distribution is proposed,which is the key of numerical homogenization.It is found by the comparison between the model and tests that the prediction with real fiber orientation distribution is more accurate(with 6.9% relative error)than that with random fibers(with 21.6% relative error).A multiscale fracture model is proposed to predict the interfacial fracture behavior for FDM printed polymeric composites.Based on the effective mechanical property from micro-and/or meso-scale,the finite element model is constructed for the macroscopic structure to calculate the three-dimensional J-contour integral to characterize the fracture performance of the interlayer interface.Integrated with cohesive zone model(CZM),the multiscale fracture model is further used to predict the fracture procedure of the interlayer interfaces.Moreover,a G-Code driven thermal analysis model is proposed and integrated with molecular dynamics theory,to study the effects of material thermal property and printing parameters.A FDM process-oriented integrated methodology is proposed for process planning and structure design.Firstly,a macroscopic mechanical performance simulation model driven by the G-Code is proposed.Then,a structure design(based on topological optimization)-process planning(based on the load path)-performance simulation(based on the prediction model above)-real manufacturing flow is established,to achieve high-performance parts.A minimum flexibility beam is taken as a benchmark to illustrate the effectiveness of the proposed methodology.The results show that the toolpath has a significant effect on the macroscopic mechanical properties.The structural stiffness of the beam with concentric path(S-3)and linear path with 0 degree(S-2)is 57.6%and 37.7% higher than that with linear path of 90 degree(S-1),respectively.In conclusion,a series of results have been achieved in the study of process-structure-properties of FDM polymers and polymeric composites,which has certain practical significance for guiding process optimization and composite design,and provides theoretical basis for improving macroscopic mechanical properties and realizing the design and manufacturing of high-performance structures.