Research On Numerical Simulation And Defect Prediction For The Filling Process Of Resin Transfer Molding

Resin Transfer Molding(RTM) has become one of the main process technologies for fiber-reinforced resin matrix composites, the quality of the final products is determined largely by the infiltration of fiber preform by resin during the mold filling stage. At present, a lot of experiments need to be carried out to determine the mould structure and mold filling parameters of the RTM process, which causes high cost, low efficiency and the quality is hard to be guaranteed. To slove the above problems, the preform geometry structure, permeability, mold filling process and defect formation are deeply studied in this dissertation based on the state-of-the-art of RTM process digitalization technology and the research results of the related fields. In addition, the outcomes of this dissertation are applied in the RTMSimu system in terms of practical requirements.The geometry structure of the preform is one of the main factors for the RTM process, building the preform geometry model is the foundation of process simulation and defect prediction. After detailed analyzing the multiscale structural characteristics of the woven preform, the geometry models of the three scales are established. On the micro-scale, the fiber random distributions on the front and rear cross sections of the tow segment are obtained by a Monte Carlo moving method, then the preliminary model of the tow segment is built through the straight linear sweep operation, after adjusting the center path lines of fiber models by Bezier method, the micro-geometry model of yarn close to the real structure is established which contains both axial and radial random factors. On the meso-scale, the cross section shape and crimp state of yarns in a unit cell are analysed, the geometry parameters of the unit cell are determined by the principle of minimum potential energy, and the deformation processes of fabric shear and compression are studied, the geometry models of the unit cell at different deformation states are built. On the macro-scale, a novel fishnet algorithm based on geometry information(GIB-fishnet algorithm) is proposed, geometry information of surface such as tangent vector and normal curvature is used to determine the position of the fabric node, the draping model of 2D textile fabric on 3D curved surface is established.The permeability of textile preforms is a crucial input for the simulation of the mold filling stage of the RTM process, which describes the resin flow resistance in the preform. Based on the multiscale geometry model of the preform, the permeability prediction methods for the fiber bundle and unit cell are researched. The micro permeability is predicted by simulating the resin flow between monofilaments, the results show that the disordered fiber arrays have certain influence on the micro permeability. A mathematical model for the dual scale flow in unit cell is established, the influence of yarn crimp and extrusion on micro permeability is studied and the numerical method for solving the resin flow governing equations based on finite difference method is built, the pressure and velocity distributions of the flow are computed to predict the meso permeability. On the basis of the permeability prediction method of the orthogonal unit-cell, the body-fitted coordinates system is employed to convert the governing equations from physical domain to computational domain to simulate the resin flow in the sheared unit-cell and predict its permeability. The laws of meso permeability with respect to shear and compression deformations are studied. The comparisons between prediction results and literature and experimental values are conducted to verify the prediction model and the solving method mentioned above.Since the resin-air two phase flow is difficult to be directly solved, the mold filling simulation algorithm for the RTM process based on VOF(Volume of Fluid) technology is studied. The VOF model for the resin-air two phase flow in the RTM mold filling process is establised by adding the fluid-solid drag term to Navier-Stokes equation, and the model is numerical solved based on VOF technology, simulation examples indicate that the algorithm has high accuracy. For the RTM processes with preform deformation, in order to avoid solving the coupled resin flow/preform deformation equations directly, the dynamic mesh model and master-slave element method are employed to implement the full three-dimensional mold filling simulation, so the viscoelastic compaction model can be used to calculate the preform deformation, which improves the simulation precision. According to the simulation examples, the dynamically changing dimensions of the flow domain can be fully reflected in the results, and the sequential injection strategy can also be successfully simulated.Predicting the molding defects is an important task of filling simulation for RTM. Dry spot is one of the primary defects during RTM, the formation and evolutionary process of dry spot are analysed, the formation of dry spot is simulat ed based on VOF technology. The void formation mechanism during mold filling is studied and a void prediction model is proposed, the model analyzes the difference between the resin flow velocities inside and outside the fiber tows to determine the location and size of the voids. Particular attention is paid on the influence of flow direction and fabric shear on the impregnation of the unit cell, so their effects on the generation and size of voids have been obtained. The correctness of the above model is validated by experiments.On the basis of the results of the present study, the RTM mold filling simulation system(RTMSimu) is developed. The database system and object-oriented programming language are employed to achieve the prototype system development, the parameter driving and module integration of preform geometry modelling, permeability prediction, mold filling simulation and defect prediction are both realized. The modelling and simulation of a car hood are implemented to demonstrate the main interfaces and operation procedure of the RTMSimu system, the technical indicators of the system are synthetically analyzed and the feasibilityof the system is preliminary validated.