Research On Compressibility And Seepage Property Characterization Methods Of Preform For FRP

Low mold filling efficiency,unstable product quality,and difficulty in molding large-sized and large-thickness components integrally have become outstanding problems faced in liquid composite molding process and have weakened its competitive advantages.Nowadays computer-based virtual manufacturing technology can be used to improve the microstructure uniformity,product quality and manufacturing precision through optimizing the production process.It is a feasible way to enhance its level of composite design,production and application and can be helpful to solve the problems mentioned above.The compressibility and permeability of fabric preform are the basic parameters in simulation of liquid composite molding process,and the preconditions for prediction of molding pressure,fiber volume content and mechanical properties accurately.Therefore,to obtain the relevant properties of fabric preform using an effective and reliable test system is not only a prerequisite for optimizing the manufacturing process,improving the product performance and developing an integrated manufacturing method,but also laying foundation for the application of liquid composite molding process in high-end manufacturing field,which is of great significance.In this paper,the glass fiber multiaxial non crimp fabric and twill fabric are selected as the research objects.Research work has been done on the structural evolution and influencing factors of the fabric preform during compression and stress relaxation.Then the results are used in the strength design of through-thickness permeability(Kz)test system which designed and fabricated based on Darcy's law.The experimental process control,data acquisition and processing software were programmed,which greatly reduced the workload of testing and analysis.The main research work and conclusions can be concluded as follows:The stress prediction models are developed to describe the compression process and stress relaxation process of preform.The results show that the relationship between compressive stress and fiber volume fraction is accurately predicted by the modified Gutowski model established in this paper.The parameters have clear physical meanings and can be used to compare the compressibility of different preforms.A Maxwell multi-scale parallel model is proposed to describe the stress relaxation behavior of preforms as well.These models,which can provide essential information for the simulation of the molding process,the strength design of Kz test system and analysis of the test results,are in good agreement with experiments.A new through-thickness permeability Kz test system,based on Darcy's law in one-dimensional porous medium flow and with real time data acquisition realized by embedded low cost sensor,has been fabricated and series of validated experiments have been done in order to obtain high precision.The validation results show that its design and manufacture precision can meet the requirements that the strain should be less than 0.5%.Effective measures were taken to maintain the long-term stability of the injection pressure.The application of the sensors greatly improves the test efficiency and accuracy and solve the problem of low test efficiency,low repeatability,long experiment period.A standard operating procedure is proposed for the Kz test as well.A software,which can be helpful to achieve efficiency and accuracy on data acquisition and processing,are developed by Python.The functionality of the software is as follows: storing all the raw data in one file,filtering the experimental data and output experimental curves,generation of test reports and summarization.The software brings automatization,high-level efficiency and accuracy to the Kz test system.The workload of data analysis is significantly reduced.The Kz test system has been verified by experiments which proved the accuracy and reliability of hardware and software.The result illustrates that the dispersion of Kz at high compression rate level is significantly lower than low compression rate level.As the injection pressure increasing,the through-thickness permeability Kz decreases significantly,showing a significant tendency of pressure dependent.