Research On The Preformability And Viscoelasticity Of Preform In HP-RTM For The Fiber Reinforced Polymer Composites

Fiber reinforced polymer(FRP)composites have been widely used in automotive body lightweight for its excellent mechanical properties and high cost performance ratio.High pressure resin transfer molding(HP-RTM)has become the research focus of automotive structural parts forming technology for its high molding efficiency and good quality.In HP-RTM,the preform of the fiber reinforcements not only has an important effect on the penetration process of resin in the subsequent processes,but also affects the mechanical properties of the composite structure.In this thesis,the preformability of fiber reinforcements and the viscoelasticity in the process of forming have been studied.The purpose is to provide guidance for the preform processes of fiber reinforcements and the forming of composite materials.The main results of the research are as follows:The representative volume element is selected according to the geometrical structure of the fiber,and then the initial thickness theory of the layers is established.Combined with the experiment of fiber preform,the thickness deformation of yarns during its preforming is investigated,and the thickness deformation evolutions are pointed out.The effects of nesting on the initial thickness,thickness in compression and thickness deformation of fiber preform during compression processes are analyzed.The relationship between nesting and the process of fabric preform is obtained.Combined with the basic theory of viscoelasticity,the Maxwell model is improved and a continuous fiber viscoelastic model is established.In this model,different structural parameters are used to characterize the viscoelasticity of the three processes of compression,stress relaxation and spring back of the continuous fibers.The correctness of the model is verified by experiments,and the relevant parameters are obtained by fitting the experimental curves.The physical meaning of the parameters in the model is studied.According to the relationship between the time constants of the three stages of compression,stress relaxation and spring back in the model,the viscoelastic relationships between these three stages are analyzed.Combined with the production processes of composite materials,viscoelastic experiment of continuous fiber under wet condition is carried out,and the viscoelastic experimental curves are obtained.Comparing and analyzing the continuous viscoelasticity of fiber under dry and wet conditions,the differences between these two conditions are pointed out.Based on the theoretical model established in this paper,the influences of viscous medium on viscoelasticity for continuous fiber are investigated by analyzing the difference of model parameters between these two conditions.The conclusions of research for continuous fiber preform and its viscoelasticity provide guidance for the preforming process of continuous fiber reinforced polymer composites and the following subsequent processes.