Research on Constitutive Model of Woven Fabric Thermoplastic Prepregs and Its Application in Thermo-Stamping Simulation
Author:Gong You Kun
Supervisor:peng xiong qi
Woven fabric reinforced thermoplastic(WFRTP)composites are undoubtedly one of the most important options for automobile lightweighting.Thermo-stamping process of WFRTP prepregs has received extensive attention recently because of its high efficiency,low-cost,easy to automatic and so on.However,thermo-stamping process of WFRTP composites involves large deformation,anisotropy and multi-field coupling phenomenon,which prevents direct extension of the theory foundation and equipment from metal stamping to WFRTP stamping.Therefore,by combining experimental study with theoretical analysis,this dissertation aims to study mechanical behavior of WFRTP during thermo-stamping,to develop a proper constitutive model and to establish an analysis model for WFRTP prepregs in thermo-stamping based on FEM(finite element method)and continuum mechanics.Based on forming experiment,numerical simulations of thermo-stamping process of WFRTP prepregs with the proposed model are carried out to study deformation patterns and mechanical behaviors of the WFRTP prepregs in thermo-stamping.The influence of mold design parameter and forming process parameters on the thermo-stamping process is investigated.Typical defects and its feature as well as its effect rules druing the WFRTP thermo-stamping process are studied.These research efforts lay a theoretical foundation for the application of thermo-stamping process on the WFRTP components manufacturing and make it possible that WFRTP composites adapt to characteristics of mass production,low cost and high efficiency in the automotive industry,accelerate the application of WFRTP composite on lightweight automobile in China.The research works in this dissertation mainly focus on the following aspects.(1)For the better characterization of mechanical properties of woven fabric reinforcements as a solid foundation for subsequent constitutive model study of WFRTP prepregs,this dissertation studied its shear-tension coupling property systematically by experiment,theory analysis and numerical simulation with balance plain woven carbon fabric as the research object.The research result shows that shear-tension coupling,which is nonlinear and affects the picture-frame test data obviously(shear stiffness increase with the pre-tension along fiber direction),has little influence on the forming results of woven fabric reinforcement through the comparison of stamping experiment with different blank holder force,which also validates the accuracy and applicability of the proposed constitutive model for woven fabric reinforcements.Therefore,shear-tension coupling can be neglected in the following WFRTP prepregs constitutive model development for purpose of simplification.(2)Based on the theory of continuum mechanics,a hyperelastic constitutive model considering fiber-fiber-matrix interaction is developed to describe the large deformation and aniso tropic behavior of woven fabric thermo plastic composite prepregs during thermo-forming process.In this model,a free energy function representing the material behavior of melting prepreg is additively decomposed into three parts nominally representing the energy contributions from the fiber yarns,matrix resin and their interaction,respectively.There are only five parameters which make the constitutive model very simple and easy to implement.The proposed constitutive model is demonstrated on a balanced 2×2 twill glass/PP prepreg.The specific forms of the free energy functions are determined by fitting uni-axial tensile and bias extension tests of the prepregs at forming temperature.Through the user subroutine modules UANISOHYPER_INV in ABAQUS,the developed model is then applied to simulation of a benchmark double dome thermo-forming,demonstrating that it is suitable for predicting the large deformation behavior of woven prepregs during thermo-forming process.The effects of matrix resin on forming process are also invest igated.However,the proposed simple model cannot characterize the influence of some important process parameters such as forming temperature and forming rate due to the limit of the experiment data in reference.(3)The thermo-stamping process of woven fabric reinforced thermoplastic composites(WFRTP)involves large deformation,anisotropy and multi-field coupling phenomenon.In order to characterize these mechanical behaviors of WFRTP in forming,a novel lamination model combining thermoplastic resin matrix and woven fabric reinforce ment is establ ished based on FEM(finite element method)and continu um mechanics.The WFRTP is modeled as a laminated structure with impregnated woven fabric layers sandwiched between two thermoplastic resin layers.The thermo-mechanical coupling and viscosity resulted from melted resin matrix is characterized by an isotropic visco-hyperelastic model,while the impregnated woven fabric reinforcement is defined by a previously developed anisotropic hyperelastic model.With the combination of two user subroutine for matrix and reinforcement,the proposed lamination model is validated on bias-extension experiment of woven carbon fabric reinforced PPS prepregs at different temperature and displacement rate.It is also demonstrated on forming simulation of a WFRTP over a double-curvature mold.The effects of processing parameters including forming temperature and blank holder force on wrinkling are investigated.The proposed model is simple and easy for material parameter determination.It provides a theoretical foundation for the numerical simulation and processing optimization of WFRTP forming.