Study Of Improved Woven CFRP Bimodulus Model And Its Application In Connection Structure Analysis

With the development of high-speed railway and the continuous improvement of the vehicle speed,reducing vehicle weight,the industry has attracted more attention on reducing weight,reducing energy consumption and improving transport capacity based on the lightweight.The mass of carbody accounts for about 22% of the whole vehicle mass.The lightweight design of the carbody using high-strength composite materials is the key way to reduce the mass.With the comprehensive performance of high strength,low density,corrosion resistance etc,carbon fiber reinforced plastic(CFRP)can meet multiple design indicators of rail vehicles on the premise of weight reduction,such as vibration and noise reduction,impact energy absorption and fatigue resistance,etc.To the promotion and application of CFRP,the connection technology is critical.However,compared with traditional metal materials,the design criteria,failure modes,simulation methods,evaluation and verification of CFRP connection structures,especially the woven carbon fiber reinforced plastic(WCFRP)are inadequate,the accumulated research are insufficient,and the necessary theory and test data are lacking.Therefore,in this thesis,on the basis of relevant research at home and abroad,the constitutive model suitable for WCFRP was studied.The application and verification of the constitutive model were completed based on the secondary development of software.A large number of WCFRP connection structures were tested,the structural performance and macro/micro failure modes were explored.The failure mechanism of WCFRP connection structure is obtained.Submodels of complex connection structure in WCFRP carbody are established.The efficient accurate design and the verification method for complex connection structure in large components are proposed and implemented.The details are as follows:(1)Static tensile and compression tests with laminate level were carried out on WCFRP to obtain the test data and macro/micro failure modes.The stress and deformation distribution of fiber and matrix were obtained by establishing mesoscopic finite element(FE)model with two stacking sequences.The deviation analysis of tensile modulus and compressive modulus was carried out combining the theoretical formula.The tension and compression deviation mechanism of WCFRP at multi-scale was obtained,and it is necessary to propose a bimodulus constitutive model.Then,the deviation of tensile and compressive modulus of WCFRP with different fiber volume fraction and different braided angle was simulated by the mesomodel.The variation rule of the deviation rate with different influencing factors was obtained.(2)Based on the principal stress criterion and track stress criterion,the traditional isotropic bimodulus constitutive model,two anisotropic bimodulus constitutive models suitable for WCFRP were proposed aimed at the characteristics of composite materials.Through the secondary development of LS-DYNA software,the construction of two bimodulus constitutive models was completed,and the leap of constitutive model from micro to macro stage was realized.Three-point bending test with two stacking sequences was designed to obtain the relationship between load,strain and vertical displacement,as well as the failure mode of the material.Through FE simulation,the differences of bending properties under the two bimodulus constitutive models and the traditional single-modulus model were explored.The load,strain,failure factor and region distribution of tension and compression were extracted from the calculation models.By comparing the experimental results,the most accurate bimodulus constitutive model was obtained,which would applied to the subsequent research.At the same time,the FE analysis of three-point bending with different thickness,stacking sequences and indenter radius was carried out to explore the bending properties under two bimodulus constitutive models.(3)The mechanical properties variation law was studied on two WCFRP single joints,bonded joint and bolted joint.The tensile tests of bonded joint and bolt joint were conducted with four stacking sequences,two lapping patterns(single lap and single strap butt lap)and two bolt number(two-bolted and four-bolted).The force displacement curves,macro/micro failure modes and hole edge strain of the structure were recorded.The micromorphology of the fracture was observed by scanning electron microscopy.The FE modeling and simulation of single lap bonded joint and two-bolted joint were carried out using the bimodulus constitutive model.The accuracy and applicability of the connection structure FE model are verified.(4)Tensile tests of two-bolted and four-bolted hybrid joints were conducted with four stacking sequences and two lapping patterns.The force displacement curves,macro/micro failure modes and hole edge strain were recorded.Combined with the experimental data of bonded joint and bolted joint in Chapter 4,the strength,stiffness and bolt load distribution with different connection modes were analyzed.The properties variation law and design method of WCFRP connection structure with multi-influence factors were summerized.The simulation and analysis of the two-bolted hybrid joint were carried out using the bimodulus constitutive model.The load-displacement curves and failure modes were compared between the simulation and the test.Based on the FE simulation,the failure process,tension and compression area distribution and hole edge strain distribution of bonded joint,two-bolted joint and hybrid joint were compared and analyzed.The failure mechanism of WCFRP connection structure was revealed.(5)To use the submodel simulation method,the whole carbody model was firstly simulated,then two connection structures in stress concentration areas were selected for fine modeling.Cohesive element was adopted for adhesive layer,and solid element was adopted for bolt considering the element failure.The boundary conditions of the submodels were extracted from the calculation results in whole carbody.Finally,according to the simulation results of submodels,the stress and strain distribution,the tension and compression area distribution and failure modes of the complex connection structure in CFRP carbody,a new idea and efficient accurate method were provided for the design and verification of the connection structure.