The Failure Mechanical Properties And Optimization Of Carbon Fiber Composite T-Joint For Lightweight Automobile Structures

Composite structures have excellent properties,which are widely used in the vehicle body and panel.It makes contribution to the lightweight vehicle development as well as the energy conservation and environment.Typically,the lightweight vehicle structure can be achieved from three aspects:structural optimization,lightweight manufacturing and use of novel material or material substitution.As an alternative,carbon fiber materials have been considered as one of key potential candidate solutions to achieve lightweight vehicle structure.The bearing structure in vehicle body is made of thin-walled beam.The joint is not only an important bearing structure but also a main factor on the properties of vehicle body.In this thesis,the carbon fiber materials and T-joint was combined in car body.The manufacturing process for bearing structure in composite vehicle body is explored.The failure mechanism in static and dynamic load is explored and the mechanical behaviors are studied.The factors affecting mechanical behaviors of this structure are defined by means of analytical,numerical and experimental methods.The the optimization methods are carried out.Then the plan is made to assess the parts in car body including material selection,material performance and structure design.This dissertation mainly includes the following research contents:(1)In view of the characteristics of complex geometrical shape and difficult molding of carbon fiber reinforced thermoplastic(CFRTP)T-joint,the slow camping molding method and technological parameters are studied.The T-joint is made of twill woven carbon fiber(CF/PC)and manufactured by high temperature and pressure molding process.First,a special mold is designed and manufactured for precise die assembly and easily demolding in and after the manufacturing process.The laminate thickness of the T-joint is controlled.Nest,the optimum forming time and temperature are found in the manufacturing process.The slow camping method can effectively reduce the area and degree of fiber wrinkle in the fillet region.At last,the material properties for CFRTP are measured in dynamic and static conditions.There exist quite large differences in majority of the measured mechanical properties using static and dynamic tests except for three parameters:density,Poisson's ratio,warp(weft)compressive strength.(2)An analytical,FE and experimental study is presented on mechanical behavior of single hat-shaped composite T-joints subjected to out-of-plane bending and bending-torsion coupling.An analytical model including the stiffness of bending and bending-torsion coupling is developed to analyze the experimental results and to establish the associated failure criteria.A failure mode map is constructed to realize quick design of composite T-joints.In CFRTP T-joint tests,shear failure occurred on the surface of hat-shaped shell.In the bottom plate,two failure modes:(a)laminate buckling and(b)laminate crushing,are considered.In particular,both laminate buckling and laminate crushing are observed during the experiment with laminate crushing being the final failure mode,which can be considered to be the most important failure mode of the fabricated T-joint.The results in the experiment show that the progressive damage of laminate causes the damping of transverse bending stiffness of the T-joint.The failure results include matrix cracking,matrix/fiber deboning of transverse fibers,delimitation and fiber breakage leading to transverse rupture.In addition,the bending and bending-torsion stiffness discrepancies in analytical and FE model closely match the experimental results.Numerical simulations based on the finite element method(FEM)and the Hashin damage criteria also accurately predict the flexural modulus,the peak load and failure locations of the T-joint obtained in the test.(3)An analynical,experimental and numerical investigation is presented into the failure characteristics of single hat-shaped thin-walled tubular T-joints made of twill-woven(CFRTP)under impact loading.One flat panel and one hat-shaped shell are joined together via(a)adhesive bonding and(b)hybrid bonding and riveting,to form the T-shaped thin-walled tubular joints.Drop weight impact tests are conducted for the T-joints with three impact velocities.Load-time/displacement curves are measured and four distinct failure modes were identified through analyzing damage images and micrographs.It is noted that the specific energy absorption(SEA)capability increases with impact velocity and can be affected significantly by the deformation of the hat-shaped shell,but appears to be less sensitive to the joining forms.Nonlinear finite element analyses based on selected progressive damage model are conducted to predict the failure and energy absorption characteristics of the T-joints under an impact loading.There exists a good correlation between the analynical,simulation and experimental results,which validates the present analynical and numerical modelling method.(4)The composite T-joint is then put into a "bead-properties" combination muti-level optimization referring a steel T-joint in vehicle body.A multi-objective and multi-level design optimization methodology is presented based on the above results on geometry and lay-ups for composite T-joints in automobiles to achieve car body lightweight design.The non-dominated sorting genetic algorithm ?(NSGA-?)is introduced to search for global optimum solution,and radial basis function(RBF)approximations for the objective functions were applied to reduce the computational cost.The proposed method is validated by an obtained optimum design of a composite T-joint with reduced weight and improved overall stiffness and strength behavior.The bending stiffness model and quick design method in chapter 3 are also validated in this situation.Gradient based bead optimization further improves the mechanical properties of T-joint.The mechanical behaviors of T-joint after "bead-properties" combination optimization are siginificantely enhanced,and the cost is reduced,which shows excellcient results for integrate optimization.In this research,the overall theoretical framework of failure mechanism carbon fiber T-joint is constructed.The failure mechanism for composite T-joint in quasi-static and dynamic condition is researched."Bead-properties" combination optimization design for T-joint is conducted,and can thus provide a guidance for vehicle bearing structure design.All of above are full of important engineering application value.