Research On The Theory Of The Multi-right Angles Thin-walled Beam And Application In Autobody Crashworthiness Design

Altough the passive safety of vehicle has been improving,the autobody design needs to be discussed deeply for the increasing requirement of the safety regulation and the concerning on the lightweight.The crashworthiness of the autobody is decided by the frame structure of the thin-walled beam structure involved with the safety,the section and the material of the thin-walled beam and the connection.The section and the material of the thin-walled beam impact on the energy dissipation of itself essentially in a crash.In a crash,crushing and bending are two basic deformation modes disppating the crash energy.So,the theories of the crush and bending are the structure design foundation.In order to reach higher requirement of crashworthiness and lightweight,the multi-right angles thin-walled beam is discussed in recent years.For the more number of the right angles,the load capacity of the multi-right angles thin-walled beam is higher than the rectangular thin-walled beam.Since the twelve-right angles section can be symmetrical and has larger space of lightweight,the section has been used in vehicle.So the crashworthiness theory of multi-right angles thin-walled beam,especially the twelve-right angles thin-walled beam,is instructive and meaningful to design the structure involved with safety.The thesis is supported by the National Key Technology R&D Program 'Lightweight Design and Development of the White Body of the Car' and the National Natural Science Foundation of China 'Study on the Conceptual Design for Vehicle Passive Safety System Based on the Crash Mechanics'.The research of the crashworthiness theory of the multi-right angles thin-walled beam and the application in the conceptional phase of the autobody crashworthines is carried out.In the research of the crashworthiness theory of the multi-right angles thin-walled beam,the theoretical analysis of the mechanism of deformation and energy dissipation is focused on,the simplified theoretical model of the fold and the local deformation of bending is set up,so that the crushing force and bending moment of the thin-walled beam can be evaluated quickly.Firstly,the theory of crush and bending of the multi-right angles beam is researched.The Super-folding Element(SE),which is composed by five kinds of energy dissipation mechanism and is used in the crush theory of the rectangular section thin-walled beam,is used in the derivation of the crush theory of the multi-right angles thin-walled beam,and the combination between the material,section property(shape and size)and the crushing force is obtained.The range of application of the crush theory is evaluated initially while the twelve-right angles section is treated as a special case.When the bending theory is deviated,the mechanism of tensile deformation is considered besides the plastic hinge and the rolling hinge lines which are used in the bending theory of the rectangular section thin-walled beam.The simplified theoretical model of the twelve-right angles thin-walled beam bending around y axis and z axis is built and the combination between the material,section property and the bending moment is obtained.Then the range of application of the bending theory of the twelve-right angles thin-walled beam is evaluated initially.Then,the crush and bending theory of foamed aluminum filled multi-right angles beam is studied.In crushing,the impact of the aluminum foam on the effective crushing length and the reaction between the foam and thin-walled beam is considered,and the combination between the foam material,the beam material,the section property and the crushing force is obtained.The theory is verified while the twelve-right angles section is treated as a special case.In bending,the moment of the aluminum foam and the twelve-right angles thin-walled beam is researched respectively.The material characteristic of the foam is simplified,and the location of the neutral axis of the foam is calculated.Then the bending moment of the foam is obtained.The ratio of the energy dissipation of the foam and the thin-walled beam is fitted based on the finite element analysis.At last,the combination between the foam material,the beam material,the section property and the bending moment is obtained while the beam bends around y axis and z axis.In addition,the crush and bending theory of externally fiber-reinforced metal multi-right angles beam is studied.The composite material characteristic is simplified.The simplified composite material characteristic and the metal material characteristic is used to calculate the fully plastic bending moment and the yield membrane force when two parts are adhered.Then the fully plastic bending moment and the yield membrane force are used in the crushing theory and the new effective crushing distance is considered so that the crushing theory of externally fiber-reinforced metal multi-right angles beam is obtained.In the same way,the revised fully plastic bending moment and the yield membrane force are used in the metal multi-right angles beam bending theory and the bending theory of externally fiber-reinforced metal multi-right angles beam is obtained.At last,the crushing and bending theory is verified by finite element analysis while the twelve-right angles thin-walled beam is treated as a special case.At last,the crushing and bending theories of the twelve-right angles thin-walled beam are used in the conceptional design of the autobody crashworthiness.Based on the body deformation characteristics of autobody thin-walled beam members,the crashworthiness of the whole passenger car and the commercial vehicle cab is decomposed into the sub-objects,that is the crushing force and the bending moment by the performance driven design.The crushing and bending theories of the twelve-right angles thin-walled beam,the aluminum foam filled twelve-right angles thin-walled beam and the externally fiber-reinforced metal twelve-right angles beam are applied in the design of the substructures cross sections and the choice of the material.Based on the sub-objects of the sub-structures of the car and cab,the cross section of the rail of the car,the energy absorber,the floor rail and the under bracket of the mount of the cab is designed and their material is chosen by the theories.The lightweight of the passenger car rails,the optimized crashworthiness of the passenger car and cab is achieved.The crushing and bending theory of multi-right angles thin-walled beam is extended from the theory of the rectangular thin-walled beam.Since the multi-right angles thin-walled beam theory can reflect the connection betweent the crushing force,the bending moment and the material,the cross section of the thin-walled beam quantificationally,the theory is suitable for the conceptional design of the autobody crashworhiness.Based on the crushing force and the bending moment,the cross section and material of sub-structures can be selected quickly while the requirement of crashworthiness and the lightweight can be satisfied meanwhile by using the theory.