Research On Crashworthiness And Failure Mechanisms Of Carbon-fiber Reinforced Polymer Composite Thin-walled Structures

With the rapid growth of the number of transportation vehicles in China,the crash of vehicle and crash landing of airplane emerge in endlessly.In addition,China is still facing increasingly serious problems such as energy shortage and environmental pollution.It is of great significance to improve the structural crashworthiness and reduce the structural weight for energy conservation,environmental protection and vehicle collision safety.Carbon fiber reinforced polymer composites(CFRP)thin-walled structures are widely used in the crashworthiness and light-weight design of vehicle and aerospace structures due to advantages of high specific strength,high specific stiffness,high specific energy absorption,etc.However,failure modes of CFRP thin-walled structures under crushing or impact loads are quite complex and uncontrollable.Micro damage or manufacturing defects may cause unstable failure modes such as local buckling,brittle fracture and interlaminar delamination,greatly reducing the structural crashworthiness.Thus,the wide applications of CFRP thin-walled structure in vehicle and aerospace are restricted.To overcome the above problems,it is urgent to extensively reveal the failure mechanisms and energy-absorption characteristics of materials and structures and conduct the crashworthiness design of CFRP thin-walled structures to improve the energy-absorption characteristics.According to the existing advanced design methods,failure triggering mechanism,bonding-patch reinforcement and interface toughening design have significant effects on improving energy-absorption of CFRP thin-walled structures(or perforated CFRP thin-walled structures).However,there are relatively few systematic studies on these aspects,and there are many problems to be further studied and explored.In this study,material behaviors,structural responses,modeling methods,failure models,analysis and design are systematically studied.Firstly,the micro,meso and macro damage models of CFRP thin-walled structures are proposed.Secondly,the micro and multi-scale failure mechanisms of CFRP thin-walled structures are studied,and the structural failure and energy-absorption mechanisms of typical CFRP thin-walled structures under crushing load are revealed.Then,the failure triggering mechanism design method is adopted to improve the crashworthiness of CFRP thin-walled structures.Finally,the crashworthiness of the perforated CFRP thin-walled structures is improved by the combination of bonding patch reinforcement and bonding interface toughening.This dissertation has carried out and completed the following research works.(1)The macro-scale,meso-scale and micro-scale damage models and the multi-scale coupling method between meso-scale and micro-scale are proposed for CFRP.Combined with the failure modes and damage evolution characteristics of CFRP constituent materials/structures,the micro-scale damage model considering fiber,matrix and interface and the meso-scale progressive damage models of bundle,matrix and interface are established.Then,according to multi-scale effects,the Bridge model-based multi-scale damage coupling method is proposed by embedding the trans-scale stress transfer model and the trans-scale damage transfer model into the meso damage model.Finally,based on the assumption of homogeneity and continuity of CFRP thin-walled structures,a macro-scale homogeneous progressive damage model considering longitudinal tension/compression,transverse tension/compression,shear failure and delamination is proposed based on the fracture energy method.(2)The micro-,meso-and macro-scale finite element models of CFRP thin-walled structure are established to simulate the material behaviors and structural responses and reveal the material and structural failure mechanisms.Firstly,the micro-and meso-scale representative unit-cell(RUC)finite element models of CFRP thin-walled structure are established to simulate and predict the material behaviors of CFRP thin-walled structure.Then,the stress distribution,damage evolution characteristics of constituent materials including fiber and matrix are revealed,and the multi-scale damage coupling mechanism between micro-scale and meso-scale are explored.Finally,the macro finite element models of typical vehicle and aerospace thin-walled CFRP structures(such as circular tube,square tube and cap-shaped plate)are established to simulate the structural crashworthiness responses which are compared with the relevant experimental results.The axial crushing failure mechanisms of circular tube,square tube and cap-shaped plate and the oblique crushing failure mechanisms of circle tube are revealed.(3)To improve the crashworthiness of CFRP thin-walled structures,the material degradation triggers,external plug triggers and their coupling trigger mechanisms are designed,and the failure trigger mechanisms are then revealed.Firstly,single material degradation trigger and gradient material degradation trigger mechanisms are proposed to improve the initial crushing behavior of CFRP square tubes.In order to make the materials fully destroyed to dissipate energy,the external trigger mechanisms are proposed to induce the progressive crushing failure process and improve the energy-absorption of the structure.Then,combining the chamfer trigger,groove external trigger and gradient material degradation trigger,the multi-coupling trigger mechanisms are designed to further improve the crashworthiness of CFRP square tube,and failure trigger mechanisms are revealed.Finally,the effect of triggering mechanism on the crushing failure of perforated CFRP square tubes is studied.The influences of the number of holes,edge distance of hole and the position of holes on the energy-absorption characteristics of perforated CFRP square tubes are analyzed.The principle of transition from medium high fracture to progressive failure mode of perforated CFRP square tubes induced by appropriate triggering mechanism is revealed.(4)Combined with the design method of bonding interface toughening and patch reinforcement,the mechanical properties of perforated CFRP thin-walled structure and the crashworthiness of perforated CFRP square tube are improved,and the patch reinforcement and bonding interface toughening mechanisms are revealed.Patch bonding method is used to enhance the perforated CFRP thin-walled structure.The pure resin interface,Aramid-pulps(AP)interface,carbon nanotube(CNT)interface and AP-RPC_CNTinterface are designed to improve the properties of bonding interface,exerting the role of patch adequately.The studies on the bending tests and simulations of parameter analysis are carried out to compare the structural bending performances of different samples.Then,the reinforcement mechanism of patch,toughening mechanism of bonding interface and the principle of improving interfacial adhesion by RPC_CNTsurface treatment are revealed.Finally,the bonding patch reinforcement method is used to improve the crashworthiness of perforated CFRP square tubes.By means of simulation,the effects of patch bonding interface strength,patch layer number and bonding position on the crashworthiness of perforated CFRP square tubes are studied,and the corresponding crushing failure behaviors and bonding interface failure process are analyzed.