Experimental Tests And Multi-scale Modelling On Crashworthiness Of Fiber Hybrid Composite Thin-walled Structures

With the increasing of car ownership,series of social problems appear in recent years such as excessive energy consumption,environmental pollution and so on.It has been concluded that vehicle body accounts for 30%-40% of the whole automobile weight.Thus,it becomes as an important content and development trend to reduce energy consumption by using lightweight materials in vehicle design.The Carbon fiber reinforced polymer(CFRP)shows great potential in application of automobile body due to its superior performance in terms of specific strength/stiffness,formability,corrosion resistance and energy absorption characteristics.However,compared with traditional metallic materials,drawbacks exist for the CFRP composite such as its higher cost.Also,lack of ductility would cause the sudden failure of the CFRP structure without any warning,which shows weak performance in structural damage monitoring.Replacing partial CFRP layers with more ductile fiber seems to be an effective approach to address the issue of poor ductility and high material cost.While research on hybridization design is still lacked in state of art,with less attention focused on the real structure and loading conditions for automobile body.This manuscript focuses on the possibility of fiber hybridization in crashworthiness design with typical thin-walled(TW)structures,aiming to simulate the real deformation response in traffic accidents.Effect of hybrid design variables on crashworthiness of TW structures is thus investigated with experimental and numerical methods,such as hybrid ratio,fiber type and stacking sequence.Basing on the homogenization theory,a multi-scale computation framework is established in this work,aiming to bridge the computational relationship between structural characteristics in micro/meso-scale and crashworthiness in macro-scale through sequential numerical tests.Detailed contents are presented in the following:(1)A multi-scale computational model is proposed for structures reinforced by plain woven carbon fiber fabric.Basing on the continuum damage mechanics theory,a progressive damage constitutive model is established to describe the mechanical response of constituents in micro/meso-scale,such as fiber yarn/bundle cracking and delamination via user subroutine.With continuum damage mechanics constitutive law coded in macro-scale,the finite element model is established for TW structures which could be possible to describe the intra-layer cracking and inter-layer delamination failure mode.(2)Research on the effect of fiber hybridization on three-point bending performance of laminates.Composite laminates composed of CFRP,basalt fiber reinforced polymer(BFRP)and glass fiber reinforced polymer(GFRP)are prepared with vacuum assisted resin transfer molding process.An experimental study is conducted to investigate the effect of design variables,such as hybrid ratio,layup and fiber type on mechanical properties and failure mode.Failure mechanisms of hybrid laminates after tests are depicted via scanning electron microscopy method.The flexural modulus and stress distribution across thickness direction are predicted with the classic laminate theory,aiming to reveal the details of strain,damage and fracture response information.A finite element model is established to predict the bending failure mechanisms of hybridized laminates.Finally,a performance versus cost efficiency analysis is conducted in depth for potential engineering applications.(3)Research on the effects of fiber hybridization on the low velocity impact resistance.Interply hybrid specimens used are fabricated in a sandwich-like stacking sequence using vacuum assisted resin transfer molding process.Low velocity impact tests are carried out for composite laminates to simulate loading conditions of thin-walled structures used in automobile body.Effect of hybrid variables on impact resistance are then analyzed with impact response and failure modes of laminates.A finite element model is developed and validated for CFRP/BFRP hybrid laminates.Stress distribution among different plies are also analyzed to evaluate the hybridized enhancement mechanism.(4)Research on the effect of fiber hybridization on axial crashworthiness of hat-shaped energy absorption structure.Typical double hat-shaped composite tubes composed of plain woven carbon and/or glass fiber prepreg are prepared by thermal-forming process.Quasi-static and dynamic axial crushing tests are conducted to simulate deformation response of energy absorption structure in real traffic accidents.The crushing load-displacement curves and progressive deformation photographs are then obtained.The sensitivity of collapse mode and crashworthiness to hybrid ratio,stacking sequence and loading rate is thus analyzed.(5)The establishment of multi-scale computation model to predict the axial crushing response of hat-shaped composite tubes.A multi-scale model is constructed basing on homogeneous concept and finite element method,which bridges the computational relationship between structural characteristics in micro/meso-scale and crashworthiness in macro-scale.Effective properties of fiber bundle are determined through micro-mechanical tests,which considers the random distribution of fibers,elasto-plastic damage of matrix phase and interface properties.Then a finite element model in meso-scale is developed to investigate the homogeneous properties of lamina and validated with experimental results.Finally,the crushing process of CFRP tube is simulated based on stacked shell model with material properties predicted from last scale.The numerical simulations are validated with corresponding experimental results and show good agreement.It can be concluded that the multi-scale computational framework is capable to correlate structural crashworthiness in macro-scale and constituents' characteristics in micro-/meso-scale.Thus,experimental tests can be partially replaced and design efficiency can be improved.