| The CFRP(carbon fiber reinforced plastics)laminates,which show high specific strength,high specific modulus,fatigue resistance,and corrosion resistance and other excellent properties,become preferred alternatives to push the tide of lightweight design of rail vehicles.After local impact,local damage is produced and its performance could be largely reduced.Therefore,in the popularization and application of carbon fiber laminates in rail vehicles,the damage behavior caused by occasional impact bodies of different sizes and speeds and the corresponding reinforcement methods should be considered.The systematic recognition of the impact damage behavior and complex evolution law related to the characteristics of variable impactor,constraint boundary,lay-ups and reinforced configuration of CFRP laminates becomes the most important challenge in the impact resistance design of CFRP structures.Therefore,this work investigated the local impact dynamic response,damage characteristics and strengthening mechanism of CFRP laminates in depth.The main research content and the novelties are as follows:1.The effects of mass and size of impactor on dynamic deformation response and damage of carbon fiber laminates under impact load was studied.With the help of the improved local impact test system of vehicle structure,the impact tests of carbon fiber laminates with different impact sources in the large kinetic energy range were carried out.The dynamic damage process of the carbon fiber laminates was captured by high-speed camera.The quantitative delamination damage assessment was performed using an ultrasonic C scanner.By programming VUMAT subroutines,an explicit simulation model of progressive damage based on Chang-Chang criterion was established with Abaqus software.This model was validated by experiment results.Further study revealed the dynamic impact damage mechanism of the size and mass effects of impactors on CFRP laminates.2.The effects of loading rate,the scale and boundary condition of CFRP laminates under impact loading were analyzed,and the corresponding numerical models were established with the strain rate and boundary effect considered.The correlation between CFRP’s modulus and strength with strain rate was established and implemented by VUMAT subroutine.The numerical model was validated by impact experiments,which carried out studies of CFRP laminated beam and plate structures subjected to local impact loads.The correlation analysis of deformation mode,delamination and failure morphology of CFRP laminates with its aspect ratio,boundary condition and load carrying rate was carried out.The ductile-brittle transition,energy absorption and dissipation of CFRP laminates affected by size effect and boundary effect under different local loads were revealed.3.An in situ analysis method for the delamination behavior of crossply CFRP laminates based on multi-scale representative volume element(RVE)simulation model was proposed.Based on the crack propagation theory of fracture mechanics and the equivalent stress analysis method,the relationship between the critical crack propagation stress and the fracture energy of cross-ply composites was established.A numerical model based on RVE was established.Combining with experimental observations,the evolution mechanism of the complex process of matrix cracks propagation in cross-ply CFRP laminates was revealed.Parametric studies was applied to study the in-situ reinforcement effect of thin-ply laminated composites under out-of-plane shear load.The effects of layer thickness,matrix properties and fiber-matrix interface properties on the crack formation and propagation in the laminates was explored.Based on the findings,The delamination resistance design of CFRP laminates under impact load was carried out.4.The relationship between failure modes,residual strength and hybrid structure design of CFRP sandwich reinforced structures was established.The correlation between the typical failure modes and structural parameters of two typical CFRP sandwich structures,CFRPaluminum alloy hybrid laminates and CFRP-aluminum honeycomb sandwich structure,were obtained by experimental and theoretical analysis.Through comparative analysis,the failure mode dominated by hybrid structural configuration was characterized.The outstanding performance of post-impact residual compressive strength of "aluminum alloy + CFRP+ aluminum alloy"(AFA)laminates was found.By means of equivalence and normalization,the corresponding relationship between local impact failure modes and residual strength of CFRP honeycomb sandwich structure were established.On this basis,a parametric damage prediction map was drawn to predict the post-impact residual strength and damage mode of CFRP sandwich structures.5.The local impact resistance design of CFRP laminated panel for rail vehicle body was carried out.Reasonable reinforcement strategies of ribsembedded panel were proposed.By using the numerical method above mentioned,the comprehensive impact resistance performance of various CFRP design schemes with the same weight was compared in terms of delamination,penetration resistance and displacement response.In order to obtain a reasonable reinforcement design strategy for CFRP reinforced panels,the effects of thin-ply,curved surface configuration,relative position of ribs and metal-hybrid on impact resistance was analyzed in detail.For illustration to the engineering application,a scheme of CFRP curved stiffened panel for rail vehicles was given and its impact strength was checked.This work systematically analyzed the effects of the impactor difference,the body boundary and the lay-up configuration of CFRP laminates on local impact damage.The impact resistance of carbon fiber body panels of different configurations was comprehensively evaluated based on application scenarios.Potential impact resistance strategies of CFRP structures were proposed as references for the application of CFRP laminates in rail vehicle body.There are 141 figures,28 tables and 206 references in the dissertation... |
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