Research On Mechanical Properties Of Reinforced Recycled Plastic Composite Sleeper

As an important component in the railway track structure,the sleeper plays an essential role in supporting the rails,transferring the load downward,and maintaining the shape and position of the track.Its mechanical properties are directly related to the reliability of the railway track and the running safety of the train.In recent years,due to the high strength-to-weight ratio,excellent insulation properties,outstanding corrosion resistance,durability,and operability,composite sleepers have gradually gained popularity.However,insufficient strength is one of the main defects of composite sleepers,which hinders its wide application to a certain extent.The mechanical properties of composite sleepers are not only affected by the composition and proportion of their raw materials,but also by their production process and structural design.Researchers all over the world have carried out various optimization research of composite sleepers from the aspects of material and structure,but few of them have taken the production process into consideration,and the relevant research on the FEM(Finite Element Method)model for composite sleepers is also relatively scarce.In this paper,the recycled plastic composite sleeper is studied from the aspects of material mechanical properties and structural mechanical properties.Recycled polyethylene,polypropylene,glass fiber,magnesium hydroxide,and maleic anhydride grafted polypropylene are selected as the raw materials for the composite sleeper.The influence of glass fiber content,molding thickness,and environmental moisture conditions on material properties was explored through mechanical testing;on this basis,the glue-laminated composite sleeper structure was designed,and bar reinforcement and carbon fiber fabric reinforcement were selected as structural optimization methods.The effects of individual segment orientations,different reinforcing bars,and carbon fiber fabric on the flexural performance of sleepers were investigated through three-point bending tests,and the spike pull-out resistance of vertically and horizontally arranged sleepers was compared.Finally,the brittle cracking model and the plastic deformation model on FEM software Abaqus are compared and analyzed based on the results of the three-point bending test,and the more suitable one for the composite sleeper material is selected for the parametric analysis of structural properties.The main results and conclusions are as follows:1.Based on the mechanical test,taking the compression molding process into consideration,the influence of glass fiber content,molding thickness,and environmental moisture conditions on the material’s mechanical properties was explored.The proportion of sleeper material components and the parameters of the production process can affect the mechanical properties of the sleeper to a certain extent,and the performance stability of the material under different environmental moisture conditions is related to the reliability of composite sleepers in the extreme rainfall weather.In terms of the effect of glass fiber content,the tensile,compressive,and bending properties of the specimens all improved with the increase of glass fiber content,while the shear properties did not change.When the glass fiber content is too high,fiber agglomeration will form local defects on the sample and reduce the mechanical properties of the sample.In addition,glass fiber can also play a role in reducing the linear expansion coefficient of the material and increasing the thermal stability of the material.The optimal material formulation of recycled plastic composite sleepers in this study was determined as PP: PE: Mg(OH)2: MAH-g-PP =8: 2: 2: 0.75,and the optimal glass fiber content was 44%.In terms of the influence of the molding thickness,the increase in the molding thickness of the sample will lead to a decrease in the density and a significant drop in the mechanical properties.Moreover,the increase in the molding thickness leads to the obvious delamination problem.Due to the influence of the interfacial delamination,the failure mode of the sample also changes,and problems such as interfacial sliding and splitting occur,which affects the bearing capacity of the sample.By analyzing the test results,the optimal molding thickness selected in this study is 10 mm.In terms of the influence of environmental moisture conditions,the water absorption test result of the material is0.254%,which means almost no water absorption.After 72 hours of immersion,the tensile,compressive,flexural,and shear properties of the wet specimen are almost the same as those of the dry specimen,but the surface of the clamped part of the wet tensile specimen flakes after immersion.2.Based on the three-point bending test and the pull-out test of spikes,the influence of the individual segment orientations,different reinforcing bars,and carbon fiber fabric on the flexural performance and the spike pull-out resistance of the composite sleeper was investigated.Referring to the previous structural optimization research on the mechanical properties of composite sleepers,the glue-laminated structure was proposed.Steel bar reinforcement,GFRP bar reinforcement,and carbon fiber fabric reinforcement were selected to perform structural mechanical tests.The three-point bending test results show that the individual segment orientations do not affect the flexural modulus of specimens,while the flexural strength of the horizontally arranged specimen is lower than that of the vertically arranged specimen because of the interlayer slip.For specimens with the same individual segment orientation,both steel bar and GFRP bar can effectively improve the flexural properties of the specimens.Among them,the flexural modulus of steel bar reinforced specimen is the highest,and the flexural strength of GFRP bar reinforced specimen and that of steel bar reinforced specimen are not significantly different.For the specimens with different individual segment orientations and different reinforcing bars,since the carbon fiber fabric can limit the interlayer sliding of the specimens to a certain extent,the specimens wrapped by carbon fiber fabric have higher flexural strength and flexural modulus.After wrapping,the flexural strength of the steel bar reinforced specimen is higher than that of the GFRP bar reinforced specimens when their segment orientations are the same.The results of the spike pull-out test show that compared with the vertically arranged specimens,the spike pull-out resistance and failure displacement of the horizontally arranged specimens increase significantly,while the vertically arranged specimens were completely split from the middle during the pull-out process.3.Based on the three-point bending tests results,Brittle Cracking Model and Plastic Deformation Model were compared,and the sensitivity analysis with FEM of the structural mechanical performance of the sleeper was carried out.Finite element simulation has been widely used to study the mechanical behavior of composite sleepers,but most of the research only explores the elastic-plastic deformation of the sleeper.There is a lack of research on the constitutive model suitable for failure behavior simulation of composite sleepers.In this paper,by comparing the numerical three-point bending analysis of Brittle Cracking Model and Plastic Deformation Model combined with the VUSDFLD subroutine,it is found that both models can well simulate the deformation behavior and brittle fracture of the composite material in the elastic stage,while Plastic Deformation Model combined with the VUSDFLD subroutine can better simulate the plastic deformation of the composite material.Therefore,Plastic Deformation Model combined with the VUSDFLD subroutine is selected to establish the three-point bending model of smallscale glue-laminated composite sleeper.The effects of steel bar reinforcement,GFRP bar reinforcement,and carbon fiber fabric reinforcement on the performance of composite sleepers under ideal bonding conditions are explored when the different individual segment orientations are ignored.The numerical results show that after ignoring the bonding interfaces,steel bars,GFRP bars,and carbon fiber fabric can all significantly improve the bending resistance of recycled plastic composite sleepers.The numerical results of flexural strength and flexural modulus of composite sleepers under almost all reinforcing conditions are higher than the corresponding test results.Finally,the reinforcement layouts of 4C25 steel bars and 6Φ40 GFRP bars are selected,and the numerical three-point bending models of the full-scale recycled plastic composite sleeper reinforced with steel bars and GFRP bars are established.The numerical results show that the flexural modulus of both the GFRP bar reinforced and steel bar reinforced composite sleepers can reach more than 15000 MPa,which meets the relevant regulations for the flexural performance of composite sleepers published by China National Railway Group Limited.However,the flexural strengths of the GFRP bar reinforced and steel bar reinforced composite sleepers are both less than 200 MPa,which fails to meet the relevant requirements of China National Railway Group Limited.