Interfacial Stress Analysis Of Graphene Reinforced Cement-based Materials

The microstructure and nanostructure of cement matrix composites play a crucial role in its macroscopic mechanical properties and durability.Experiment results showed that grapene can be used to improve the structure of cement hydration products and improve mechanical properties of concrete.Understanding the interaction mechanism of the two different materials and its effect on the overall mechanical properties is the premise of its application.The thesis studied the distribution of matrix normal stress and reinforcement normal stress and interfacial shear stress at the interface of graphene reinforced cement-based composites.The shear lag model combined with the cohesive theory was used to analyze the distribution of stresses at the interface which are assumed to be the bonded,damaged and debonded.A representative volume element containing one layer of graphene sheet and two layers of cement matrix was selected to establish a two-dimensional mechanical model subjected to tensile stress at both ends of the matrix.The analytical expressions of stresses at the interface of composites were obtained.The distribution laws of stresses along the direction of the interfacial length were studied.The principal factors influencing stresses distribution were determined,and the influence laws of the interfacial length and the interfacial stiffness on the stresses distribution were investigated.In this study,the finite element method(FEM)was used to verify the theoretical model.The FEM model of representative volume element was built.The parameters in FEM model were given,and the tensile simulation was carried out.The distribution laws of the matrix normal stress and the reinforcement normal stress along the direction of the cross-section were obtained.The distribution laws of the stresses at the interface along the direction of the interfacial length were obtained.The shear lag model was verified through comparing the results of FEM simulation with those of the theoretical model.The causes of errors were also analyzed.The finite element model containing three layers of graphene sheet was bulit.The influence of upper and lower boundary loads of representative volume element on the theoretical analysis results were investigated.The distribution laws of the matrix normal stress and the reinforcement normal stress along the direction of the cross-section were obtained through the tensile simulation.The distribution laws of the stresses at the interface along the direction of the interfacial length were obtained.It is found that the upper and lower boundary loads of representative volume units have little influence on the stresses distribution,which further verified the theoretical model.The following is the main conclusions of this thesis:(1)The shear lag model combined with the cohesive theory was used to derive the analytical expressions of the matrix normal stress,the reinforcement normal stress and the interface shear stress at the interface of the composite at the interface which is assumed to be bonded,damaged and debonded.(2)The theory analysis results agree well with the finite element results,which indicates that the analytical expressions of the stresses at the interface,can reflect the distribution laws of the stresses along the direction of the interfacial length.(3)Larger interfacial length or interfacial stiffness are conducive to reducing the matrix normal stress and increasing the reinforcement normal stress.Therefore,the selection of graphene sheets with larger length or the functionalization of graphene sheets can help to alleviate the cracking of cement-based materials and improve the durability of composites.(4)The variation of the interfacial stiffness along the interface in accordance with the law of gradual increase from the middle to the both ends contributes to the normal stresses to be uniformly distributed along the direction of the interfacial length.As a conclusion,the graphene sheets are functionalized in a manner that the functionalization degree increased gradually from the middle to the both ends,so that the functionalized derivatives of graphene can help to acquire high-performance composites.The research results can provide ideas for the selection of graphene sheet size in composites,provide a reference for the arrangement of functional groups at different positions on the graphene sheet when acquiring functional derivatives of graphene,and provide theoretical basis for obtaining high-performance graphene reinforced cement-based composites.