Research On The Dynamic Mechanical Properties Of Carbon Nanotubes Reinforced Ultra High Toughness Cementitious Composites

Cementitious material is a quasi-brittle and heterogeneous composite material.It is widely used in civil engineering due to the excellent characteristics such as easy availability of materials,low cost and convenient construction.However,traditional cementitious materials are prone to brittle failure under impact loads such as explosions,and the flying fragments will cause great damage to the internal personnel and equipment.Ultra-high toughness cementitious composites(UHTCC)is a kind of fiber reinforced cementitious composite with remarkable tensile strain hardening and multi-cracking characteristic.Due to the excellent tensile deformation and impact energy dissipation capacity,UHTCC has broad application prospects in improving the impact resistance of important structures such as protective engineering.Carbon nanotubes(CNTs)is a kind of nanomaterial with excellent mechanical properties and good electromagnetic wave-absorbing properties.Combining CNTs and UHTCC is expected to produce a new cementitious composite with advantages of both materials,thus improving the electromagnetic wave-absorbing capacity and impact resistance of protective structures.This dissertation focused on the dynamic compressive performance and the dynamic splitting tensile performance of CNTs-UHTCC,and the main research contents are as follows:(1)The quasi-static and dynamic compression experiments of CNTs-UHTCC at different CNTs dosage were carried out.The microscopic pore structure of the materials was studied by XCT technology and mercury injection technology,and the two-dimensional planar morphology of the materials was studied by scanning electron microscopy(SEM).The results showed that under the quasi-static load,the compressive strength reached the peak value when the CNTs content is 0.2wt.%.Under dynamic loads,CNTs-UHTCC showed significant strain rate sensitivity.A prediction model between the dynamic compressive strength increase factor and strain rate of CNTs-UHTCC was established and showed good agreement with the experimental results.Well-dispersed CNTs can optimize the void structures of UHTCC matrix and reduce the porosity,while the poor dispersion of CNTs at high dosage will cause deterioration of UHTCC.(2)The quasi-static and dynamic splitting tensile experiments of CNTs-UHTCC at different CNTs dosage were carried out,and the synergistic effect between CNTs and polyvinyl alcohol fiber(PVA)was studied based on SEM technology,as well as the failure mode of PVA fiber at different strain rates.The results showed that the splitting tensile strength of CNTs-UHTCC at 0.2wt.% is the highest under both quasi-static and dynamic loads.A prediction model was established between the dynamic splitting strength increase factor and the strain rate of CNTs-UHTCC at different dosage,and the prediction model were in good agreement with the experimental results.The microscopic analysis showed that CNTs could not only exert the bridging effect of fibers to prevent the proliferation and expansion of cracks,but also improve the bonding performance between PVA fibers and cement matrix.With the increase of strain rate,the failure mode of PVA fiber transmitted from pulling-out to breaking.(3)The dynamic constitutive model of CNTs-UHTCC was studied and verified by numerical simulation.Based on the experimental results and relevant literature data,the strength surface parameters,damage evolution parameters,strain rate effect parameters and high pressure equation of state parameters of the K&C model were calibrated.The numerical simulation of the dynamic compression and dynamic splitting experiments was carried out in LS-DYNA using the calibrated parameters.The simulated damage evolution process and failure morphology of the specimen were in good agreement with the experimental results.