Research On Properties Of CNT/PDMS Flexible Sensing Material And Its Monitoring Of Structural Crack

In the usage of major structural engineering,such as high-rise buildings,large bridges,large sports venues,dams,large oil production platforms,and especially the bridges used as important nodes of transportation network,crack can seriously influence the integrity and safety of structures,caused by the loads and environment.Cracks can characterize the stage change of structural bearing capacity,and hold the key signal of structural dangerousness.If the occurrence and propagation of structural crack is timely and effectively monitored,the using life of structures will be lengthened significantly.The mostly used methods in monitoring of structural crack are optical fiber sensor,image processing,piezoelectric sensors,memory alloy,the sensing skins,and so on.However,the above-mentioned methods also have some disadvantages.For example,optical fiber sensor is costly,difficult in construction,and easily damaged in the reparations of concrete structures.It is very important that flexile sensor with large tensile strain was used to monitor the crack propagation when the width of crack gets to the higher value.Two tensile flexible sensing materials were prepared in this paper,such as multi-walled carbon nanotubes/polydimethyl siloxane(MWCNTs/PDMS)composite and transparent conductive SWCNT thin film,which are low cost,good durability,large tensile strain and good sensitivity.The percolation threshold,the relationship between voltage and current,the relationship between temperature and resistance,the relationship between moisture and resistance,and tensile properties of MWCNTs/PDMS composite were studied,and the mechanism of tensile sensitivity was also analyzed.The results showed that the MWCNTs/PDMS composite had obvious phenomena of percolation.The curves of voltage-current and temperature-resistance of MWCNTs/PDMS composite were approximately linear,and the effect of relative humidity can be neglected.The sensitivity factor of MWCNTs/PDMS composite was higher when the content of carbon nanotubes was near the percolation threshold,and the tensile property shows good repeatability.The relationship between the relative electrical conductivity of MWCNTs/PDMS composite and the tensile strain can be expressed by ln(?/?0)=A?.In addition,the quantitative relationship between the conductivity of MWCNTs/PDMS composite and the content of carbon nanotubes was obtained based on the tunneling effect theory and the effective media model.Transparency,sheet resistance,and the effects of acid treatment,temperature and tensile strain on the properties of transparent conductive SWCNT thin film were studied.The results showed that transparency and sheet resistance were decreased with the increased layer of single-walled carbon nanotubes random conductive network.Nitric acid can effectively remove the surfactant SDBS attached to the walls of carbon nanotubes,and the surfactant was much more easily removed when transparent conductive SWCNT thin film experienced a certain cycles of stretching-releasing.The resistance of transparent conductive SWCNT thin film decreased linearly with the increase of temperature.The effect of strain history on transparent conductive SWCNT thin film showed that irreversible resistance occured,and the irreversible resistance was greater with larger tensile strain.The resistance of transparent conductive SWCNT thin film more quickly reached stable state under a larger tensile strain.When transparent conductive SWCNT thin film with sandwich structure experienced 50 cycles of stretching-releasing,the relationship between the resistance change ratio and tensile strain was linear,and holds better sensitivity.The stable morphology caused by cycles of stretching-releasing was analyzed using atomic force micrograph.The application of MWCNTs/PDMS composite in monitoring of concrete structural crack was studied.The results showed that the relationship between the resistance change ratio of MWCNTs/PDMS composite and the compressive strain of reactive powder concrete was linear under the axial compress loading.The resistance change ratio of MWCNTs/PDMS composite was exponentially changed with the deflection of reactive powder concrete under axial bending loading.The first crack occurrence of reactive powder concrete can be determined by the initial change of resistance of MWCNTs/PDMS composite.The relationship between the resistance change of MWCNTs/PDMS composite and crack width of polypropylene fiber concrete can be expressed by an exponential equation under the falling ball impact load.Two-parameter Weibull distribution could evaluate the anti-impact property of polypropylene fiber concrete reasonably.Transparent conductive SWCNT thin film with sandwich structure shows good tensile sensitivity in the tensile strain range of 0~100%,and can be used as a strain sensor.MWCNTs/PDMS composite can be used as a sensing material to monitor the compression strain and deflection of concrete,the occurrence and propagation of concrete crack.The paper has theoretical and practical significance for the engineering application of new sensor elements in the area of structural health monitoring.