Research On Prestressing Detection Technology Of Steel Strand Based On Electromagnetic Standing Wave

Prestressing technology is widely used in modern bridges.Prestressed steel strands have made considerable contributions in delaying bridge cracks and increasing bridge life.However,when the prestress in the prestressed steel strand is insufficient,the structure is greatly damaged.During the construction and use of the prestressed structure,the prestress will be gradually lost,so that the load that the structure can recover the deformation is smaller and the structural safety performance is worse.The residual prestress is an important indicator of a prestressed bridge,especially the in-service structure.It should be detected and monitored to help detect problems and repairs in time to avoid major accidents.In this paper,the existing methods for detecting prestress are analyzed,and the advantages and disadvantages of each method are summarized.Based on this,a prestress detection method suitable for in-service structures is proposed: the prestressing detection method based on electromagnetic standing wave is accomplished.The theoretical calculation of prestressing by standing wave method is used to explore the experimental method of prestressing by standing wave method.The feasibility of the standing wave method for measuring prestressing method is verified.The main research contents and results are as follows:(1)Solving the equivalent magnetic permeability of the medium when the electromagnetic wave propagates at the interface between the steel strand and the air.The combined effect of air and steel strands on electromagnetic wave propagation is equivalent to the single action of a single medium,and the equivalent permeability is calculated by conformal mapping and distributed inductance.Provide a basis for the establishment of electromagnetic wave propagation models.(2)Established a propagation model of electromagnetic waves in steel strands.The propagation of electromagnetic waves in steel strands is equivalent to the propagation of the interface between the ideal conductor and the medium,and the surface wave transmission line model is established accordingly.The model is used to obtain the expression and distribution of electric and magnetic fields in steel strands,and the influence of magnetic permeability on wavelength is further analyzed.(3)The mechanism of forming electromagnetic standing waves in steel strands is analyzed.Refraction and reflection occur when a medium interface is encountered during electromagnetic wave propagation.When the transmission line terminal is open,total reflection will occur,and the reflected wave and the incident wave will be superimposed to form a standing wave.The amplitude of the fixed point of the transmission line is only related to the position of its opposite input end.Changing the wavelength changes the relative position and the voltage amplitude changes.(4)Explain in detail the principle of standing wave method for measuring prestress of steel strands.According to the principle of magnetostriction,the magnetized material changes its magnetic permeability under the action of an external force,which in turn causes a change in the wavelength of the standing wave,causing the terminal voltage to change.Solve the relationship between the standing wave voltage and the stress.(5)The method of measuring the stress of steel strand by standing wave method is explored.Explore experimental methods and select the appropriate experimental setup and experimental protocol.After the experiment,it is concluded that when the tensile force is applied to the steel strand,the terminal voltage of the strand decreases with the increase of the tensile force,and when the tensile force is released in the strand,the terminal voltage increases with the decrease of the tensile force,and the voltage It changes with the cosine law as the stress changes.