Numerical Simulation And Experimental Study Of Acoustic Elastic Effect In Steel Strand

Steel strand is widely used in civil engineering structures,such as prestressed concrete Bridges,cable-stayed Bridges and arch Bridges.On the one hand,the stress level of anchored steel strand is seriously degraded due to creep and relaxation under the condition of long-term service.On the other hand,the temperature drop,shrinkage and creep of prestressed concrete structures will lead to the change of prestress.After long-term operation,the prestressed steel strand suffers from corrosion to a certain extent,the residual tension decreases,the prestressed concrete bridge bends too much in the middle and bottom of the span,the beam body cracks,which seriously threatens the bridge’s safety,resulting in the serious degradation of the structure’s durability,safety and bearing capacity.In order to carry out nondestructive testing of the working stress of steel strand by combining the ultrasonic guided wave technology with the theory of acoustic elasticity,this paper adopts the method of theoretical analysis,finite element simulation and experimental research to study the propagation characteristics of ultrasonic guided wave in steel strand under different stresses.According to the theory of solid state acoustic elasticity,the variation law of the mode wave velocity of ultrasonic guided wave L(0,1)with the tension of steel strand is studied,which provides theoretical basis and technical support for the nondestructive testing of steel strand working stress.This paper mainly studies the following aspects.(1)Numerical simulation of ultrasonic guided wave propagation in high strength steel wire and steel strand.Based on transient dynamics establish three-dimensional geometric model,model parameter setting was introduced in detail,unit grid partition,incentive load Settings.The propagation law of ultrasonic guided wave in high strength steel wire and steel strand is intuitively observed by the time history signals of acceleration and displacement of the node.The numerical simulation of ultrasonic guided wave lays a foundation for the subsequent numerical simulation of acoustic elastic effect.(2)Combined with the ultrasonic guided wave,the acoustic elastic effect of high strength steel wire and steel strand is numerically simulated based on the equivalent acoustic elastic method.The second order elastic constant and density under different stress are converted into equivalent second order elastic constant and equivalent density according to the sound elastic formula of body wave.In the simulation of ultrasonic guided wave,the variation of group velocity was analyzed according to the time history signal of acceleration under different equivalent stresses.At the same time,the variation of guided wave modes in the frequency-wave number domain under different equivalent stresses is analyzed.The variation of phase velocity with equivalent stress is obtained indirectly.(3)Contact under the action of the steel strand tension effect has carried on the detailed analysis,under the actual pull force,wire contact area contact effect.Frequency and wave number cloud near 200 k Hz and 600 k Hz frequency on the modal split ends,modal split level does not change drastically with the increase of stress change.From the perspective of energy,as the stress increases,the color of the frequency-wavenumber cloud image gradually becomes lighter and the energy decreases.(4)The tensioning experiment of steel strand and the ultrasonic guided wave experiment were carried out,and the signals with frequencies of 100 k Hz,200 k Hz and300 k Hz were excited respectively.The results show that the flight time of guided wave increases linearly with the increase of tensile force,that is,the velocity of first wave decreases with the increase of tensile force.The flight time of guided wave signals with different frequencies has a similar law with the variation of tension.Line fitting was carried out for the experimental curves of two steel twisted wires.The coefficient R2 of the fitting line was not less than 0.8504,indicating that the linear law was strong.The variation of flight time with tension during loading and unloading is mainly due to the influence of residual stress of steel strand after unloading.(5)In the experiment,the greater the tensile force of steel strand,the greater the elongation.The minimum variation of guided wave flight time due to the amount of elongation is 16.5% of the total variation.Therefore,the influence of elongation cannot be ignored in the experiment,but it does not affect the variation trend of guided wave flight time with tensile force.The velocity of the wavefront wave decreases with the increase of frequency and has the dispersion characteristic similar to that of the wavefront wave in steel wir.Comparing the finite element and experimental results of steel strand,the experiment verifies the acoustic elastic effect of steel strand.