Study On Uniaxial Stress Identification Method Of Steel Strand Based On The Longitudinal Mode Of Ultrasonic Guided Wave

Steel strands are the main tensile members of cable bridges and prestressed concrete structures,and their stress status is an important indicator for evaluating structural safety.The traditional uniaxial stress detection methods of steel strands have developed,such as vibration frequency method,pressure sensor method,embedded sensor method and magnetic flux detection method,but they have applicability and limitations.Ultrasonic guided wave method has been developed and applied in the non-destructive testing of pipelines and anchors due to its strong penetration,wide detection range,and high efficiency.Some scholars have also conducted research on steel strand stress testing based on ultrasonic guided wave method,but the propagation mechanism is not clear enough,there are problems such as excitation frequency selection,signal energy attenuation and low stress sensitivity.Based on the above,this paper proposes a uniaxial stress identification method for steel strands based on the longitudinal mod e of ultrasonic guided wave.The main research contents are as follows:Theoretical analysis of uniaxial stress identification based on the first-order longitudinal mode of ultrasonic guided waves.In this paper,the steel strand is simplified into a round bar,and the dispersion equation of the longitudinal mode of the ultrasonic guided wave is established based on the wave theory of the round bar.On this basis,combined with the theory of acoustic elasticity,GUIGUW numerical calculation software is used to study the sensitivity of the stress identification feature to the uniaxial stress.It is found that group velocity could be used as the characteristic quantity of uniaxial stress identification,and the method for identifying the uniaxial stress of the steel strand based on the change of group velocity was proposed.Furthermore,the relationship between the stres s sensitivity of group velocity and frequency in typical diameter steel round bars are analyzed,which reveals that there is a positive correla tion and a negative correlation between the group velocity and the uniaxial stress at different excitation frequencies.The optimal method of excitation frequency for uniaxial stress identification of steel strand is proposed.Establishment of uniaxial stress identification system for steel strand and the study of the influencing factors on the system.According to the detection method of single sending and single receiving,the hardware system of steel strand uniaxial stress identification is built,and the software system is built for the design and generation of ultrasonic guided wave signal as well as the requirements of signal processing and data acquisition.On this basis,the parameters of the system are selected reasonably,and the optimal excita tion frequency is determined by the curve of the relationship between the stress sensitivity and the frequency obtained by the theory and the test.The attenuation law of the signal energy over the sound path is obtained by using the multiple reflection of the ultrasonic guided wave in the steel strand.At last,the control method of time-of-flight error of ultrasonic guided wave signal is proposed.Identification and verification of uniaxial stress based on first-order longitudinal mode of ultrasonic guided wave.The relationship between group velocity and uniaxial stress in different diameter steel bars is obtained by using the uniaxial stress identification system.On this basis,the loading test s and verification of different diameter steel strand are ca rried out to realize the identification of uniaxial stress of steel strand.Finally,the influence of the tensile deformation of the steel strand on the uniaxial stress identification is studied.