Tension Measurement Of Steel Strand Based On Nonlinear Guided Wave

Seven-wire strand is a critical part of prestressed concrete bridge structure.However,the tension of the strand will inevitably decrease with time.The loss of prestress in bridge will lead to mid-span bending,web cracking and other diseases,which will endanger the public safety of traffic.The theory of contact acoustic nonlinearity can establish a mathematical relationship between the ultrasonic characteristics and the mechanical state of the structural interface,which is potential to be used in the tension measurement of steel strand.Theoretical analysis,finite element simulation and experiments are combined to study the characteristics of nonlinear ultrasonic guided waves in steel strand under different stress levels.A method of tension evaluation based on the second harmonic and the sideband in guided waves was established,which provides the theoretical basis and technical support for the nondestructive testing of the stress in strand.The following contents are mainly studied.The model of contact acoustic nonlinearity for interwire surface in tensioned strand was established.Considering the nonlinear stress-strain conditions,the governing equation of interwire surface was derived and the displacement of transmitted and reflected waves were obtained using perturbation analyses on the incident conditions for one wave and double waves.Combining the transfer relationship between the axial tension of steel strand,the contact stiffness at interwire surface and the nonlinear waves,the mapping formula from the parameters of nonlinearity to the tension of strand is proposed.The two kinds of nonlinear parameters decrease monotonically with the increase of tension in a power function law,which depends on the material properties of the wire interface and is independent of the incident amplitude.Compared with the second harmonic wave,the tension identification index based on the sideband has higher theoretical robustness under attenuation and instrument interference.The nonlinear ultrasonic phenomena and law in a steel strand were simulated.Through the comparison of guided waves for of single wire and parallel wires,it was proved that the interwire contact is the generating condition of nonlinear waves.The nonlinear ultrasonic propagation of tensioned steel strand was analyzed more carefully,the time-domain,frequency-domain and modal vibration laws of the nonlinear wave field of steel strand under different loading and incident conditions were analyzed.By extracting and fitting the curves of the nonlinear coefficient of the simulated signal under different tension levels,the validity of the theoretical model in the strand is verified.For multiple simulation conditions,the vibration mode of the mixing ultrasonic field is the most stable,in which the nonlinear coefficient of sideband has the highest sensitivity to the tension variation under high stress level.A method for measuring the tension of steel strand using mixing ultrasonic guided waves was proposed.Based on the simulation results of nonlinear wave of steel strand,the ultrasonic study of tensioned steel strand is carried out.Based on the simulation experience,the ultrasonic study of tensioned steel strand is carried out and the measurement scheme suitable for the actual conditions is proposed.The curves of nonlinear parameters decreasing monotonically with tension was obtained using the measured second harmonics and sideband harmonics.These two kinds of coefficients increase with the increase of the conduction distance and do not vary with the incident power when the power is large enough.The concrete has significant influences on nonlinear wave field in the embedded strand.In comparison,the nonlinear parameter of sidebands varies more regularly with the tension,and is less affected by the incident amplitude and conduction attenuation.