Study Of Axial Stress Response Of Bending Modes In Pipe

Pipelines are widely used in various fields of industrial life,and the damage or defects caused by stress will affect the overall performance of the structure.In more serious cases,it will not only directly cause greater economic losses,but also may even pose a threat to human safety.If the stress can be detected in advance and relevant measures can be taken,this loss can be effectively avoided.Because of its long detection distance,sensitivity to stress and single point excitation,ultrasonic guided wave nondestructive testing has gradually become one of the nondestructive testing and monitoring technologies with great application potential.However,at present,the theoretical research and application of ultrasonic guided wave nondestructive testing technology for pipelines are mostly concentrated in axisymmetric modes.Nonaxisymmetric guided waves are less studied because of their numerous modes and obvious dispersion.The study of non-axisymmetric guided wave propagation characteristics can further improve the non-destructive testing and stress monitoring technology of pipelines.And there are few studies on the influence of pipeline structural stress on guided waves.Therefore,it is of great significance and prospect to use ultrasonic guided waves to monitor the stress of pipelines.When using ultrasonic guided waves to monitor stress,it is necessary to understand the characteristics and changes of guided waves under the stress of materials.Therefore,this paper studies the influence of axial stress on the propagation characteristics of guided waves(especially non-axisymmetric guided waves)in circular tubes.According to the acoustic elastic theory,the isotropic pipe subjected to axial stress or the transversely isotropic pipe with the intrinsic symmetry axis along the axial direction can be approximated as transversely isotropic.Firstly,this paper introduces five equivalent elastic modulus of stress-induced transversely isotropic media,and gives the expression.Then,the sound field of the pipeline subjected to axial stress is theoretically solved.By introducing the sound field potential function,the dispersion equations under different models are solved and calculated by using the boundary conditions of five models(empty pipeline in air,empty pipeline under water load,oilfilled pipeline in air,water-filled pipeline model with soil outside and intrinsic transverse isotropic medium pipeline model).In this paper,the Newton iteration method is used to solve the dispersion curve.The bending mode of the first three models,the water-filled pipeline model with soil outside,the dispersion curve of the guided wave of the intrinsic transversely isotropic pipeline and the phase velocity change curve under axial stress are numerically simulated.The full-wave waveform of the bending mode in the oil-filled pipeline model in the air is simulated by the real axis integration method.According to the phase velocity curve of guided wave under the action of stress,the phase velocity of the bending mode has a response in the whole frequency range under the action of tensile stress for the empty pipeline model in the air,and the phase velocity decreases.The greater the stress,the greater the change of the phase velocity,and there is a flat area sensitive to stress in the special frequency band for individual modes.The response of the bending mode of the empty pipe model under water load to the axial stress is basically the same as that of the empty pipe model in air.For the oil pipeline model in the air,under different stresses,the phase velocity changes of the fourth and subsequent high-order modes of the bending mode are basically zero at low and high frequencies,and the change is large at the flat area,and the phase velocity flat area corresponds to the group velocity peak area.It is found that the phase velocity of guided wave is very insensitive to stress,and there is no peak area of group velocity,so there is no flat area of phase velocity.In addition,in view of the anisotropy of actual pipelines,this paper studies and analyzes the axial stress response characteristics of guided waves in transversely isotropic aluminum and degraded isotropic aluminum pipelines with 9 independent third-order elastic modulus.It is found that the response characteristics of the guided wave to the axial stress are basically similar to those of the steel pipe,and the phase velocity change is more obvious than that of the steel pipe.Under the same stress,the change of phase velocity is obviously larger than that of isotropic aluminum tube.Therefore,in the actual stress detection,the intrinsic anisotropy in the pipeline may not be ignored.The results of the full wave waveform simulation show that under the oil-filled pipeline model in the air,the arrival time of the first wave increases after the tensile stress is applied,which indicates that the tensile stress causes the velocity of the first wave to decrease,but the multi-order mode of the mode wave is mixed together,the composition is very complex,and the arrival time change is not easy to distinguish.Based on the analysis of the numerical results of the sound field in the pipe structure under stress,it further provides a theoretical basis for the stress monitoring technology of the circular pipe structure using ultrasonic guided waves.