Research On Quantitative Detection Method Of Surface Cracks In Thick-walled Pipes Based On Electromagnetic Acoustic Surface Waves

Thick-walled pipe,which can withstand high temperature and high pressure,is the key component of the pipeline system in boiler and widely used in the thermal power generation.Production accidents caused by leakage or bursting of such pipes seriously threaten the safety of workers and state property.Therefore,it is very necessary to use nondestructive testing(NDT)technology to monitor the health of thick-walled pipes,and finally to prevent serious losses from pipe failure.However,the conventional NDT methods are inefficient and not appropriate for quantitative test,the general ultrasonic guided wave technologies are difficult to detect thick-walled pipes with large diameter.Surface wave,with advantages of far propagation distance,being sensitive to surface crack,and can propagate in thick specimen,is of great significance for the improving of flaw detection level in thick-walled pipes.Therefore,this thesis develops the surface wave detection method based on electromagnetic acoustic technique,so as to realize the rapid and accurate detection for the surface cracks in thick-walled pipes.In the development of high-performance surface wave electromagnetic acoustic transducers(EMATs),the relationship between the center frequency and acoustic field directivity of EMATs and the parameters of meander-line-coil is studied experimentally.The factors that affect the signal to noise ratio(SNR)of signals received by EMATs in pulse-echo mode are analyzed,and the variation curve of SNR with the space between meander-line-coil and magnet is obtained.The influences of the coil layers,wire splits,and the ratio of magnet height to width on the excitation performance of EMATs are researched by simulation.Moreover,by improving the assembling technology,the service life and practicability of EMATs are improved.Experiments reveal that,the developed surface wave EMATs possess high SNR and energy conversion efficiency.For the propagation characteristics of surface wave in specimen,surface waves in steel plate and pipes are simulated by the finite element method(FEM).It is found that the surface wave in steel plate is non-dispersive,and the simulation results of wave velocity and phase are almost identical to the measurements.The group velocity of surface wave in pipes is constant,but the phase velocity,attenuation characteristic and penetration depth are related to the curvature of pipe and the propagation direction of surface wave.Meanwhile,the scattering characteristics of surface waves at the rightangle edge were studied,the influence of chamfer size and rounding size on reflection and transmission coefficients is obtained.Aiming at defect detection for thick-walled pipes,the sensitivity of surface wave to different defects is studied experimentally,and a scanning imaging system based on automatic scanning robot is integrated.By optimizing the test frequency and scanning step,defects on the inner and outer walls of different pipes are detected.According to surface wave scattering characteristics at right-angle edge,a new detection method for inner wall circumferential cracks in special-shaped pipes with small diameter-depth ratio is proposed.In the method,EMATs are innovatively placed on the end face of pipe and along radial direction,and transmitted surface wave is used for detection.It shows that the propagation ability of transmitted wave satisfies the detection requirement,and the detection process is independent with the pipe curvature,which improves the defect detection level of the special-shaped pipes with small diameter-depth ratio.Finally,a slot depth quantitative detection method based on multi-frequency and multi-parameter surface wave detection is developed,and a simulated crack depth quantitative detection method based on arrival time difference between low-frequency transmitted wave and delay transmitted wave is proposed.FEM simulation is used to study the influence of slot depth and width on surface wave reflection and transmission coefficients,then the multi-frequency characterization curve is obtained.The effect of wire length in the coils on measurement accuracy of reflection/transmission coefficients is analyzed experimentally.Furthermore,the depth estimations of slots and simulated cracks is acquired.The results show that the reflection coefficient of the latter is small,and the error of estimations is large at the large depth crack.The relationship between arrival time difference of low-frequency transmitted wave and delay transmitted wave and crack depth is studied by decomposing and reconstructing the transmitted waves.Combining the reflection and transmission coefficients and the time difference between transmission wave components,the depth of simulated crack is detected quantitatively.The application of surface wave EMATs improves the measurement repeatability,and the multi-frequency detection expands the quantitative detection range of defect depth.In addition,the use of the time difference of two components in transmitted wave reduces the requirement of transducer installation accuracy.