Detection of internal crack growth in polyethylene pipe using guided wave ultrasonic testing

Despite the success of guided wave ultrasonic inspection for internal defect detection in steel pipes,its application on polyethylene(PE)pipe remains relatively unexplored.The growth of internal cracks in PE pipe severely affects its pressure-holding capacity,hence the early detection of internal cracks is crucial for effective pipeline maintenance strategies.This study extends the scope of guided wave-based ultrasonic testing to detect the growth of internal cracks in a natural gas distribution PE pipe.Laboratory experiments and a finite element model were planned to study the wave-crack interaction at different stages of axially oriented internal crack growth with a piezoceramic transducer-based setup arranged in a pitch-catch configuration.Mode dispersion analysis supplemented with preliminary experiments was performed to isolate the optimal inspection frequency,leading to the selection of the T(0,1)mode at 50-kHz for the investigation.A transmission index based on the energy of the T(0,1)mode was developed to trace the extent of simulated crack growth.The findings revealed an inverse linear correlation between the transmission index and the crack depth for crack growth beyond 20%crack depth.

Propagation Characteristics of Ultrasonic Guided Waves in Grouted Rockbolt Systems with Bond Defects under Different Confining Conditions

A rockbolt acting in the rock mass is subjected to the combined action of the pull-out load and confining pressure, and the bond quality of the rockbolt directly affects the stability of the roadway and cavern. Therefore, in this study, confining pressure and pull-out load are applied to grouted rockbolt systems with bond defects by a numerical simulation method, and the rockbolt is detected by ultrasonic guided waves to study the propagation law of ultrasonic guided waves in defective rockbolt systems and the bond quality of rockbolts under the combined action of pull-out load and confining pressure. The numerical simulation results show that the length and location of bond defects can be detected by ultrasonic guided waves under the combined action of pull-out load and confining pressure. Under no pull-out load, with increasing confining pressure, the low-frequency part of the guided wave frequency in the rockbolt increases, the high-frequency part decreases, the weakening effect of the confining pressure on the guided wave propagation law increases, and the bond quality of the rockbolt increases. The existence of defects cannot change the strengthening effect of the confining pressure on the guided wave propagation law under the same pull-out load or the weakening effect of the pull-out load on the guided wave propagation law under the same confining pressure.

The Inspection of Level Crossing Rails Using Guided Waves

Level crossing rails are high risk areas due to the combination of the limited effectiveness of current inspection methods and high corrosion rates which often exist.This paper discusses the current UK standard practices for the periodic inspection of level crossing rails using visual(VT)and conventional ultrasonic(UT)methods.The limitations of these methods are discussed and how these limitations affect the overall maintenance program for level crossings.A new inspection method,guided wave testing(GWT)is then described with particular emphasis on its advantages for inspecting level crossings.Finally,a review is given of the current Network Rail trial of GWT on level crossings using the G-Scan system,with representative results which demonstrate the effectiveness of GWT for this application.

Temperature Dependence of Ultrasonic Longitudinal Guided Wave Propagation in Long Range Steel Strands

Ultrasonic guided wave inspection is an effective non-destructive testing method which can be used for stress level evaluation in steel strands.Unfortunately the propagation velocity of ultrasonic guided waves changes due to temperature shift making the prestress measurement of steel strands inaccurate and even sometimes impossible.In the course of solving the problem,this paper reports on quantitative research on the temperature dependence of ultrasonic longitudinal guided wave propagation in long range steel strands.In order to achieve the generation and reception of a chosen longitudinal mode in a steel strand with a helical shaped surface,a new type of magnetostrictive transducer was developed,characterized by a group of thin clips and three identical permanent magnets.Excitation and reception of ultrasonic guided waves in a steel strand were performed experimentally.Experimental results shows that in the temperature range from-4 ℃ to 34 ℃,the propagation velocity of the L（0,1） mode at 160 kHz linearly decreased with increasing temperature and its temperature dependent coefficient was 0.90（m·s-1 ·（℃）-1） which is very close to the theoretical prediction.The effect of dimension deviation between the helical and center wires and the effect of the thermal expansion of the steel strand on ultrasonic longitudinal guided wave propagation were also analyzed.It was found that these effects could be ignored compared with the change in the material mechanical properties of the steel strands due to temperature shift.It was also observed that the longitudinal guided wave mode was somewhat more sensitive to temperature changes compared with conventional ultrasonic waves theoretically.Therefore,it is considered that the temperature effect on ultrasonic longitudinal guided wave propagation in order to improve the accuracy of stress measurement in prestressed steel strands.Quantitative research on the temperature dependence of ultrasonic guided wave propagation in steel strands provides an important basis for the compensation of temperature effects in stress measurement in steel strands by using ultrasonic guided wave inspection.

High Precision Ultrasonic Guided Wave Technique for Inspection of Power Transmission Line

Due to the merits of high inspection speed and long detecting distance, Ultrasonic Guided Wave（UGW） method has been commonly applied to the on-line maintenance of power transmission line. However, the guided wave propagation in this structure is very complicated, leading to the unfavorable defect localization accuracy. Aiming at this situation, a high precision UGW technique for inspection of local surface defect in power transmission line is proposed. The technique is realized by adopting a novel segmental piezoelectric ring transducer and transducer mounting scheme, combining with the comprehensive characterization of wave propagation and circumferential defect positioning with multiple piezoelectric elements. Firstly, the propagation path of guided waves in the multi-wires of transmission line under the proposed technique condition is investigated experimentally. Next, the wave velocities are calculated by dispersion curves and experiment test respectively, and from comparing of the two results, the guided wave mode propagated in transmission line is confirmed to be F（1,1） mode. Finally, the axial and circumferential positioning of local defective wires in transmission line are both achieved, by using multiple piezoelectric elements to surround the stands and send elastic waves into every single wire. The proposed research can play a role of guiding the development of highly effective UGW method and detecting system for multi-wire transmission line.

Crack Detection in Pipes with Different Bend Angles Based on Ultrasonic Guided Wave

Pipeline plays an indispensable role in process industries,because the progressing crack-like defects of in it may result in serious accidents and significant economic losses.Therefore,it is essential to detect the cracks occurred in pipelines.The axial crack-like defects in elbows with different angle are inspected by using the T(0,1)mode guided waves,in which different configurations including 45°,90°,135°and 180°(straight pipe)are considered respectively.Firstly,the detection sensitivity for different defect location is experimentally investigated.After that,finite element simulation is used to explore the propagation behaviors of T(0,1)mode in different bend structures.Simulation and experiment results show that the crack in different areas of the elbow can affect the detection sensitivity.It can be found that the detection sensitivity of crack in the middle area of the elbow is higher compared to the extrados and intrados of the elbow.Finally,the mode conversion is also investigated when the T(0,1)crosses the bend,and the results show that bend is a key factor to the mode conversion phenomenon which presents between the T(0,1)mode and F(1,2)mode.

Numerical Perspective of Second-Harmonic Generation of Circumferential Guided Wave Propagation in a Circular Tube

The effect of second-harmonic generation （SHG） by primary （fundamental） circumferential guided wave （CGW） propagation is investigated from a numerical standpoint. To enable that the second harmonic of the primary CGW mode can accumulate along the circumferential direction, an appropriate mode pair of primary and double frequency CGWs is chosen. Finite element simulations and evaluations of nonlinear CGW propagation are analyzed for the selected CGW mode pair. The numerical simulations performed directly demonstrate that the response of SHG is completely generated by the desired primary CGW mode that satisfies the condition of phase velocity matching at a specific driving frequency, and that the second harmonic of the primary CGW mode does have a cumulative effect with circumferential angles. The numerical perspective obtained yields an insight into the complicated physical process of SHG of primary CGW propagation unavailable previously.

GUIDED CIRCUMFERENTIAL WAVES IN DOUBLE-WALLED CARBON NANOTUBES

A model of guided circumferential waves propagating in double-walled carbon nanotubes is built by the theory of wave propagation in continuum mechanics, while the van der Waals force between the inner and outer nanotube has been taken into account in the model. The dispersion curves of the guided circumferential wave propagation are studied, and some dispersion characteristics are illustrated by comparing with those of single-walled carbon nanotubes. It is found that in double-walled carbon nanotubes, the guided circumferential waves will propagate in more dispersive ways. More interactions between neighboring wave modes may take place. In particular, it has been found that a couple of wave modes may disappear at a certain frequency and that, while a couple of wave modes disappear, another new couple of wave modes are excited at the same wave number.

Experimental Observation of Cumulative Second-Harmonic Generation of Circumferential Guided Wave Propagation in a Circular Tube

The experimental observation of cumulative second-harmonic generation of the primary circumferential guided wave propagation is reported. A pair of wedge transducers is used to generate the primary circumferential guided wave desired and to detect its fundamental-frequency and second-harmonic amplitudes on the outside surface of the circular tube. The amplitudes of the fundamental waves and the second harmonics of the circumferential guided wave propagation are measured for different separations between the two wedge transducers. At the driving frequency where the primary and the double-frequency circumferential guided waves have the same linear phase velocities, the clear second-harmonic signals can be observed. The quantitative relationships between the second-harmonic amplitudes and circumferential angle are analyzed. It is experimentally verified that the second harmonics of primary circumferential guided waves do have a cumulative growth effect with the circumferential angle.

An analytical approach to reconstruction of axisymmetric defects in pipelines using T(0,1)guided waves

Torsional guided waves have been widely utilized to inspect the surface corrosion in pipelines due to their simple displacement behaviors and the ability of longrange transmission.Especially,the torsional mode T(0,1),which is the first order of torsional guided waves,plays the irreplaceable position and role,mainly because of its non-dispersion characteristic property.However,one of the most pressing challenges faced in modern quality inspection is to detect the surface defects in pipelines with a high level of accuracy.Taking into account this situation,a quantitative reconstruction method using the torsional guided wave T(0,1)is proposed in this paper.The methodology for defect reconstruction consists of three steps.First,the reflection coefficients of the guided wave T(0,1)scattered by different sizes of axisymmetric defects are calculated using the developed hybrid finite element method(HFEM).Then,applying the boundary integral equation(BIE)and Born approximation,the Fourier transform of the surface defect profile can be analytically derived as the correlative product of reflection coefficients of the torsional guided wave T(0,1)and the fundamental solution of the intact pipeline in the frequency domain.Finally,reconstruction of defects is precisely performed by the inverse Fourier transform of the product in the frequency domain.Numerical experiments show that the proposed approach is suitable for the detection of surface defects with arbitrary shapes.Meanwhile,the effects of the depth and width of surface defects on the accuracy of defect reconstruction are investigated.It is noted that the reconstructive error is less than 10%,providing that the defect depth is no more than one half of the pipe thickness.

Guided Wave Based Damage Detection Method for Aircraft Composite Structures under Varying Temperatures

Guided waves based damage detection methods using base signals offer the advantages of simplicity of signal generation and reception,sensitivity to damage,and large area coverage;however,applications of the technology are limited by the sensitivity to environmental temperature variations.In this paper,a Spearman Damage Index-based damage diagnosis method for structural health condition monitoring under varying temperatures is presented.First,a PZT sensor-based Guided wave propagation model is proposed and employed to analyze the temperature effect.The result of the analysis shows the wave speed of the Guided wave signal has higher temperature sensitivity than the signal fluctuation features.Then,a Spearman rank correlation coefficient-based damage index is presented to identify damage of the structure under varying temperatures.Finally,a damage detection test on a composite plate is conducted to verify the effectiveness of the Spearman Damage Index-based damage diagnosis method.Experimental results show that the proposed damage diagnosis method is capable of detecting the existence of the damage and identify its location under varying temperatures.

Borehole guided waves in a non-Newtonian(Maxwell) fluid-saturated porous medium

The property of acoustic guided waves generated in a fluid-filled borehole surrounded by a non-Newtonian （Maxwell） fluid-saturated porous formation with a permeable wall is investigated. The influence of non-Newtonian effects on acoustic guided waves such as Stoneley waves, pseudo-Rayleigh waves, flexural waves, and screw waves propagations in a fluid-filled borehole is demonstrated based on the generalized Biot-Tsiklauri model by calculating their velocity dispersion and attenuation coefficients. The corresponding acoustic waveforms illustrate their properties in time domain. The results are also compared with those based on generalized Biot＇s theory. The results show that the influence of non-Newtonian effect on acoustic guided wave, especially on the attenuation coefficient of guided wave propagation in borehole is noticeable.

Nonlinear inversion of ultrasonic guided waves for in vivo evaluation of cortical bone properties

Ultrasonic guided waves(UGWs),which propagate throughout the entire thickness of cortical bone,are attractive for the early diagnosis of osteoporosis.However,this is challenging due to the impact of soft tissue and the inherent difficulties related to multiparametric inversion of cortical bone quality factors,such as cortical thickness and bulk wave velocity.Therefore,in this research,a UGW-based multi-parameter inversion algorithm is developed to predict strength-related factors.In simulation,a free plate(cortical bone)and a bilayer plate(soft tissue and cortical bone)are used to validate the proposed method.The inversed cortical thickness(CTh),longitudinal velocity(V_(L))and transverse velocity(V_(T))are in accordance with the true values.Then four bovine cortical bone plates were used in in vitro experiments.Compared with the reference values,the relative errors for cortical thickness were 3.96%,0.83%,2.87%,and 4.25%,respectively.In the in vivo measurements,UGWs are collected from the tibias of 10 volunteers.The theoretical dispersion curves depicted by the estimated parameters(V_(T),V_(L),CTh)match well with the extracted experimental ones.In comparison with dual-energy x-ray absorptiometry,our results show that the estimated transverse velocity and cortical thickness are highly sensitive to osteoporosis.Therefore,these two parameters(CTh and V_(T))of long bones have potential to be used for diagnosis of bone status in clinical applications.

Single SH guided wave mode generation method for PPM EMATs

Using periodic permanent magnet(PPM)electromagnetic acoustic transducers(EMATs),different shear horizontal(SH)guided wave modes can form simultaneously in some situations,which can interfere with the inspection.The main cause of this phenomenon(typically named multiple modes)is related to the frequency bandwidth of excitation signals and the transducer spatial bandwidth.Simply narrowing the frequency bandwidth cannot effectively limit the number of different SH modes.Previous researches showed that unnecessary SH wave modes can be eliminated by using dual EMATs.However,in practical applications,it is more convenient to change the excitation frequency than to use dual EMATs.In this paper,the stress boundary conditions of the PPM-EMAT are analyzed,the analytical expression of SH guided wave is established,and the magnitude of SH guided wave mode under continuous tone and tone-burst input is obtained.A method to generate a single SH mode by re-selecting an operating point is proposed.Furthermore,the influence of the frequency bandwidth of the tone-burst signal is analyzed.Finally,a single SH mode excitation is achieved with tone-burst input.

Three-Dimensional Scattering of an Incident Plane Shear Horizontal Guided Wave by a Partly through-Thickness Hole in a Plate

We investigate the three-dimensional （3D） scattering problem of an incident plane shear horizontal wave by a partly through-thickness hole in an isotropic plate, in which the Lamb wave modes are also included due to the mode conversions by the scattering obstacle in the 3D problem. An analytical model is presented such that the wave fields are expanded in all of propagating and evanescent SH modes and Lamb modes, and the scattered far-fields of three fundamental guided wave modes are analyzed numerically for different sizes of the holes and frequencies. The numerical results are verified by comparing with those obtained by using the approximate Poisson/Mindlin plate model for small hole radius and low frequency. It is also found that the scattering patterns are different from those of the SO wave incidence. Our work is useful for quantitative evaluation of the plate-like structure by ultrasonic guided waves.

Characterization of inner layer thickness change of a composite circular tube using nonlinear circumferential guided wave:A feasibility study

The feasibility of using the nonlinear effect of primary circumferential guided wave(CGW)propagation for characterizing the change of inner layer thickness of a composite circular tube(CCT)has been investigated.An appropriate mode pair of the fundamental and double-frequency CGWs(DFCGWs)has been selected to enable the second harmonics of primary wave mode in the given CCT to accumulate along the circumferential direction.When changes in the inner layer thickness(described as the equivalent inner layer thickness)take place,the corresponding nonlinear CGW measurements are conducted.It is found that there is a direct correlation between change of equivalent inner layer thickness of the CCT and the relative acoustic nonlinearity parameter(Δβ)measured with CGWs propagating through one full circumference,and that the effect of second-harmonic generation(SHG)is very sensitive to change in the inner layer thickness.The experimental result obtained demonstrates the feasibility for quantitatively assessing the change of equivalent inner layer thickness in CCTs using the effect of SHG by primary CGW propagation.

Multi-Mode Guided Waves Based Reference-Free Damage Diagnostic Imaging in Plates

Probability-based diagnostic imaging(PDI)is one of the most well-known damage identification methods using guided waves.It is usually applied to diagnose damage in plates.The previous studies were dependent on the certain damage index(DI)which is always calculated from the guided wave signals.In conventional methods,DI is simply defined by comparing the real-time data with the baseline data as reference.However,the baseline signal is easily affected by varying environmental conditions of structures.In this paper,a reference-free diagnostic imaging method is developed to avoid the influence of environmental factors,such as temperature and load conditions.The DI is defined based on the mode conversion of multi-mode guided waves with realtime signals without baseline signals.To improve the accuracy of diagnosis,two terms are included in the reference-free DI.One is called energy DI,which is defined based on the feature of signal energy.The other is called correlation DI and is defined based on the correlation coefficient.Then the PDI algorithm can be carried out instantaneously according to the reference-free DI.The real-time signals which are used to calculate DI are collected by the piezoelectric lead zirconate titanate(PZT)transducers placed on both sides of a plate.The numerical simulations by the finite element(FE)method on aluminum plates with PZT arrays are performed to validate the effectiveness of the reference-free damage diagnostic imaging.The approach is validated by two different arrays:a circle network and a square network.The results of diagnostic imaging are demonstrated and discussed in this paper.Furthermore,the advantage of reference-free DI is investigated by comparing the accuracy of defined reference-free DI and energy DI.

Confined aquifer as wave-guide and its responses to geo-acoustic waves

On the basis of the hydro geological model of a confined aquifer, the propagation mechanism of geo acoustic waves along the confined aquifer outlined as a plate wave guide is proposed. The harmonic frequency equation for geo acoustic propagation along confined aquifer as waveguide is derived from Biot theory. The basic frequency of the confined aquifer with a deep well for geo acoustic observation, located at Juxian county, Shandong province, China, is 35.0 Hz. By Wigner distribution of geo acoustic signals observed at Juxian geo acoustic well, the frequencies of geo acoustics are basically the integral multiple of the basic frequency. The results show that the responses of the confined aquifer to geo acoustic waves are characterized by frequency selection and frequency dependence. Only the waves whose frequency f is the integral multiple of basic frequency can propagate as guide waves in the aquifer, that is , the aquifer responds to the waves.

A New Guide Wave Inspection System Using Three Polarized Transverse Wave EMATs without Any Couplant

A guide wave is provided with the characteristic of long range propagation in the axis direction of a pipe, so it is possible to detect many defects over a large pipe area at once. At present, there is a technique to generate a guide wave using a piezoelectric element (PZT). However, the transverse wave-transducer using PZT needs to require a high viscosity couplant because the transverse wave cannot travel into typical liquid like water or oil. A guide wave inspection system that uses an electromagnetic ultrasonic transducer (EMAT) which does not require any couplant has then been developed to solve this trouble. First, a guide wave into a pipe, L, T and F-mode, can be transmitted and received by a polarized shear horizontal transverse wave propagating to the thickness direction when the vibration direction has been adjusted to the best direction. At next stage, the three EMATs for L, T and F-mode with different polarized vibration directions were piled up to improve the performance at the same position under the permanent magnet to inspect the pipe at the same condition. Next, the system with the EMATs can be confirmed to be able to detect three guide wave modes signal with enough intensity. Finally, the detection performance using the test pipes with any artificial defects has been done by the developed pipe inspection system, and any drilled holes and any notches can be detected. It is indicated that the developed system could be useful in real industrial field.

High-Frequency Guided Wave Scattering by a Partly Through-Thickness Hole Based on 3D Theory

We present a theoretical investigation of the scattering of high frequency S0 Lamb mode from a circular blind hole defect in a plate based on the 3D theory. The SO wave is incident at the frequency above the A1 mode cut-off frequency, in which the popular approximate plate theories are inapplicable. Due to the non-symmetric blind hole defect, the scattered fields will contain higher order converted modes in addition to the fundamental SO and AO modes. The far-field scattering amplitudes of various propagating Lamb modes for different hole sizes are inspected. The results are compared with those of lower frequencies and some different phenomena are found. Two-dimensional Fourier transform （2DFT） results of transient scattered Lamb and SH wave signals agree well with the analytical dispersion curves, which check the validity of the solutions from another point of view.