Morimasa Murase

Defect imaging with guided waves in a pipe.

Guided wave techniques are expected to become an effective means for rapid, long-range inspection of pipes. Such techniques still have many practical difficulties in application, however, due to the complex characteristics of guided waves such as dispersion and their multimodal nature. A defect imaging technique is developed in this study to overcome the complexities of guided wave inspection. Received signals are separated into single-mode waveforms with a mode extraction technique and then spatial waveforms on the pipe surface at an arbitrary time are reconstructed. The predicted waveforms can provide a defect image at the moment when an incident wave arrives at a defect region, which is based on a time-reversal technique. This defect imaging technique is experimentally verified using eight signals detected at eight different circumferential positions. Images of artificial defects are obtained with one-hole and two-hole test pipes, and increasing the frequency of incident waves increases the resolution of the images. Holes and pipe ends are recognizable in the images, but the reconstructed images contain some errors in the area behind the defects where guided waves do not propagate or do not reflect back to the receiving transducers.

Rapid thickness measurements using guided waves from a scanning laser source.

Guided waves have been effectively used for rapid inspections of plates and pipes. However, the guided-wave technique is not generally used for measuring the remaining thickness in a plate and a pipe due to the difficulties in guided-wave motion. Instead, time-consuming and costly direct contact thickness measurements are still used in practice. This study describes a thickness measurement technique using the A0 mode of a Lamb wave generated by a laser source. A finite element analysis of Lamb wave revealed that this mode propagates with small reflections and mode conversions at a rounded shallow defect and has larger amplitude at thinner regions. Using these characteristics, it is experimentally demonstrated that the distributions of plate thickness were obtained from the amplitude of A0 mode generated by a scanning laser source and received by an angle-beam transducer. The resulting distribution images were obtained at extremely high speed compared to the conventional thickness measurements.

Visualization and Modal Analysis of Guided Waves from a Defect in a Pipe

Many commercial guided wave pipe inspection techniques use an axisymmetric torsional mode T(0,1) as well as a longitudinally vibrating L(0,2) mode as incident waves since the torsional mode has prominent characteristics of low attenuation and nondispersion. However, reflected waves from defects in a pipe contain many modes other than the incident torsional mode, and the complex reflected wave propagation sometimes prevents us from locating defects in the field. Therefore, we carried out a visualization of guided wave propagations around the full circumference of pipes to explain the behavior of reflected waves from defects that occur in pipe inspections. In this study, a large number of RF signal measurements at many locations on the curved surfaces of pipes were performed using a laser Doppler vibrometer (LDV) and a 6-axis robot arm that scanned over the curved surfaces of two pipes with circumferential and oblique defects. The visualizations of guided wave propagations from defects are shown for both in- and out-of-plane vibrations to analyze the mode conversions that occur in pipes around these defects when using the axisymmetric torsional mode T(0,1) as an incident wave in pipe inspections.

Application of Nonlinear Ultrasonic Measurement for Quality Assurance of Diffusion Bonds of Gamma Titanium Aluminum Alloy and Steel

A new nonlinear C-scan acoustic microscope (NCAM) has been developed and applied to inspect diffusion bonds of gamma titanium aluminum alloy–chromium molybdenum steel (γ TiAl–steel). The large mismatch of acoustic impedance gives strong ultrasonic reflection and masks a small signal from imperfections at the interface. However, NCAM detects indications of minute cracks or gaps, which are rarely observed by a conventional pulse–echo inspection method. Application of high-power ultrasound, with amplitude ten times greater than the conventional method, results in generation of second harmonic signals. All A-scope signals were stored in a computer, and frequency components were analyzed by fast Fourier transformation (FFT). As a result, no indication of fundamental frequency component was observed on a C-scan image, whereas clear indications of the second harmonic components were observed on the C-scan image. The minute cracks or gaps were confirmed at the indicated area by micrographic analysis following the inspection. Moreover, a quality assurance process for the γ TiAl–steel bond with a combination of linear and nonlinear ultrasonic responses is proposed.

ANALYSIS OF GUIDED WAVE PROPAGATION BY VISUALIZING IN‐PLANE AND OUT‐OF‐PLANE MODES

Guided wave techniques in nondestructive evaluation provide practical application for a long range of pipe inspection. On the contrary, the phenomenon of mode conversions around defects are not been explained experimentally, and visualization of guided wave propagation is significantly useful to describe behavior of mode conversions around defects from the measured RF signals in a pipe. This paper presents visualization of guided waves around defects in a pipe using many waveforms detected by Laser Doppler Vibrometer (LDV) scanning equipped with a robot arm. These waveforms were detected at many points on a surface of a pipe from two different directions. As result of the measurements, incident wave and reflected wave of the in‐plane and out‐of‐plane vibrations can be separated in the visualization results. The images clearly visualize the reflected waves from a circumferential notch and an oblique stretching notch that strongly indicate the existence of mode conversions around defects.

Group Velocity Measurement of Various Lamb Waves with Two-Beam Laser Interferometer

Multimode Lamb waves in a 1.0-mm-thick aluminum plate were excited by a line-focused Q-switched neodymium yttrium aluminum garnet (Nd:YAG) laser and the waves were received at two different positions by a laser interferometer. Using the phase spectrum method and moving average, we measured the dispersion curves of the group velocities of various modes of the Lamb waves and compared the measured results with the theoretical dispersion curves. The measured dispersion curves of the modes (S0, S1, A1, A2, and A3) of the Lamb waves agree well with the theoretical dispersion curves in the velocity range of 2000–4000 m/s and frequency x thickness range of 0.2–8.5 MHz mm. This method may be effective for material characterization of thin plates and pipes.

Development of Calculation Software for Guided Wave Propagation In a Pipe

Pipe screening technique with guided waves has become widely applied to pipe inspection in actual industrial plants. In most cases, however, guided wave propagating in a pipe is still difficult for inspection technicians due to its complex characteristics. Authors have developed the fast calculation technique for guided wave propagation using a semi‐analytical finite element method (SAFEM). This study is on development of software for guided wave propagation with the SAFE calculation. A parameter input interface was made with Visual C++, and a core execution file was coded with Fortran 90. Visualization of data set obtained by the calculation was carried out with a free visualization tool OPEN DX. Using this software including a preprocessor and a postprocessor, many kinds of guided wave simulation have become feasible even by a non‐expert in guided waves.

LASER BASED MEASUREMENT OF PLATE THICKNESS USING A FUNDAMENTAL ANTISYMMETRIC MODE OF LAMB WAVE

A guided wave technique for measurement of remaining thickness distribution is presented for the purpose of maintenance of large structures like oil storage tanks and pipe networks. This study used a laser source to excite the guided wave at many locations on aluminum plates with defect that have high ratio of A0 mode transmission and small mode conversions. The A0 mode was used to interrogate the through‐thickness over the rastered areas in the test plates. It was expected that many modes of Lamb wave were excited in intact and defect regions of the plates through the thermoelastic stresses from the laser source. The fundamental antisymmetric mode A0 was extracted to indicate the thickness distribution in aluminum test plates with the defect. The results exhibit greater amplitude distributions of A0 mode over thinner regions around defects and smaller amplitudes distributions over the intact regions.

Simultaneous measurement of group velocities of various mode’s Lamb wave

Multimode’s Lamb waves in stainless steel plates of 0.490 and 5.042 mm thickness were generated by 2.25 or 5 MHz broadband transducers with three plastic wedges of different angles. With the time domain signals received at two different positions, the group velocities of various modes (S0-S6,A1-A7) were calculated by using the phase spectrum. The measured dispersion curves of the group velocity agree well with a certain part of each theoretical curves in velocity range of 2000–4000 m/s and in frequency x thickness range of 2.5–22.5 MHz mm.

Mode Extraction and Defect Detection Using a Multi‐Channel Array with Magnetostrictive Transducer and Mode Extraction

This study presents that mode extraction and defect detection using a multi‐channel array with magnetostrictive transducers. In order to detect multi‐modes guided waves and extract them, we use transducer array with many transducers in the circumferential direction. The signals detected in the multi transducers are processed in data analyzing system developed by LabVIEW. The mode extraction technique gives the results that crack locations in both longitudinal and circumferential directions are detected.