Jing Mu

Guided wave propagation and mode differentiation in hollow cylinders with viscoelastic coatings

Guided wave propagation theories have been widely explored for about one century. Earlier theories on single-layer elastic hollow cylinders have been very beneficial for practical nondestructive testing on piping and tubing systems. Guided wave flexural (nonaxisymmetric) modes in cylinders can be generated by a partial source loading or any nonaxisymmetric discontinuity. They are especially important for guided wave mode control and defect analysis. Previous investigations on guided wave propagation in multilayered hollow cylindrical structures mostly concentrate on the axisymmetric wave mode characteristics. In this paper, the problem of guided wave propagation in free hollow cylinders with viscoelastic coatings is solved by a semianalytical finite element (SAFE) method. Guided wave dispersion curves and attenuation characteristics for both axisymmetric and flexural modes are presented. Due to the fact that dispersion curve modes obtained from SAFE calculations are difficult to differentiate from each other, a mode sorting method is established to distinguish modes by their orthogonality. Theoretical proof of the orthogonality between guided wave modes in a viscoelastic coated hollow cylinder is provided. Wave structures are also calculated and discussed in view of wave mechanics in multilayered cylindrical structures containing viscoelastic materials.

GUIDED WAVE PROPAGATION IN CURVED PLATE‐LIKE STRUCTURES

Ultrasonic guided waves have been demonstrated as a promising tool in nondestructive evaluation (NDE) and structural health monitoring (SHM), especially for plates and plate‐like structures. However, many plate‐like structures subject to NDE and SHM are curved. The curvature plays an important role in guided wave propagation. The objective of the work presented in this paper is to investigate the influence of curvature on guided wave propagation. A semi‐analytical finite element (SAFE) technique is employed to analyze the dispersion curves for plate‐like structures with different curvatures. The changing of the wave structures with curvature is investigated as well. Physical insight on the dispersion curve variations introduced by the plate curvature variation is obtained through wave structure analysis.

Defect circumferential sizing by using long range ultrasonic guided wave focusing techniques in pipe

Ultrasonic guided waves have the advantage of propagating over very long distances due to their natural ability of following differently shaped wave guides. This advantage makes guided waves a good candidate for long range non-destructive evaluation (NDE). The recently developed guided wave focusing techniques in pipes employ a circumferentially distributed phased array. By applying different amplitude and time-delay inputs to the phased array, guided wave energy can be successfully focused at any predetermined locations in pipes. Guided wave focusing techniques improve penetration power and circumferential resolution. As a result, excellent defect detection and accurate axial and circumferential location have been achieved. In addition, defect circumferential sizing in pipes is realized by doing circumferential scans at defect distances. A circumferential scan is accomplished by focusing at 44 angles around the circumference of a pipe at a specific axial distance. In a circumferential scan, the maximum amplitude of the defect echo was recorded and then plotted with respect to each focal angle. This produces an experimental angular profile of the reflected wave. The theoretical angular profiles of the reflected wave is obtained by calculating the focused guided wave energy impinged onto defects with different cross sectional areas (CSA). By comparing the theoretical and experimental profiles, the circumferential extent of the defect can be effectively measured.

PHASED-ARRAY FOCUSING WITH LONGITUDINAL GUIDED WAVES IN A VISCOELASTIC COATED HOLLOW CYLINDER

Guided wave phased‐array focusing techniques have been studied and applied in the long‐range guided wave inspection of industrial pipelines. Advantages include longer inspection distance, greater wave penetration power and higher detection sensitivity. For reasons of protection, safety and thermal efficiency, a large percentage of pipes are coated and/or encased and buried underground. A phased‐array focusing study for guided waves is now considered on pipelines with viscoelastic coatings. In this paper, longitudinal guided wave focusing as well as axisymmetric wave propagation is studied in a bare pipe and a pipe with a viscoelastic coating. A finite element model is studied. First, an investigation on whether the coating has an affect on the axisymmetric guided wave propagation is reported. Based on the result of a single channel, phased array focusing with 8‐channel segments is studied. This study provides a very useful tool and guidance for the analysis and examination of guided wave focusing in a rea...

DEFECT SIZING IN PIPE USING AN ULTRASONIC GUIDED WAVE FOCUSING TECHNIQUE

Defect circumferential location, circumferential length and depth are studied with an ultrasonic guided wave focusing inspection technique. Differently shaped defects, such as a planar saw cut, a volumetric through‐wall hole, and a volumetric spherical shape corrosion are studied. By focusing on 44 circumferential positions around the pipe at a specific distance, the maximum amplitude of the defect echo is recorded with respect to each circumferential focal position. Circumferential lengths of the planar saw cut and volumetric through‐wall hole are then measured by comparing the experimental results with theoretical calculations. It is shown that this measurement technique works well with the planar saw cut and volumetric through‐wall hole defects. In addition, it is shown that reflections from defects with the same cross sectional area (CSA), but different shapes, might be very different.

Long Range Ultrasonic Guided Wave Focusing in Pipe Using a Phased‐Array System

Guided wave focusing in pipe is discussed from three aspects: theoretical calculations, numerical FEM simulations, and experimental verifications. The guided wave focusing techniques are used to provide us with higher wave penetration power and defect circumferential resolution. An ultrasonic multi‐channel phased‐array system is employed to achieve guided wave focusing at a predetermined position in a pipe. When appropriate amplitudes and time‐delays are applied to different channels of the phased array system, a constructive interference can be realized at the pre‐determined focal spot. FEM simulations are performed to verify the theories and visualize the focusing phenomenon. The increase in penetration power is verified by experiments.

GUIDED WAVE NORMAL MODES IN HOLLOW CYLINDERS WITH VISCOELASTIC COATINGS

Guided wave propagation in free hollow cylinders with viscoelastic coatings is solved by a semi‐analytical finite element (SAFE) method. Guided wave dispersion curves and attenuation curves including both axisymmetric and flexural modes are obtained. The dispersion and attenuation curves agree quite well with analytical solutions. It is shown that the guided wave modes in a viscoelastic coated hollow cylinder are orthogonal to each other. This orthogonality is derived analytically and implemented into the numerical calculations to realize mode sorting. The mode sorting result also serves as verification to the orthogonality relation. Some characteristics of the dispersion curves and their corresponding practical inspection values of such a study are discussed.

Long Range Ultrasonic Guided Wave Focusing in Pipe for Large Defect Circumferential Length Analysis

Planar saw cuts and volumetric elliptical corrosion with different circumferential lengths are studied using ultrasonic guided wave focusing techniques in steel pipe. A circumferential scan is achieved by focusing at 44 circumferential positions around the pipe at a specific distance. The maximum amplitude of the defect echo is recorded with respect to each circumferential focal position in producing an experimental angular profile of the reflected wave. The maximum amplitude of the angular profiles occurs at the angle of the defect circumferential location. The energy reflected from the defects with different cross sectional areas (CSA) for each focusing position can be calculated from the focused angular profile. Comparison between the experimental angular profiles and the theoretical ones provides us with a good estimate of the defect circumferential length.

Long Range Ultrasonic Guided Wave Focusing in Pipe for Small Defect Detection, Location and Circumferential Length Analysis

Ultrasonic guided wave focusing techniques in steel pipe are used to detect and measure the axial as well as circumferential locations of three differently shaped defects (planar saw cuts, a volumetric through-wall hole, and a volumetric elliptical corrosion). The axial location of a defect was obtained by measuring the arrival time of the defect echo. Defect circumferential location was determined by focusing at 44 circumferential positions around the pipe at a specific distance. The maximum amplitude of the defect echo was recorded with respect to each circumferential focal position in producing an angular profile of the reflected wave. The maximum amplitude of this experimental angular profile occurs at the angle of the defect circumferential location. The energy impingement onto the defect for each focusing position can be calculated from the focused angular profile and the defect cross sectional area. Comparison between the experimental angular profiles and the theoretical ones provides us with a good estimate of the defect circumferential size.Copyright

Long Range Guided Wave Natural Focusing Pipe Inspection

Long range ultrasonic guided wave nondestructive evaluation can be used to inspect pipelines over fairly long distances. Partial loading of transducers around the circumference leads to a non-axisymmetric energy distribution. At particular axial distances and frequencies, the ultrasonic energy is naturally focused at some spots around the circumference via constructive wave interference. This so called “natural focusing” phenomenon can be used to improve guided wave sensitivity for a defect since more energy is sent to the defect. However, defects located in other places could possibly be missed, unless we can move the natural focusing points throughout the pipe. We have done this by frequency and circumferential angle tuning for specific circumferential loading lengths. In order to utilize the natural focusing phenomenon to enhance detection sensitivity, a frequency and angle tuning (FAT) technique is employed to extend the area that is scanned by focal energy. It is observed that the natural focal points at a fixed axial distance move with frequency variation and circumferential excitation length change. In this paper, the natural focusing phenomenon with FAT is theoretically calculated and experimentally investigated. The results show that the natural focusing inspection technique can sufficiently inspect an entire pipe with FAT. Some sample inspection data is compared by applying axisymmetric excitations, FAT, and phased array focusing.Copyright