Ultrasonic guided wave inspection is expanding rapidly to many different areas of manufacturing and in-service inspection. The purpose of this paper is to provide a vision of ultrasonic guided wave inspection potential as we move forward into the new millennium. An increased understanding of the basic physics and wave mechanics associated with guided wave inspection has led to an increase in practical nondestructive evaluation and inspection problems. Some fundamental concepts and a number of different applications that are currently being considered will be presented in the paper along with a brief description of the sensor and software technology that will make ultrasonic guided wave inspection commonplace in the next century.

Ultrasonic guided waves, due to their capability of interrogating a large structure from a single sensor position, have been proven as a promising tool for structural health monitoring. In this article, we present two imaging approaches of utilizing guided wave leave-in-place sensors for early detection of defects in plate-like structures. The first approach is based on a guided wave tomographic technique, in which the region surrounded by a sparse sensor array is monitored. The second one is a phased array approach, in which sensors are attached to a structure in a compact format to form an array. The region subjected to inspection and monitoring is the region outside the array. Both techniques have shown excellent capability of determining damage size, location, and severity.

Recent developments in guided wave generation, reception, and mode control show that increased penetration power and sensitivity are possible. A tone burst function generator and appropriate signal processing are generally used. Variable angle beam and comb-type transducers are the key to this effort. Problems in tubing, piping, hidden corrosion detection in aging aircraft, adhesive and diffusion bonding, and ice detection are discussed. Additionally, sample configurations, inspection objectives, and logic are being developed for such sample problems as defect detection and analysis in lap splice joints, tear straps, cracks in a second layer, hidden corrosion in multiple layers, cracks from rivet holes, transverse cracking in a beam, and cracks in landing gear assembly. Theoretical and experimental aspects of guided wave analysis include phase velocity, group velocity, and attenuation dispersion curves; boundary element model analysis for reflection and transmission factor analysis; use of wave structure for defect detection sensitivity; source influence on the phase velocity spectrum, and the use of angle beam and comb transducer technology. Probe design and modeling considerations are being explored. Utilization of in-plane and out-of-plane displacement patterns on the surface and longitudinal power distribution across the structural cross-section are considered for improved sensitivity, penetration power, and resolution in nondestructive evaluation. Methods of controlling the phase velocity spectrum for mode and frequency selection are available. Such features as group velocity change, mode cut-off measurements, mode conversion, amplitude ratios of transmission, and reflection factors of specific mode and frequency as input will be introduced for their ability to be used in flaw and material characterization analysis.

Tremendous interest has surfaced recently on the use of guided waves in pipe inspection in the oil, chemical, and power generating industries. Relatively long lengths of piping can be inspected for corrosion and cracking from a single probe position. This saves a great deal of time and money compared to using more standard point-by-point normal beam inspection procedures. Pipes can be inspected without removing insulation or tar coatings by controlling the guided wave modes and frequencies used to carry out the study. This paper will review the history and state of the art of the guided wave techniques in piping. Benefits and limitations of the various methods will be pointed out along with a vision of future directions in the area of pipe inspection.

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.

The work presented in this paper shows that Fiber Bragg Grating (FBG) optical fiber sensors can potentially be used as receivers in a long-range guided wave torsional-mode pipe inspection system. Benefits over the conventional pulse-echo method arise due to reduced total travel distance of the ultrasonic guided wave reflections, since reflections from defects and structural features do not need to propagate a full round trip back to the transmitting collar. This is especially important in pipe configurations with high attenuation, such as coated and buried pipelines. The use of FBGs as receivers instead of conventional piezoelectric or magnetostrictive elements also significantly reduces cabling, since multiple FBG receivers can be placed along a single optical fiber which has a diameter on the order of only around 100 μm. The basic approach and sample results are presented in the paper. Additionally, a brief overview of some topics in ultrasonic guided waves is presented as a background to understand the inspection problem presented here.