Ultrasonic Guided Waves in Structural Health Monitoring

Ultrasonic Guided Wave inspection and structural health monitoring is being considered today in such natural wave guide structures as plates, multi-layer structures, rods, rails, piping and tubing, an interface, and curved or flat layers on a half space. 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. Computing power today is also making dreams come true, where only a vision was possible decades ago. A principal advantage of guided waves is inspection over long distances with excellent sensitivity from a single probe position. There is also an ability to inspect hidden structures and structures under water, coatings, insulations, and concrete. Basic theoretical aspects of dispersion curve analysis, wave structure, source influence, sensor types and instrumentation possibilities and commercialization ventures will be discussed along with a variety of practical applications on ship hull, containment structures, aircraft, ice detection, pipelines, rail, overlap joints, and crystal manufacture. Phased array focusing in pipes and across elbows will be highlighted. Computational aspects of FEM and BEM analysis for defect classification and sizing analysis will be outlined. Future directions of leave in place sensors and wireless activity will also be presented. Introduction Ultrasonic guided waves are becoming more commonplace in industry because of the tremendous advances being made in the mathematics and mechanics of wave propagation that allows us to understand the unusual behavior characteristics that could become a major benefit in ultrasonic non-destructive testing methodologies. For the plenary talk given at the Asian Pacific Non-Destructive Testing Conference a great deal of material was covered on guided waves of which only a limited amount of information can be presented in this summary paper. Nevertheless, this summary paper serves as an instrument of knowledge for those interested and who want to get involved in ultrasonic guided wave analysis. The first three references include very basic material associated with ultrasonic guided waves in solid media along with some basic principles of dispersion curve analysis and an interesting example of the utilization of wave structure in guided wave analysis that allows us to perform guided wave testing of water loaded structures. References 4 and 5 contain very large literature surveys of a lot of very significant work that has been carried out in guided wave mechanics over the last few decades. A vision of ultrasonic guided wave inspection potential is also outlined in those papers. To add to the basic concepts of ultrasonic guided waves visualization schemes are often quite useful. One interesting example is presented by Hayashi and Rose [6]. To think of the utilization of ultrasonic guided waves we can consider a variety of different natural wave guides as outlined in Table 1. Guided wave inspection is a natural for any of these structures so when you really think about it guided waves can be applied to many, many structures very quickly and efficiently. An understanding of the basic wave mechanics and wave propagation principles for various sensors and mode types is essential, though, if one is to carry out some reliable tests. The benefits of guided waves are illustrated in Table 2. The most interesting one of course is to be able to inspect over long distances from a single probe position. Table 1. Natural Waveguides Plates (aircraft skin) Rods (cylindrical, square, rail, etc.) Hollow cylinder (pipes, tubing) Multi-layer structures Curved or flat surfaces on a half-space Layer or multiple layers on a half-space An interface Key Engineering Materials Online: 2004-08-15 ISSN: 1662-9795, Vols. 270-273, pp 14-21 doi:10.4028/www.scientific.net/KEM.270-273.14 © 2004 Trans Tech Publications Ltd, Switzerland All rights reserved. No part of contents of this paper may be reproduced or transmitted in any form or by any means without the written permission of Trans Tech Publications Ltd, www.scientific.net. (Semanticscholar.org-13/03/20,17:37:36) Title of Publication (to be inserted by the publisher) Table 2. Benefits of Guided Waves Inspection over long distances from a single probe position By mode and frequency tuning, to establish wave resonances and excellent overall defect detection and sizing potential. Often greater sensitivity than that obtained in standard normal beam ultrasonic inspection or other NDT techniques. (Beam focusing is on the horizon for even improved sensitivity.) Ability to inspect hidden structures and structures under water, coatings, insulations, and concrete with excellent sensitivity. Cost effectiveness because of inspection simplicity and speed. Ultrasonic guided waves can be produced in a structure by a variety of different techniques including angle beam transducers, comb type transducers, EMATs and magnetostrictive type sensors. The utilization of a comb-type transducer outlined by Rose and Quarry [7] is an interesting one to consider. Comb transducers can produce surface and guided waves in any structure and material including very low wave velocity composite materials where generation possibilities with an angle beam technique is not even possible. Other benefits of a comb transducer are associated with overall size and low profile height and cost. A sample phase and group velocity dispersion curve is presented in Figure 1. Every natural wave guided has associated with it a set of dispersion curves that presents to us the wave propagation possibilities in that structure. Details and analysis can be found in [1]. A sample set of wave structures are illustrated in Figure 2. At each point of a dispersion curve there is a different wave structure. The wave structure is associated with sensitivity, penetration power, and the ability to propagate in a water loaded structure, for example. In Figure 3 is an interesting concept associated with an ability to get on to a particular point in a dispersion curve. When studying the dispersion curve it is easy to understand that there is a corresponding frequency bandwidth associated with the abscissa value, but there is also a phase velocity bandwidth as illustrated in Figure 3 associated with the ordinate value on the phase velocity dispersion curve. This means that we are actually exciting a fairly large zone and multiple modes could propagate in a structure at the same time. Details on the wave mechanics of this source influence can be found in [1]. A variety of different applications and other aspects of guided wave inspection are presented in this paper in the following paragraph.