Title of Invention

NON-DISPERSIVE HIGHER-ORDER MODES CLUSTER ULTRASONIC GUIDED WAVE (HOMC-GW) TECHNIQUE FOR MEDIUM RANGE INSPECTIONS

Abstract ABSTRACT 1569 CHE 2€07 A method of propagation of ultrasonic guided wave modes travelling long distiances between the walls of structures for detecting, locating, sizing identifying defects and making measurements remote from the excitation point, characterised by the steps of generating higher order guided wave modes having smaller wavelengths; determining the optimum wedge angle for a specific frequency-thickness combination of the said structure, for generating the modes clusre: obtaining a non-dispersive wave by selecting a region in the dispersion curve where all the modes converge to nearly the same velocity value and are thus atmost non- dispersive; the signals received after travel of the said non-dispersive wave through the said structure being, thereafter, measured and interpreted in the known way.
Full Text

The non-dispersive propagation of higher order ultrasonic guided wave modes traveling long distances along the wall of sti-ucturcs, such as flat, cylindrical, and complex shape structures capable of detecting, locating, sizing, and characterizing defects located far, from the excitation point. These higher frequency modes posses the ability to inspect for defects, anomalies, and damage in materials and structures in the range of 0.1-10 meters along the line of sight. These higher order guided wave modes exhibit small dispersion and have ability to provide improved imaging of small size defects due to the higher frequency of operation. These wave modes may also be used to measure the state of the material. The material state that may be measured includes, but not limited to:
(a) Elastic constants
(b) Rheological properties of fluids that is in contact with the boundaries of the material sustaining the guided waves
(c) Flaws and anomalies such as cracks, wallthickness reduction, porosity, voids, . delaminations, gouging, etc
(d) Thickness of layers and materials

(e) Material non-uniformity
(f) Depth profiles of elastic^ visco-elastic and aji-elastic material properties
(h) Stress and stress profiles
(i) Material degradation including plastic deformation, hydride embritlement, creep, fatigue, etc.
(k) Process parameter profiles such as temperature, pressure, flow, etc
(1) Process parameter profiles such as.temperature, pressure, flow, etc
(m) Surface properties such as roughness, treatments, hardness, etc.
The application of this technology can be extended to various industiies including, but not limited to (a) power plant inspection, (b) oil and gas industry, chemical, fertilizer, petrochemical, cement, and mining, structures and components, (c) aircraft and aerospace structures and components, (d) automobile components, (e) infrastructurai components and structures such as bridges, dams, road, etc., (f) transportation structures such as rails, wheels, axles, engines, etc.

Related Background
The ultrasonic guided waves, unlike longitudinal and transverse bulk wave modes, are a manifestation of geometrical confinement of acoustical waves by one or more boundaries. [1,2]. In many instances, these waves travel long distances, depending on the frequency and mode characteristics of the wave, and follow the contour of the structure in which they are propagating. Usually, these waves not only propagate along the length of the structure but also cover the entire thickness and circumference (in the case of cylinders and rods). The use of guided wave modes is potentially a very attractive solution to the problem of inspecting the embedded portions of structures because they can be excited at one point on the structnre, propagated over considerable distances, and received at a remote point on the structure, in a pitch-catch mode, as schematically illustrated in Figure 1 for an elbow pipe. The received signal contains information about the integrity of the material that lies between the transmitting and receiving transducers. Alternative approaches, where the receiving and transmitter are co-located, similar to a pulse-echo method is also possible.
Since there are several types of guided waves, there are many ways to classify them. The first classification can be based on the type of structure in which it is generated. These include (a) Plate waves, (b) Cylindrical waves, (c) Rod waves, etc., depending upon the type of structure. The wave mode characteristics for each of the above type is distinctly different, but can be theoretically predicted if the material properties are well known. The second method of classification of the wave mode is based on the nature of the particle vibration with respect to the direction of wave propagation (like in the case of bulk waves). In this type of classification, the types include (a) Extensional or Longitudinal waves, (b) Shear-horizontal waves, (c) Flexural waves, and (d) Torsional waves [3-5]. Here, the first two types are similai- to the Longitudinal and Shear wave vibration. The Flexural waves are modes where the structure flexes in a wave like pattern and the Torsion waves exist when the particle motion is circumferential in nature while. the wave moves along the structure.
Also, the wave modes can also be broadly classified into symmetric and anti-syrmnetric modes based on the type of symmetry of the displacement profile exhibited by the wave during propagation. This classification is based on whether the out-of-plane displacement in a structure is symmetric about the *neutral' axis of the bounded structure i.e., if the two outer particles simultaneously move away from die center axis, then it is a symmetric mode and if they move together, then it is anti-symmetric. This is well illustrated in Figure 3.
Finally, for a give type of guided wave, there are many orders of modes that can exist. The modes have mode shapes are analogous to vibration modes in a beam. These modes are numbered numerically from zero representing the basic fundamental modes to the higher order modes representing more complex behavior. '^

Thus, while defining a guided wave mode, a complete description will require the specification of all of the above classifications. For instance, a cylindrically guided, flexural, anti-symmetric, fundamental mode would represent a guided wave that is traveling along the length of a cylindrical structure that has a fundamental flexural type particle vibration direction that is not symmetric about the axis.
The multi-modal nature of these wave modes can be used advantageously since each mode has different sensitivity to a particular type of defect and hence by comparing the wave propagation of different modes, i.c^ by using one as a reference mode and the other as a sensing mode, defect and/or material characterization becomes feasible. One of the key aspects of guided wave modes is Dispersion, i.e., wave velocity is not a constant for a given material. It additionally depends on geometry (thickness) and frequency of the wave. In most cases, this becomes one parameter, frequency-thickness product (f^d). The consequence of dispersion is that a compact broad-banded signal will not retain its shape while propagating and will elongate considerably with distance of travel. This is
because of the fact (hat a broad band ultrasonic pulse comprises of a range,of frequencies (depending on the bandwidth and central frequency of the pulse) and since each frequency is traveling at a different velocity, the pulse duration increases.
The velocity of the wave mode for a single frequency is called as its Phase velocity. It must be apparent that the measurement of phase velocity by traditional velocity measurement techniques (such as pulse-overlap, zero-crossing, etc.) is difficult, due to the change in the pulse shape. Hence, a different definition of velocity called the Group velocity is used while measuring the velocities of a dispersive ultrasonic pulse by traditional methods. The Group velocity cannot be faster than the Phase velocity and the dispersive nature of these modes can be theoretically computed if the material properties are known.
The dispersion curves can be plotted to gain understanding of the types of modes that are generated and their dispersive nature. These curves are also used for interpretation of the signal. The dispersion curves are represented in different manner in literature. The most useftil representation for NDE application is shown in Figure 3. In this representation, the velocity of the wave is plotted as a fiinction of frequency of the wave. Each curve represents a guided wave mode. The wave velocity that is plotted can be either phase or group velocity. But, from our previous discussion, it can be concluded that the group velocity is representative of measurements made with dispersive wave pulses and hence is more useful. Several (in fact, often too many) modes can get generated unless carefiil attention is not paid to the method of generation and reception. Any of the three types of

modes are possible in a structure depending on the geometry of the waveguide (i.e., Torsional, Longitudinal and Flexural). The slope of these ciHves indicates the dispersive nature of the wave mode. Hence, a curve with a steep.slope is very dispersive and may be avoided during NDE. Consequently, a flat region of a curve means the mode is non-dispersive and the wave pulse will propagate effectively and measurements are possible. The cylindrically guided waves can be generated and measured using several mechanisms. These wave modes can be generated using circular ring-type array transducers [14] for pipes, or comb-transducer configuration [15] for tubes, or like in the regular weld inspection using an array of variable angle beam transducers located around
the circumference of the cylinder. Alleyne and Cawley [14] reported the development of a dry coupled piezoelectric transducer system for the excitation of the axially symmetric L(0,m) modes in pipes. It comprises a ring of piezoelectric eleiiients that are damped individually to the pipe surface; no coupling fluid is required at tlie low ultrasonic frequencies used here.
Additionally, non-contact methods using Electro-Magnetic Acoustic Transducers (EMAT) has also been reported [16]. These can be located either to the inside or the outside surface of the pipe/tube. The cylindrically guided wave teclinique has been modified to generate and detect wave-modes without the physical contact with the pipe walls [17]. This is accomplished by using the magnetostrictive property of steel pipes, where the pipe material acts as the transducer, and a coil that encircles the pipe couples the excitation energy into the pipe. A separate receiver coil is used to pick up the signals from the guided waves. These two methods have the advantage of being able to generate and receive waves without physical corilact with the structure. But, it has been reported that mode isolation and identification of received signals is more complex.
Technique Description
The apparatus primarily involves (a) a means for the generation of non-dispersive guided waves in the structure, and (b) a means for the measuieraent of and interpretation of the signals received. A typical apparatus is illustrated, in Figure 2

It becomes very important to choose Ihe optimum angle for a particular frequency-thickness combination for identifying the optimal mode(s) for defect detection. Since 'several modes may be generated in this region, sufficient care must be taken during the wedge manufacturing so that only the desired mode is generated. Also, the size of the commercial probe used in these studies was selected to be relatively large (25 mm or larger) in order to minimize the beam divergence. The conventional approach to guided wave inspection involves the use of tlie limdamental order (low order) modes since these can be generated without interference from higher order modes. However, the frequency of generation of such modes is relatively low (leading to long wavelengths) and hence, Umhs the resolution of the inspection to sinaUer defects.
The novelty of this apparatus involves the use of higher order modes that have smaller wavelengths and hence improved resolution to smaller defects. The difHculties previously encountered with multiple modes are resolved here by selecting a region in the dispersion curve where all the modes converge to the same velocity and arc non-dispersive. This is illustrated in the Figure 3 and the region selected is encircled. This selection allows the employment of higher order modes without the difficulties of multiple modes traveling at different velocities and hence dispersion in the modes. There is no report of any previous use of this region in the dispersion curve for Nondestructive Testing/Evaluation applications and hence constitutes a novel technique that is useful for medium range inspection applications with higher resolution of defect detection compared to the conventional guided wave techniques earlier reported. Both the group and the phase velocities of ail modes arc similar and hence improves time resolution for defect location and velocity measurement for NDT/E and measurement applications. It must be noted that as the frequency increases further, the guided wave modes cease to exist and degenerates into a single bulk wave mode traveling at the shear wave velocity in the material. Hence, the frequency bandwidth region of the generation and reception of this Higher Order Modes Cluster Guided Wave Technique (HOMC-GW) is limited

to the region between the dispersive behavior of the modes and the degeneration of these modes into the shear mode.
A Typical Apparatus
A typical setup for employing the HOMC-GW technique for generation of tliese modes in a cylindrical guided wave approach for defect detection is shown in Figure 2 using the curved coupling "wedge". It becomes very impoitant to choose the optimum angle for a particular frequency-tliickness combination for identifying the optimal mode(s) for defect detection. Also, the size of the commercial probe used in these studies was selected to be relatively large (25 mm or larger) in order to minimize the beam divergence. This system is a standard ultrasonic pulser/ receiver with commercial PANAMETRICS rV102 and
V104, 25.4 mm diameter) transducer interfaced to the PC using a bus,powered National Instruments PCMCIA NI 5102 based data acquisition cai'd to acquire data.
Wedge Design: —— . -— —.. :..._.. _„
A suitable coupling "wedge" becomes absolutely necessary for correct matching of the flat transducer to the curved pipe surface so that maximum ultrasonic energy can be transmitted into the pipe specimen. The wedge is also necessary for directing the ultrasonic beam at the required angle to the pipe suiface so that that the beam divergence is minimized. A typical wedge is shown in Figure 4 and will depend on the size of the pipe and the type of transducers used. Phased array transducers may also be used for this generation. For other forms of excitation of such modes, like EMATs, and Magnetostrction means, the inode selection is less complicated.
Typical Illustrative Experimental Results
TYPICAL APPLICATION: On Pines for inaccessible location inspection
The A-scan signals obtained using the 1 MHz conventional (circular transducer of 25 mm diameter), the 2.25 MHz that generates the HOMC-GW (circular transducer of 25

mm diameter) and the 2.25 MHz linear phased array transducers that generates the HOMC-GW were compared to demonstrated the improved detection and sizing of defects for pipe inspection under in-accessible pipe support regions and for gouging applications in crude transport pipes. Comparisons are made for the two defect cases i.e., EDM notches and 1.5 mm diameter pin holes. The expected/estimated theoretical amplitude of the smaller defects is taken to be 20%. 40%. 60% and 80% of the reference amplitude. The estimated theoretical amplitude of the signals was compared with the experimentally obtained amplitudes from the respective notches and the percentage eiTors were estimated.
A-scan defect signals obtained from the EDM notches using tlie three transducer configurations are shown in Figures 5(a-c), respectively. It is observed. that signals obtained from the 2.25 MHz conventional transducer were much less dispersive when compared to the signals from tlie 1 MHz conventional transducer. This result occurs because, at 1 MHz and the chosen wedge angle combination, the wave mode being excited is primarily A3 mode. However, along with A3, other modes are also being excited collaterally. This creates a signal that is both dispersive and which contains a lot -of smaller amplitude elastic modes that give a noisy appearance to the signal, hi the case of the HOMC-GW technique using the 2.25 MHz conventional transducer and the chosen wedge angle combination, one primarily excites a cluster of modes. Due to the size and shape of the wedge several modes including Si, Ai and A2 modes are also excited collaterally and travel at almost the same velocity in the structure. In this case, however, these modes interfere constructively over a very long path length and give a strong single peak signal (actually, the dispersion curves predicts two extremely close modes, but only non-dispersive mode was observed with a slightly broadened peak). It was also noted that the HOMC-GW technique using the 2.25 MHz phased array transducer signals were less contaminated by noise in comparison to the 1.0 MHz conventional transducer based single mode technique signals. Thus, it is seen that HOMC-GW technique using a transducer of a large diameter and also a higher frequency is capable of detecting significantly smaller defects and also generating signals with higher Signal to Noise Ratio (SNR). The B-scan images of the EDM notches for the three transducer configurations are shown in Figures 6(a-c).

TYPICAL APPLICATION: Qu Tank Floors
The typical aboveground Storage tank consists mainly of the
1. Annular plate
2. Bottom plate
3. Shell
4. Floating roof
The annular plate joins the shell plate of the tank to the bottom plate. Usually the annular plate rests on a concrete ring wail about 2 feet from the ground. The annular area of tlie ring wall is filled witli materials like sand, crushed stones etc. The tank shell on top of the annular plate, has plates of decreasing thickness with the thickest one at the bottom. The top surface of the annular and bottom plates are coated with materials like GRP to prolong their life from wear and corrosion.
The vertical shell of the tank is welded to tlie annular plates as shown in the Figure 8. The annular plates are welded to tank bottom plates on the horizontal plane.
The maximum stress in a tank bottom exists at the toe of the inside shell-to-bottom fillet weld at the annular plate. The region of the amiular plate near the shell-to-bottom fillet weld ( both on the top and bottom side ) is subjected to metallurgical change due to the irregular heating and cooling arising from the welding process, which makes the region more vulnerable to corrosion when compared to other regions of the annular plate.
Some of the typical defects which occur in annular plates of storage tank with tlieir reasons are listed below
1. Corrosion /Wall thinning at the bottom surface of the annular plate, near the shell-to-bottom plate fillet weld as shown in the figui-e.
.2. Corrosion on the bottom surface of annular plate due to water seepage through the protruding area of the annular plate outside the shell.
3. Stress Corrosion cracks on the liquid side of the annular plate
4. Corrosion on the bottom surface of the annular plate which is in contact with the compacted foundation sand.
5. Pitting corrosion on the liquid side of the annular plate.

The leakage of storage tank containing any chemical product poses high risk to environment and human safety apart from the actual economic loss of the product stored. Having these serious consequences in mind storage tanks has to be inspected /monitored regularly.
However, available internal inspection teclmiques involve emptying and cleaning the tanks, which present several disadvantages;
• Heavy cost consumptive.
• The tank is out of operation for a long duration.
• High risk exposing the skilled inspection crew to hazardous chemical sludge . deposits and chemical particles.

• Transportation and storage cost of empting the fluid from the tank to be inspected . to other sites.
• The inspection time and resources involved are high.
Typical results obtained using the HOMC-GW technique is shown in Figures 9 and 10 in the A-scan and B-scan displays.The sharp/tight signals and the higher frequency allow this method to resolve defects in a much improved manner for several applications of which only two have been illustrated here.
1; The technique is a non-destructive means of making measurement using higher-order modes cluster ultrasonic guided wave excitation, ,and imaging/measurement of materials and structures of the structure/material/component.
2. The technique depends on the selection of a collection of ultrasonic guided wave modes that are non-dispersive and have similar wave speeds and are individually or co-generated.
3. The technique has the ability to detect and quantify defects at distances far from the source of generation and reception of the.guided waves. This may be of,the order of 1 cm to 10 m.
4. The technique can evaluate structure of various geometries such aS plates, pipes, rods, tubes, shells, etc. and combinations of such components.
5. The technique can be used on components and struclxires tliat have joints such as welds, bonded regions, etc.

6. The technique can be employed on structures and components where coatings and insulation ai'e present over it.
7. The technique can be used in inaccessible regions in the structure by generation on the accessible surface.
8. The technique can be used to measure depth resolved properties such as (a) moduli, (b) density, (c) stresses, (b) hardness, (c) visco-elastic propei'ties,etc.
9. The technique caii be used to measure and characterize material properties and
. used of sorting of materials and applications thereof.
10. The technique can be used for measuring the layer thicknesses on single and multi-layered structures and components.
11. The technique can be used for material degradation including plastic deformation, fatigue, creep, embrittlement, corrosion, stress con-osion crackings etc. in materials, structures and components.
12. The techmque can be used for testing and evaluation of defects in materials, during manufacture, post manufacture, in-service, life extension, and failure analysis purposes. Defect types.include delamination, disbonds, cracks, porosity, '
' voids, corrosion, wide-spread multi-site damage, etc.
13. The techmque can be used in imaging mode for all of the above properties over a large surface area of a component and/or structure.
14. The technique may also be used in point measurements that can be at single point or a series of points over a surface.
15. The technique can be used for all conducting materials including metals, composites, ceramics, etc.
16. The technique can be used for non-conducting materials by application of a layer of conducting material on to the non-conductive material. The conducting; layer may be of removal type or non-removal type. ; ^ ■
17. The technique may be used for process monitoring during manufacture or processing of materials.
18. The excitation can be any of the possible types including, but not limited to, (a) Tone Burst, (b) Spike Pulse, (c) Chirp, (d) Coded pulses (like the Golay codes, etc.), and (f) arbitrary ftmctions.

19. The mode of implementation of the method can be in through-transmission or pulse-echo or pitch-catch modes. The orientation of the excitation source/receiver and the defects can be at normal or oblique angles. .
20. The technique .is supported by analytical and numerical mathematical models that support the forward solution (prediction of the results) and the inverse solutions (quantitative interpretation of the experimentals) and parametric estimations.
,21. The technique may support signal and image processing algorithms that enhance
the signal to noise ratio (SNR) of tlie experimental data and provide
enhancements to the image, when used in an imaging mode. 22. Phased arrays, EMATs, Magnetostriction, air coupled, laser based ultrasonic
transducers are alternate possible mechanisms for the generation and reception of
these guided wave modes.


We Claim: .A method of propagation of ultrasonic guided wave modes travelling long distances between the walls of structures for detecting, locating, sizing identifying defects and making measurements remote from the excitation point, characterised by the steps of generating higher order guided wave modes having smaller wavelengths; determining the optimum wedge angle for a specific frequency-thickness combination of the said structure^ for generating the modes clusre; obtaining a non-dispersive wave by selecting a region in the dispersion curve where all the modes converge to nearly the same velocity value and are thus almost non- dispersive; the signals received after travel of the said non-dispersive wave through the said structure being, thereafter, measured and interpreted in the known way.
2. A method of propagation of ultrasonic guided wave modes travelling Jong distances between the walls of structures for detecting, locating, sizing identifying defects and making measurements

remote from the excitation point, substantially as here4in described with reference to, and as ([[ustrated by, the accompanying drawings.


Documents:

http://ipindiaonline.gov.in/patentsearch/GrantedSearch/viewdoc.aspx?id=Qgj1aRBw6oZyjj1F3qBkkA==&loc=egcICQiyoj82NGgGrC5ChA==


Patent Number 277265
Indian Patent Application Number 1569/CHE/2007
PG Journal Number 48/2016
Publication Date 18-Nov-2016
Grant Date 16-Nov-2016
Date of Filing 20-Jul-2007
Name of Patentee INDIAN INSTITUTE OF TECHNOLOGY
Applicant Address IIT P.O CHENNAI 600 036, INDIA.
Inventors:
# Inventor's Name Inventor's Address
1 PROF. KRISHNAN BALASUBRAMANIAM DEPARTMENT OF MECHANICAL ENGINEERING IIT, CHENNAI 36
2 DR. BRUCE MAXFIELD DEPARTMENT OF MECHANICAL ENGINEERING IIT, CHENNAI 36
3 J. CHANDRASEKARAN DEPARTMENT OF MECHANICAL ENGINEERING IIT, CHENNAI 36
4 L. SATYANARAYAN DEPARTMENT OF MECHANICAL ENGINEERING IIT, CHENNAI 36
5 DR. C.V. KRISHNAMURTHY DEPARTMENT OF MECHANICAL ENGINEERING IIT, CHENNAI 36
PCT International Classification Number G01B17/00;
PCT International Application Number N/A
PCT International Filing date
PCT Conventions:
# PCT Application Number Date of Convention Priority Country
1 NA