Summer Internship Report

Entitled 

“A Study on HOMC Guided Waves in Industrial Pipes”  

 

BY GEO PAUL ANTONY

06356

DEPARTMENT OF MECHANICAL ENGINEERING NATIONAL INSTITUTE OF TECHNOLOGY

HAMIRPUR-177005, HP (INDIA)

JUNE, 2009

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Indian Institute of Technology Madras, Chennai - 600036

Centre For Nondestructive Evaluation

TO WHOMSOEVER IT MAY CONCERN This is to certify that Mr. Geo Paul Antony S/O Dr. M.P.Antony of NIT Hamirpur has completed his Summer Internship in CNDE; IIT Madras during the period of 18th may, 2009 to 22nd July, 2009. The report entitled “A study on HOMC guided waves in industrial pipes” is a bonafide record of the work done by him. During the tenure of training he was found honest, hard working and sincere. He was regular and showed keen interest in learning new things. Dated: - 22nd June, 2009 Training Officer

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DECLARATION

I, Geo Paul Antony (06356), a student of Mechanical Engineering, B.Tech., 3rd year,

National Institute of Technology Hamirpur (H.P.), hereby declare that the work in the

Training report ‘A Study on HOMC Guided Waves in Industrial Pipes’ is a result of

study and investigation carried out by myself under the guidance of our aforesaid mentor.

Date: July 27th, 2009

Place: IIT Madras, CNDE

GEO PAUL ANTONY

(06356)

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ACKNOWLEDGEMENTAACCKKNNOOWWLLEEDDGGEEMMEENNTT

It is said that the knowledge is the gate way to success and to get the right

knowledge, one requires a right teacher and as an engineering student, I know very well

that without the right direction and guidance of Mr. Krishnan Balasubramaniam, it

would have been impossible to complete and present this summer internship Report.

I offer my profound gratitude to the management of IIT, Madras for giving

me this opportunity to amalgamate my theoretical knowledge with the practical

experience, in a professional environment.

I m deeply grateful to Mr. V. Chandrashekaran for all the help, co-operation

& valuable guidance he gave me.

My special thanks to Mr. Isaac for the valuable time he spent with me

teaching me the tricks of accurate measurements.

I cannot omit my gratitude to Prof. Anoop Kumar, HOD ( MED ) who gave

me this prestigious chance to do my internship for great exposure.

A special mention of my college, NIT Hamirpur, is in order, which gave me

this opportunity to gain real life hands-on experience by organizing this summer training

course.

Lastly I would like to thank my parents who stood by me at all stages of my

life and never lost confidence in me. Without their constant nudge in the right directions,

I could never have achieved anything in life.

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INDEX Si No CONTENT Pg. no.

1 INTRODUCTION (CNDE) 7

2 NDT - Non-Destructive Testing

2.1. Introduction 9

2.2. Methods of NDT 11

3 ULTRASONIC TESTING

3.1. Introduction 13

3.2. UT over the times 14

3.3. Wave Propagation 17

3.4. Piezoelectric Transducers 20

3.5. Inspection Methods 24

3.6. Data Presentation 26

4 HOMC guided waves study

4.1. Introduction 29

4.2. The Ultrasonics measurement systems 30

4.3. Experiment 34

4.4. Resulting data 35

5 CONCLUSIONS 40

6 REFERENCES 41

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1. INTRODUCTION (CNDE) CNDE – Centre for Non-Destructive Evaluation is a department in IIT, Madras

The Center for Nondestructive Evaluation (CNDE) is committed to the advancement

/development of science and technology of NonDestructive Evaluation. This objective is

being progressively fulfilled by pursuing fundamental and applied research in the

frontier areas of NDE , proposing and executing of projects of immediate pertinence to

government and other industry agencies, carrying out consultancy work for various

industries in India and abroad utilizing the state of art facilities available, organizing

educational and training programmes in traditional and advanced NDE for industries and

institutions and developing models for various nondestructive testing methods and

measurements. A devoted team of research and project co-ordinators, research scientists,

project officers and associates and PhD and MS scholars are striving untiringly to

contribute to realize the above goal. Most of our B.Tech students evince a lot of interest

to take up NDE as their field of research and they are allowed to have a free hand to work

on innovative ideas of them. It is constantly endeavored to establish reliable engineering

measurement techniques, material characterization methods etc. Emerging technologies

like MEMS, nanomaterials and others are continuously monitored with a view to provide

NDE inputs.

THRUST AREA

1. Advanced NDE Technologies.

2. Forward and Inverse Modeling.

3. Measurements while Manufacturing.

4. Structural Health Monitoring.

5. Micro-/Nano-Devices Imaging.

6. Data Analysis Algorithms.

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Research carried out on following methods:-

1. Acoustic NDE 2. Ultrasonic NDE 3. Thermal Imaging 4. Digital Radiography NDE 5. Hybrid Modality NDE 6. 6Structural Health Monitoring 7. NDE Models & Simulation 8. Materials Characterization 9. Micro & Nano Imaging 10. Signal and Image Processing & Data Fusion 11. Process Monitoring and Measurements 12. Sensors for NDT 13. Electromagnetic NDE

Ultrasonic Testing Facilities available:-

• Ultrasonic C-scan Systems 1’x1’, 2’x2’, 3’x3’, and 6’x6’ CNDE scanners

• Ultrasonic Pulser Receivers Panametrics 500, 5077, 5058, 5800, 5900

• Ultrasonic Transducers PZTs and EMATs from 50 kHz-120 MHz

• Ultrasonic Phased Array R-D Tech Omni Scan MX

• Ultrasonic Phased array immersion and contact probes up to 17 MHz

• Ultrasonic High Power Systems RITEC SNAP (5000W)

• Ultrasonic High Power Systems RITEC GA2500 with Receiver (2500W)

• Ultrasonic High Power Systems MATEC PR5000 (5000W)

• Ultrasonic High Power Systems MATEC (3000W)

• Ultrasonic Automated TOFD System CNDE development

• Impact Echo System CNDE development

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2. NDT – Non-Destructive Testing 2.1. Introduction

The field of Nondestructive Testing (NDT) is a very broad [1], interdisciplinary field that plays a critical role in assuring that structural components and systems perform their function in a reliable and cost effective fashion. NDT technicians and engineers define and implement tests that locate and characterize material conditions and flaws that might otherwise cause planes to crash, reactors to fail, trains to derail, pipelines to burst, and a variety of less visible, but equally troubling events. These tests are performed in a manner that does not affect the future usefulness of the object or material. In other words, NDT allows parts and materials to be inspected and measured without damaging them. Because it allows inspection without interfering with a product's final use, NDT provides an excellent balance between quality control and cost-effectiveness. Generally speaking, NDT applies to industrial inspections. While technologies are used in NDT that are similar to those used in the medical industry, typically nonliving objects are the subjects of the inspections.

Non-Destructive Evaluation:-

Nondestructive Evaluation (NDE) is a term that is often used interchangeably with NDT. However, technically, NDE is used to describe measurements that are more quantitative in nature. For example, a NDE method would not only locate a defect, but it would also be used to measure something about that defect such as its size, shape, and orientation. NDE may be used to determine material properties such as fracture toughness, formability, and other physical characteristics.

Background on Nondestructive Testing (NDT):-

Nondestructive testing has been practiced for many decades. One of the earliest applications was the detection of surface cracks in railcar wheels and axles. The parts were dipped in oil, then cleaned and dusted with a powder. When a crack was present, the oil would seep from the defect and wet the powder providing visual indicating that the component was flawed. This eventually led to oils that were specifically formulated for performing these and other inspections and this inspection technique is now called penetrant testing.

X-rays were discovered in 1895 by Wilhelm Conrad Roentgen (1845-1923) who was a Professor at Wuerzburg University in Germany. Soon after his discovery, Roentgen produced the first industrial radiograph when he imaged a set of weights in a box to show his colleagues. Other electronic inspection techniques such as ultrasonic and eddy current

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testing started with the initial rapid developments in instrumentation spurred by technological advances, and subsequent defense and space efforts following World War II. In the early days, the primary purpose was the detection of defects. Critical parts were produced with a "safe life" design, and were intended to be defect free during their useful life. The detection of defects was automatically a cause for removal of the component from service.

In the early 1970's, two events occurred which caused a major change in the way inspections were viewed. The continued improvement of inspection technology, in particular the ability to detect smaller and smaller flaws, led to more and more parts being rejected (even though the probability of part failure had not changed). At this time the discipline of fracture mechanics emerged, which enabled one to predict whether a crack of a given size would fail under a particular load if a particular material property or fracture toughness, were known. Other laws were developed to predict the rate of growth of cracks under cyclic loading (fatigue). With the advent of these tools, it became possible to accept structures containing defects if the sizes of those defects were known. This formed the basis for a new design philosophy called "damage tolerant designs." Components having known defects could continue to be used as long as it could be established that those defects would not grow to a critical size that would result in catastrophic failure.

A new challenge was thus presented to the nondestructive testing community. Mere detection of flaws was not enough. One needed to also obtain quantitative information about flaw size to serve as an input to fracture mechanics calculations to predict the remaining life of a component. These needs, which were particularly strong in the defense and nuclear power industries, led to the creation of a number of research programs around the world and the emergence of nondestructive evaluation (NDE) as a new discipline.

The number of NDT methods that can be used to inspect components and make measurements is large and continues to grow. Researchers continue to find new ways of applying physics and other scientific disciplines to develop better NDT methods. However, there are six NDT methods that are used most often. These methods are visual inspection, penetrant testing, magnetic particle testing, electromagnetic or eddy current testing, radiography, and ultrasonic testing. These methods and a few others are briefly described below.

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2.2. Methods of NDT

Visual and Optical Testing (VT)

Visual inspection involves using an inspector's eyes to look for defects [1]. The inspector may also use special tools such as magnifying glasses, mirrors, or borescopes to gain access and more closely inspect the subject area. Visual examiners follow procedures that range from simple to very complex.

Penetrant Testing (PT)

Test objects are coated with visible or fluorescent dye solution. Excess dye is then removed from the surface, and a developer is applied. The developer acts as blotter, drawing trapped penetrant out of imperfections open to the surface. With visible dyes, vivid color contrasts between the penetrant and developer make "bleedout" easy to see. With fluorescent dyes, ultraviolet light is used to make the bleedout fluoresce brightly, thus allowing imperfections to be readily seen.

Magnetic Particle Testing (MT)

This NDE method is accomplished by inducing a magnetic field in a ferromagnetic material and then dusting the surface with iron particles (either dry or suspended in liquid). Surface and near-surface imperfections distort the magnetic field and concentrate iron particles near imperfections, previewing a visual indication of the flaw.

Electromagnetic Testing (ET) or Eddy Current Testing

Electrical currents are generated in a conductive material by an induced alternating magnetic field. The electrical currents are called eddy currents because they flow in circles at and just below the surface of the material. Interruptions in the flow of eddy currents, caused by imperfections, dimensional changes, or changes in the material's conductive and permeability properties, can be detected with the proper equipment.

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Radiography (RT)

Radiography involves the use of penetrating gamma or X-radiation to examine parts and products for imperfections. An X-ray generator or radioactive isotope is used as a source of radiation. Radiation is directed through a part and onto film or other imaging media. The resulting shadowgraph shows the dimensional features of the part. Possible imperfections are indicated as density changes on the film in the same manner like a medical X-ray shows broken bones.

Ultrasonic Testing (UT)

Ultrasonics use transmission of high-frequency sound waves into a material to detect imperfections or to locate changes in material properties. The most commonly used ultrasonic testing technique is pulse echo, wherein sound is introduced into a test object and reflections (echoes) are returned to a receiver from internal imperfections or from the part's geometrical surfaces.

Acoustic Emission Testing (AE)

When a solid material is stressed, imperfections within the material emit short bursts of acoustic energy called "emissions." As in ultrasonic testing, acoustic emissions can be detected by special receivers. Emission sources can be evaluated through the study of their intensity, rate, and location.

Leak Testing (LT)

Several techniques are used to detect and locate leaks in pressure containment parts, pressure vessels, and structures. Leaks can be detected by using electronic listening devices, pressure gauge measurements, liquid and gas penetrant techniques, and/or a simple soap-bubble test.

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3. ULTRASONIC TESTING

3.1. Introduction

Ultrasonic Inspection is a very useful and versatile NDT method [1]. Some of the advantages of ultrasonic inspection that are often cited include:

• It is sensitive to both surface and subsurface discontinuities. • The depth of penetration for flaw detection or measurement is superior to other

NDT methods. • Only single-sided access is needed when the pulse-echo technique is used. • It is highly accurate in determining reflector position and estimating size and

shape. • Minimal part preparation is required. • Electronic equipment provides instantaneous results. • Detailed images can be produced with automated systems. • It has other uses, such as thickness measurement, in addition to flaw detection.

As with all NDT methods, ultrasonic inspection also has its limitations, which include:

• Surface must be accessible to transmit ultrasound. • Skill and training is more extensive than with some other methods. • It normally requires a coupling medium to promote the transfer of sound energy

into the test specimen. • Materials that are rough, irregular in shape, very small, exceptionally thin or not

homogeneous are difficult to inspect. • Cast iron and other coarse grained materials are difficult to inspect due to low

sound transmission and high signal noise. • Linear defects oriented parallel to the sound beam may go undetected. • Reference standards are required for both equipment calibration and the

characterization of flaws.

The above introduction provides a simplified introduction to the NDT method of ultrasonic testing. However, to effectively perform an inspection using ultrasonics, much more about the method needs to be known. The following pages present information on the science involved in ultrasonic inspection, the equipment that is commonly used, some of the measurement techniques used, as well as other information.

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3.2. UT over the times

History of Ultrasonics:-

Prior to World War II, sonar, the technique of sending sound waves through water and observing the returning echoes to characterize submerged objects, inspired early ultrasound investigators to explore ways to apply the concept to medical diagnosis. In 1929 and 1935, Sokolov studied the use of ultrasonic waves in detecting metal objects. Mulhauser, in 1931, obtained a patent for using ultrasonic waves, using two transducers to detect flaws in solids. Firestone (1940) and Simons (1945) developed pulsed ultrasonic testing using a pulse-echo technique.

Shortly after the close of World War II, researchers in Japan began to explore the medical diagnostic capabilities of ultrasound. The first ultrasonic instruments used an A-mode presentation with blips on an oscilloscope screen. That was followed by a B-mode presentation with a two dimensional, gray scale image.

Japan's work in ultrasound was relatively unknown in the United States and Europe until the 1950s. Researchers then presented their findings on the use of ultrasound to detect gallstones, breast masses, and tumors to the international medical community. Japan was also the first country to apply Doppler ultrasound, an application of ultrasound that detects internal moving objects such as blood coursing through the heart for cardiovascular investigation.

The United States produced the earliest hand held "contact" scanner for clinical use, the second generation of B-mode equipment, and the prototype for the first articulated-arm hand held scanner, with 2-D images.

Present State of Ultrasonics:-

Ultrasonic testing (UT) has been practiced for many decades. Initial rapid developments in instrumentation spurred by the technological advances from the 1950's continue today. Through the 1980's and continuing through the present, computers have provided technicians with smaller and more rugged instruments with greater capabilities.

Fig.1 - modern UT

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Thickness gauging is an example application where instruments have been refined make data collection easier and better. Built-in data logging capabilities allow thousands of measurements to be recorded and eliminate the need for a "scribe." Some instruments have the capability to capture waveforms as well as thickness readings. The waveform option allows an operator to view or review the A-scan signal of thickness measurement long after the completion of an inspection. Also, some instruments are capable of modifying the measurement based on the surface conditions of the material. For example, the signal from a pitted or eroded inner surface of a pipe would be treated differently than a smooth surface. This has led to more accurate and repeatable field measurements.

Many ultrasonic flaw detectors have a trigonometric function that allows for fast and accurate location determination of flaws when performing shear wave inspections. Cathode ray tubes, for the most part, have been replaced with LED or LCD screens. These screens, in most cases, are extremely easy to view in a wide range of ambient lighting. Bright or low light working conditions encountered by technicians have little effect on the technician's ability to view the screen. Screens can be adjusted for brightness, contrast, and on some instruments even the color of the screen and signal can be selected. Transducers can be programmed with predetermined instrument settings. The operator only has to connect the transducer and the instrument will set variables such as frequency and probe drive.

Along with computers, motion control and robotics have contributed to the advancement of ultrasonic inspections. Early on, the advantage of a stationary platform was recognized and used in industry. Computers can be programmed to inspect large, complex shaped components, with one or multiple transducers collecting information. Automated systems typically consisted of an immersion tank, scanning system, and recording system for a printout of the scan. The immersion tank can be replaced with a squirter systems, which allows the sound to be transmitted through a water column. The resultant C-scan provides a plan or top view of the component. Scanning of components is considerably faster than contact hand scanning; the coupling is much more consistent. The scan information is collected by a computer for evaluation, transmission to a customer, and archiving.

Future Direction of Ultrasonic Inspection:-

Looking to the future, those in the field of NDE see an exciting new set of opportunities. The defense and nuclear power industries have played a major role in the emergence of NDE. Increasing global competition has led to dramatic changes in product development and business cycles. At the same time, aging infrastructure, from roads to buildings and aircraft, present a new set of measurement and monitoring challenges for engineers as well as technicians.

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Among the new applications of NDE spawned by these changes is the increased emphasis on the use of NDE to improve the productivity of manufacturing processes. Quantitative nondestructive evaluation (QNDE) both increases the amount of information about failure modes and the speed with which information can be obtained and facilitates the development of in-line measurements for process control.

Fig. 2 - simulation

The phrase, "you cannot inspect in quality, you must build it in," exemplifies the industry's focus on avoiding the formation of flaws. Nevertheless, manufacturing flaws will never be completely eliminated and material damage will continue to occur in-service so continual development of flaw detection and characterization techniques is necessary.

Advanced simulation tools that are designed for inspectability and their integration into quantitative strategies for life management will contribute to increase the number and types of engineering applications of NDE. With growth in engineering applications for NDE, there will be a need to expand the knowledge base of technicians performing the evaluations. Advanced simulation tools used in the design for inspectability may be used to provide technical students with a greater understanding of sound behavior in materials. UTSIM, developed at Iowa State University, provides a glimpse into what may be used in the technical classroom as an interactive laboratory tool.

As globalization continues, companies will seek to develop, with ever increasing frequency, uniform international practices. In the area of NDE, this trend will drive the emphasis on standards, enhanced educational offerings, and simulations that can be communicated electronically. The coming years will be exciting as NDE will continue to emerge as a full-fledged engineering discipline.

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3.3. Wave Propagation

Ultrasonic testing is based on time-varying deformations or vibrations in materials, which is generally referred to as acoustics. All material substances are comprised of atoms, which may be forced into vibrational motion about their equilibrium positions. Many different patterns of vibrational motion exist at the atomic level; however, most are irrelevant to acoustics and ultrasonic testing. Acoustics is focused on particles that contain many atoms that move in unison to produce a mechanical wave. When a material is not stressed in tension or compression beyond its elastic limit, its individual particles perform elastic oscillations. When the particles of a medium are displaced from their equilibrium positions, internal (electrostatic) restoration forces arise. It is these elastic restoring forces between particles, combined with inertia of the particles that leads to the oscillatory motions of the medium.

In solids, sound waves can propagate in four principle modes that are based on the way the particles oscillate. Sound can propagate as longitudinal waves, shear waves, surface waves, and in thin materials as plate waves. Longitudinal and shear waves are the two modes of propagation most widely used in ultrasonic testing. The particle movement responsible for the propagation of longitudinal and shear waves is illustrated below.

fig. 3 - wave types

In longitudinal waves, the oscillations occur in the longitudinal direction or the direction of wave propagation. Since compressional and dilational forces are active in these waves, they are also called pressure or compressional waves. They are also sometimes called density waves because their particle density fluctuates as they move. Compression waves can be generated in liquids, as well as solids because the energy travels through the atomic structure by a series of comparison and expansion (rarefaction) movements.

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In the transverse or shear wave, the particles oscillate at a right angle or transverse to the direction of propagation. Shear waves require an acoustically solid material for effective propagation, and therefore, are not effectively propagated in materials such as liquids or gasses. Shear waves are relatively weak when compared to longitudinal waves. In fact, shear waves are usually generated in materials using some of the energy from longitudinal waves.

Modes of Sound Wave Propagation:-

In air, sound travels by the compression and rarefaction of air molecules in the direction of travel. However, in solids, molecules can support vibrations in other directions, hence; a number of different types of sound waves are possible. Waves can be characterized in space by oscillatory patterns that are capable of maintaining their shape and propagating in a stable manner. The propagation of waves is often described in terms of what are called “wave modes.”

As mentioned previously, longitudinal and transverse (shear) waves are most often used in ultrasonic inspection. However, at surfaces and interfaces, various types of elliptical or complex vibrations of the particles make other waves possible. Some of these wave modes such as Rayleigh and Lamb waves are also useful for ultrasonic inspection.

The table below summarizes many, but not all, of the wave modes possible in solids.

Wave Types in Solids Particle Vibrations Longitudinal Parallel to wave direction Transverse (Shear) Perpendicular to wave direction Surface - Rayleigh Elliptical orbit - symmetrical mode Plate Wave - Lamb Component perpendicular to surface (extensional wave) Plate Wave - Love Parallel to plane layer, perpendicular to wave direction Stoneley (Leaky Rayleigh Waves) Wave guided along interface Sezawa Antisymmetric mode

Fig. 4 - Rayleigh wave

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Surface (or Rayleigh) waves travel the surface of a relatively thick solid material penetrating to a depth of one wavelength. Surface waves combine both a longitudinal and transverse motion to create an elliptic orbit motion as shown in the image and animation below. The major axis of the ellipse is perpendicular to the surface of the solid. As the depth of an individual atom from the surface increases the width of its elliptical motion decreases. Surface waves are generated when a longitudinal wave intersects a surface near the second critical angle and they travel at a velocity between .87 and .95 of a shear wave. Rayleigh waves are useful because they are very sensitive to surface defects (and other surface features) and they follow the surface around curves. Because of this, Rayleigh waves can be used to inspect areas that other waves might have difficulty reaching.

Fig. 5 – lamb waves

Plate waves are similar to surface waves except they can only be generated in materials a few wavelengths thick. Lamb waves are the most commonly used plate waves in NDT. Lamb waves are complex vibrational waves that propagate parallel to the test surface throughout the thickness of the material. Propagation of Lamb waves depends on the density and the elastic material properties of a component. They are also influenced a great deal by the test frequency and material thickness. Lamb waves are generated at an incident angle in which the parallel component of the velocity of the wave in the source is equal to the velocity of the wave in the test material. Lamb waves will travel several meters in steel and so are useful to scan plate, wire, and tubes.

With Lamb waves, a number of modes of particle vibration are possible, but the two most common are symmetrical and asymmetrical. The complex motion of the particles is similar to the elliptical orbits for surface waves. Symmetrical Lamb waves move in a symmetrical fashion about the median plane of the plate. Wave motion in the symmetrical mode is most efficiently produced when the exciting force is parallel to the plate. The asymmetrical Lamb wave mode is often called the “flexural mode” because a large portion of the motion moves in a normal direction to the plate, and a little motion occurs in the direction parallel to the plate. In this mode, the body of the plate bends as the two surfaces move in the same direction.

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3.4. Piezoelectric Transducers

Fig. 6 - PzT

The conversion of electrical pulses to mechanical [1] vibrations and the conversion of returned mechanical vibrations back into electrical energy is the basis for ultrasonic testing. The active element is the heart of the transducer as it converts the electrical energy to acoustic energy, and vice versa. The active element is basically a piece of polarized material (i.e. some parts of the molecule are positively charged, while other parts of the molecule are negatively charged) with electrodes attached to two of its opposite faces. When an electric field is applied across the material, the polarized molecules will align themselves with the electric field, resulting in induced dipoles within the molecular or crystal structure of the material. This alignment of molecules will cause the material to change dimensions. This phenomenon is known as electrostriction. In addition, a permanently-polarized material such as quartz (SiO2) or barium titanate (BaTiO3) will produce an electric field when the material changes dimensions as a result of an imposed mechanical force. This phenomenon is known as the piezoelectric effect. Additional information on why certain materials produce this effect can be found in the linked presentation material, which was produced by the Valpey Fisher Corporation.

The active element of most acoustic transducers used today is a piezoelectric ceramic, which can be cut in various ways to produce different wave modes. A large piezoelectric ceramic element can be seen in the image of a sectioned low frequency transducer. Preceding the advent of piezoelectric ceramics in the early 1950's, piezoelectric crystals made from quartz crystals and magnetostrictive materials were primarily used. The active element is still sometimes referred to as the crystal by old timers in the NDT field. When piezoelectric ceramics were introduced, they soon became the dominant material for transducers due to their good piezoelectric properties and their ease of manufacture into a variety of shapes and sizes. They also operate at low voltage and are usable up to about 300oC. The first piezoceramic in general use was barium titanate, and that was followed during the 1960's by lead zirconate titanate compositions, which are now the most commonly employed ceramic for making transducers. New materials such as piezo-polymers and composites are also being used in some applications.

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The thickness of the active element is determined by the desired frequency of the transducer. A thin wafer element vibrates with a wavelength that is twice its thickness. Therefore, piezoelectric crystals are cut to a thickness that is 1/2 the desired radiated wavelength. The higher the frequency of the transducer, the thinner the active element. The primary reason that high frequency contact transducers are not produced is because the element is very thin and too fragile.

Characteristics of Piezoelectric Transducers:-

The transducer is a very important part of the ultrasonic instrumentation system. As discussed on the previous page, the transducer incorporates a piezoelectric element, which converts electrical signals into mechanical vibrations (transmit mode) and mechanical vibrations into electrical signals (receive mode). Many factors, including material, mechanical and electrical construction, and the external mechanical and electrical load conditions, influence the behavior of a transducer. Mechanical construction includes parameters such as the radiation surface area, mechanical damping, housing, connector type and other variables of physical construction. As of this writing, transducer manufacturers are hard pressed when constructing two transducers that have identical performance characteristics.

fig. 7 – PzT components

A cut away of a typical contact transducer is shown above. It was previously learned that the piezoelectric element is cut to 1/2 the desired wavelength. To get as much energy out of the transducer as possible, an impedance matching is placed between the active element and the face of the transducer. Optimal impedance matching is achieved by sizing the matching layer so that its thickness is 1/4 of the desired wavelength. This keeps waves that were reflected within the matching layer in phase when they exit the layer (as illustrated in the image to the right). For contact transducers, the matching layer is made from a material that has an acoustical impedance between the active element and steel. Immersion transducers have a matching layer with an acoustical impedance between the active element and water. Contact transducers also incorporate a wear plate to protect the matching layer and active element from scratching.

The backing material supporting the crystal has a great influence on the damping characteristics of a transducer. Using a backing material with an impedance similar to

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that of the active element will produce the most effective damping. Such a transducer will have a wider bandwidth resulting in higher sensitivity. As the mismatch in impedance between the active element and the backing material increases, material penetration increases but transducer sensitivity is reduced.

Transducer Efficiency, Bandwidth and Frequency:-

nt transmitters and others to be more efficient receivers. A transducer that performs well in one application will not

associated with a transducer. The frequency noted on a transducer is the central or center

Couplant:-

a material (usually liquid) that facilitates the transmission of ultrasonic energy from the transducer into the test specimen. Couplant is generally necessary

o e

Some transducers are specially fabricated to be more efficie

always produce the desired results in a different application. For example, sensitivity to small defects is proportional to the product of the efficiency of the transducer as a transmitter and a receiver. Resolution, the ability to locate defects near the surface or in close proximity in the material, requires a highly damped transducer.

It is also important to understand the concept of bandwidth, or range of frequencies,

frequency and depends primarily on the backing material. Highly damped transducers will respond to frequencies above and below the central frequency. The broad frequency range provides a transducer with high resolving power. Less damped transducers will exhibit a narrower frequency range and poorer resolving power, but greater penetration. The central frequency will also define the capabilities of a transducer. Lower frequencies (0.5MHz-2.25MHz) provide greater energy and penetration in a material, while high frequency crystals (15.0MHz-25.0MHz) provide reduced penetration but greater sensitivity to small discontinuities. High frequency transducers, when used with the proper instrumentation, can improve flaw resolution and thickness measurement capabilities dramatically. Broadband transducers with frequencies up to 150 MHz are commercially available.

A couplant is

because the acoustic impedance mismatch between air and solids (i.e. such as the test specimen) is large. Therefore, nearly all of the energy is reflected and very little is transmitted into the test material. The couplant displaces the air and makes it possible tget more sound energy into the test specimen so that a usable ultrasonic signal can bobtained.

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Fig. 8 – contact testing couplant

contact ultrasonic testing a thin film of oil, glycerin or water is generally used between the transducer and the test surface.

– immersion testing

hen scanning over the part or making precise measurements, an immersion technique is often used. In immersion ultrasonic testing both the transducer and the part are immersed

In

Fig. 9

W

in the couplant, which is typically water. This method of coupling makes it easier to maintain consistent coupling while moving and manipulating the transducer and/or the part.

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3.5. Inspection methods

ents can determine the location of a discontinuity in a part or structure by accurately measuring the time required for a short ultrasonic pulse

= 2d/t

ce from the surface to the discontinuity in the test piece, v is the velocity of sound waves in the material, and t is the measured round-trip transit time.

and 100 MHz, by means of piezoelectric transducers that generate bursts of sound waves

thickness without requiring access to both sides of a part. Accuracy's as high as ±1

ngle Beams I:-

ducers and wedges are typically used to introduce a refracted

Normal Beam Inspection:-

Pulse-echo ultrasonic measurem

generated by a transducer to travel through a thickness of material, reflect from the back or the surface of a discontinuity, and be returned to the transducer. In most applications, this time interval is a few microseconds or less. The two-way transit time measured is divided by two to account for the down-and-back travel path and multiplied by the velocity of sound in the test material. The result is expressed in the well-known relationship

d = vt/2 or v

where d is the distan

Precision ultrasonic thickness gages usually operate at frequencies between 500 kHz

when excited by electrical pulses. A wide variety of transducers with various acoustic characteristics have been developed to meet the needs of industrial applications. Typically, lower frequencies are used to optimize penetration when measuring thick, highly attenuating or highly scattering materials, while higher frequencies will be recommended to optimize resolution in thinner, non-attenuating, non-scattering materials.

In thickness gauging, ultrasonic techniques permit quick and reliable measurement of

micron or ±0.0001 inch can be achieved in some applications. It is possible to measure most engineering materials ultrasonically, including metals, plastic, ceramics, composites, epoxies, and glass as well as liquid levels and the thickness of certain biological specimens. On-line or in-process measurement of extruded plastics or rolled metal often is possible, as is measurements of single layers or coatings in multilayer materials. Modern handheld gages are simple to use and very reliable.

A

Angle Beam Trans shear wave into the test material. An angled sound path allows the sound beam to come in from the side, thereby improving detectability of flaws in and around welded areas.

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fig 10. angle beam 1

Angle Beams II:-

Angle Beam Transducers and wedges are typically used to introduce a refracted shear wave into the test material. The geometry of the sample below allows the sound beam to be reflected from the back wall to improve detectability of flaws in and around welded areas.

Fig. 11 – angle beam 2

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3.6. Data Presentation

Ultrasonic data can be collected and displayed in a number of different formats. The three most common formats are know in the NDT world as A-scan, B-scan and C-scan presentations. Each presentation mode provides a different way of looking at and evaluating the region of material being inspected. Modern computerized ultrasonic scanning systems can display data in all three presentation forms simultaneously.

A-Scan Presentation:-

The A-scan presentation displays the amount of received ultrasonic energy as a function of time. The relative amount of received energy is plotted along the vertical axis and the elapsed time (which may be related to the sound energy travel time within the material) is displayed along the horizontal axis. Most instruments with an A-scan display allow the signal to be displayed in its natural radio frequency form (RF), as a fully rectified RF signal, or as either the positive or negative half of the RF signal. In the A-scan presentation, relative discontinuity size can be estimated by comparing the signal amplitude obtained from an unknown reflector to that from a known reflector. Reflector depth can be determined by the position of the signal on the horizontal sweep.

fig. 12 – A scan and B scan

In the illustration of the A-scan presentation to the right, the initial pulse generated by the transducer is represented by the signal IP, which is near time zero. As the transducer is scanned along the surface of the part, four other signals are likely to appear at different times on the screen. When the transducer is in its far left position, only the IP signal and signal A, the sound energy reflecting from surface A, will be seen on the trace. As the transducer is scanned to the right, a signal from the backwall BW will appear later in time,

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showing that the sound has traveled farther to reach this surface. When the transducer is over flaw B, signal B will appear at a point on the time scale that is approximately halfway between the IP signal and the BW signal. Since the IP signal corresponds to the front surface of the material, this indicates that flaw Bis about halfway between the front and back surfaces of the sample. When the transducer is moved over flaw C, signal C will appear earlier in time since the sound travel path is shorter and signal B will disappear since sound will no longer be reflecting from it.

B-Scan Presentation:-

The B-scan presentations is a profile (cross-sectional) view of the test specimen. In the B-scan, the time-of-flight (travel time) of the sound energy is displayed along the vertical axis and the linear position of the transducer is displayed along the horizontal axis. From the B-scan, the depth of the reflector and its approximate linear dimensions in the scan direction can be determined. The B-scan is typically produced by establishing a trigger gate on the A-scan. Whenever the signal intensity is great enough to trigger the gate, a point is produced on the B-scan. The gate is triggered by the sound reflecting from the backwall of the specimen and by smaller reflectors within the material. In the B-scan image above, line A is produced as the transducer is scanned over the reduced thickness portion of the specimen. When the transducer moves to the right of this section, the backwall line BW is produced. When the transducer is over flaws B and C, lines that are similar to the length of the flaws and at similar depths within the material are drawn on the B-scan. It should be noted that a limitation to this display technique is that reflectors may be masked by larger reflectors near the surface

C-Scan Presentation:-

The C-scan presentation provides a plan-type view of the location and size of test specimen features. The plane of the image is parallel to the scan pattern of the transducer. C-scan presentations are produced with an automated data acquisition system, such as a computer controlled immersion scanning system. Typically, a data collection gate is established on the A-scan and the amplitude or the time-of-flight of the signal is recorded at regular intervals as the transducer is scanned over the test piece. The relative signal amplitude or the time-of-flight is displayed as a shade of gray or a color for each of the positions where data was recorded. The C-scan presentation provides an image of the features that reflect and scatter the sound within and on the surfaces of the test piece.

Fig. 13 – C scan

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High resolution scans can produce very detailed images. Below are two ultrasonic C-scan images of a US quarter. Both images were produced using a pulse-echo technique with the transducer scanned over the head side in an immersion scanning system. For the C-scan image on the left, the gate was setup to capture the amplitude of the sound reflecting from the front surface of the quarter. Light areas in the image indicate areas that reflected a greater amount of energy back to the transducer. In the C-scan image on the right, the gate was moved to record the intensity of the sound reflecting from the back surface of the coin. The details on the back surface are clearly visible but front surface features are also still visible since the sound energy is affected by these features as it travels through the front surface of the coin.

Fig. 14 – high resolution scans of US quarter

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4. HOMC Guided waves study

4.1. Introduction [3]

Pipes that rest on supports tend to develop localized pinhole-type pitting corrosion and/or stress corrosion crack-like defects; both of these may severely affect the structural integrity [4, 5]. To avoid sagging, long pipes usually rest on supports. The crevice or bottom region of pipes, at the support locations, is more prone to corrosion due to the presence of water, air, and minerals as well as additional local contact stresses. Several types of corrosion are found to occur. The more common types include the wide area corrosion that leads to wall thickness reduction, and the pitting corrosion leading to through thickness pinhole damage. Among the various type of corrosion in pipelines, pinhole-type pitting corrosion (localized in nature) is considered to be of prime importance due to the degree of difficulty of detection and the increased risk for causing leaks. The pipe surface at these support locations may appear to be normal during visual inspection of the visible surface (i.e. top and side region of pipe), while the condition of the pipe in the hidden region (i.e. bottom portion), where it is in contact with the support cannot be evaluated. These pipelines carry hazardous chemicals and gasses and the presence of a small leak or rupture in lines may lead to a catastrophic situation. Hence, these lines have to be inspected periodically to assess them for any possible damage before a catastrophic failure occurs [6, 7]. This calls for a reliable non-destructive evaluation technique that is capable of detecting and sizing the defects at such inaccessible regions. Conventional ultrasonic methods of wall thickness measurements of pipe at support locations is not feasible in the crevice region, and visual inspection is difficult and time consuming as the pipe has to be lifted out of the supports, a risky operation, particularly if the pipe is corroded. Unlike conventional ultrasonic inspection methods, axial guided waves can be used to inspect the pipe at the support locations. In this method, the guided waves are generated at the accessible regions of the pipe. Generally, guided wave techniques excite ultrasonic waves at lower frequency than used in this study and hence require the size of the defects to be relatively large in order to detect them. Hence, there is a need for a technique to detect small pitting while simultaneously imaging the defect and giving its circumferential location. Some of the methods mentioned above need advanced signal processing whereas the proposed technique will reduce the computational resources required for signal processing. The objective of this study is to describe a new method of applying higher order ultrasonic guided waves for detecting and sizing circumferential crack and pinhole-like defects that are located in inaccessible regions such as pipe. One convenient means of generating these guided waves is with wedge based piezoelectric transducers. The following transducer configuration has been used for generating these guided waves: 2.25MHZ, 1 inch dia conventional circular transducer. The guided wave was generated in the pipe by choosing the optimum coupling angle for the mode that was to be generated.

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Perspex coupling ‘‘wedges’’, shaped to match the pipe curvature, were used to define a range of coupling angles near the optimum value determined from calculations made using DISPERSEs [8, 9]. The generation and reception of the guided waves can be implemented using either the pulse–echo or through-transmission mode [10]. All the experiments in this study used the pulse–echo configuration for the detection of defects. The use of larger frequency times thickness values for the guided wave modes was investigated because of the potential for a higher frequency to produce a larger back scattered amplitude and, hence, possibly a larger signal-to-noise ratio (SNR), for small defects. As shown in the results presented in this study, the chosen guided wave modes at large values of frequency_thickness product (greater than 20 mm-MHz) have very small dispersion. It was observed that these cylindrically guided ultrasonic Higher Order Modes Cluster (HOMC) propagate distances on the order of 100 wavelengths without exhibiting noticeable dispersion (broadening) and hence they can be used to obtain a higher sensitivity (higher SNR) to scattering from small defects than can be (and which has been) achieved using the two more common guided wave modes, S0 and A0, in the range of 2 mm-MHz. 4.2. The ultrasonic measurement systems All measurements were performed on mild steel (MS) pipe specimens (commonly used in refineries and chemical plants) with the collection of artificial drill hole defects shown in Fig. 15 using the curved coupling ‘‘wedge’’ shown in Fig. 16. It becomes very important to choose the optimum angle for a particular frequency-thickness combination for identifying the optimal mode(s) for defect detection. The commercially available software DISPERSEs was used to generate the dispersion curves (which, for example, is a plot of group velocity versus frequency-thickness product) for the 10.0mm wall thickness MS pipe specimen for a transducer frequency of 2.25 MHz. The first order anti-symmetric mode (A1) was selected as the desired mode for inspection and the optimum wedge angle for inspection were estimated to be 56 degree in Plexiglas for 2.25 MHz. The dispersion curves obtained for the pipe specimen are shown in Fig. 17. Since several modes may be generated in this region, sufficient care must be taken during wedge manufacturing to ensure that primarily one mode is generated. Also, the size of the conventional probes used in these studies was selected to be relatively large (25mm or larger) in order to minimize the beam divergence. The data presented in this paper was acquired using the 2.25MHz center frequency.

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Fig. 15 – the defect test pipe

Defects are of 10 mm dia (at a depth of 25, 50, 75 and 100 % of wall thickness):-

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Fig. 16 – Wedge Designs (axial curve)

Side view Front view

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Fig. 17 – disperse curves

CLUSTERING

As you can see from the DISPERSE plot. The blue lines are the A modes (anti symmetric) and the red lines are the S modes (symmetric) You can see clearly how all the modes cluster together at angle 56. at the higher frequency thickness range. This wave is the HOMC wave. It has high energy and travels over long distances.

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4.3. Experiment The experiment involves using the HOMC guided waves to detect the defects in the above mentioned conditions. We use a NI DaQ card and a pulsar receiver for this experiment. The set up is shown in fig. 18. We also use a labview software for the data acquisition to get the A scans of the experiment.

Fig. 18 – laptop, daQ card and Pulsar receiver to excite the PzT

PROCEDURE:-

1. Place the pipe on a support such that the 10mm dia 25% thickness artificial defect hole is on top.

2. Take measurements on distances from the hole at 4, 8, 12, 16, 20 cms. 3. Affix the 2.25 MHZ PzT into the wedge. Make sure to use gel. 4. Place gel all over the pipe surface and make the wedge come in contact with it. 5. Keep the wedge at 4cm distance and acquire the A scan in the labview software. 6. Move the wedge throughout distances 8, 12, 16 and 20 cm and acquire scan. 7. Repeat the experiment with all other holes and acquire scans.

The experiment was performed for all the 4 holes. 5 readings were taken per hole as we move the wedge further and further away from the defect. The resulting A scans were obtained. This resulted in the following 20 scans.

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4.4. Resulting Data The resulting scans were obtained:- It was obtained in the following lab view A scan software designed in CNDE itself.

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10 MM 25 % (4, 8, 12, 16, 20 cm)

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10 MM 50 % (4, 8, 12, 16, 20 cm)

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10 MM 75 % (4, 8, 12, 16, 20 cm)

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10 MM 75 % (4, 8, 12, 16, 20 cm)

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5. CONCLUSIONS

1. As you can see from the scans: - the amplitude of the returning wave decreases as we move away from the defect. Maximum peak is when it’s closest.

2. We also note that the spread of defect time increases as we move away from the

defect.

3. we see that the amplitude of the returning wave is more for the deeper defect (100 % thickness)

4. Using the basic time, distance equation, we calculate the speed of the wave to be 3070 m/s. this is accurate with the A1 mode generated in non dispersive region.

5. We have used and studied the application of HOMC guided waves for defect detection in pipes.

6. The range has been accurately measured upto 20 cm with clear results. Thus proving the HOMC waves propagate over long distances.

7. We have established the importance of HOMC guided waves as a good alternative and more accurate measuring system than conventional ultrasonic testing.

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6. REFERENCES

1. www.cnde-iitm.ac.in 2. www.ndt-ed.org

3. L. Satyarnarayan, J. Chandrasekaran, Bruce Maxfield, Krishnan Balasubramaniam

Circumferential higher order guided wave modes for the detection and sizing of cracks and pinholes in pipe support regions. NDT&E International 41 (2008) 32–43

4. Woodtli J, Kieselbach R. Damage due to hydrogen embrittlement and stress corrosion

cracking. Eng Failure Anal 2000;7(6): 427–50.

5. USNRC. An approach for plant specific, risk- informed decision making: in-service inspection of piping. Draft Regulatory Guide DG- 1063, 1997; Washington, DC: US Nuclear Regulatory Commission.

6. Lowe MJS, Alleyne DN, Cawley P. Defect detection in pipes using guided waves.

Ultrasonics 1998;36:147–54.

7. R/D Tech Guideline. Introduction to phased array ultrasonic technology applications. 1st ed. Canada: R/D Tech Inc; 2004.

8. Grace OD, Goodman RR. Circumferential waves on solid cylinders. J Acoust Soc Am

1966;39:173–4.

9. Liu G, Qu J. Guided circumferential waves in a circular annulus. ASME J Appl Mech 1998;65:424–30.

10. Wang DW. Applications of guided wave technique in the petrochemical industry. AIP

Conf Proc Rev Prog QNDE 1998;18: 277–84.