TN 09 Development of long range ultrasonic techniques for ... · TN09 Placement report Development of long range ultrasonic technique (LRUT) for different engineering assets Camille-Paul

Etudiant : Camille-Paul GUIGON A2007 Suiveur UTC : Mr Abdelouahed LAKSIMI

TN 09

Development of long range ultrasonic techniques for different engineering assets

Entreprise : TWI Lieu : Cambridge (England) Responsable : Chiraz Ennaceur

TN09 Placement report Development of long range ultrasonic technique (LRUT) for different engineering assets

Camille-Paul GUIGON UTC Autumn 2007 - 1 -

Acknowledgements

All along this placement, I have been welcomed by most of the employees of

TWI, from every department. I would like to acknowledge all these people for the time, the help, the advice, and the kindness they granted me.

I would particularly like to acknowledge:

- Ms Chiraz Ennaceur, the person in charge of me at TWI, for the confidence, the autonomy, and all the knowledge she passed on to me; as well as for her limitless kindness and happiness which made this placement so interesting and enjoyable,

- Mr Tat-Hean Gan, manager of the LRU section, for his welcome and

help,

- Mr Menno Hoekstra, Mr Nathan Decourcelle, Ms Angélique Raude, Mr Slim Soua, Ms Viviane Beaugrand, Mr Lee Smith and Mr Channa Nageswaran for their kindness, great help and advice,

- Mr Alex Haig, Mr Phil Catton, Mr Yousef Gharaibeh, Ms Septimonette

Chan, Mr Vichaar Dimlaye, students, for their knowledge and help as well as the good cheer and friendship atmosphere at “the student corner”

- Ms Kena Makaya and Pi, for the Teletest equipment and their kindness

- All the people I used to meet up after work, making this placement so

unforgettable,



Contents

Acknowledgements .................................. ............................................................ - 1 - Contents .......................................... ...................................................................... - 2 - Introduction ...................................... ..................................................................... - 3 - 1. Overview of the TWI Group ......................... ................................................... - 4 -

1.1 TWI Ltd ........................................................................................................ - 4 - 1.2 TWI Granta Park .......................................................................................... - 4 - 1.3 The NDT department ................................................................................... - 5 -

2. Placement subject ................................. .......................................................... - 7 -

2.1 LRUCM ........................................................................................................ - 7 - 2.2 Assets addressed during my placement ...................................................... - 9 -

3. State of Art ...................................... ............................................................... - 12 -

3.1 NDT techniques ......................................................................................... - 12 - 3.2 LRUT: Long Range Ultrasonic Testing ...................................................... - 14 -

3.2.1 Ultrasonic Testing (UT) background ............................................................................ - 14 - 3.2.2 LRUT principles ........................................................................................................... - 19 - 3.2.3 Teletest© equipment .................................................................................................... - 22 -

4. The development of LRUT in different assets ....... ..................................... - 23 -

4.1 Rail investigation ........................................................................................ - 23 - 4.1.1 First problem ................................................................................................................ - 23 - 4.1.2 Experimental results of the rail web ............................................................................. - 24 - 4.1.3 Experimental results of the rail head ........................................................................... - 26 - 4.1.4 Experiental results of the rail Foot ............................................................................... - 27 - 4.1.5 Conclusion ................................................................................................................... - 29 -

4.2 Sheet Pile investigation .............................................................................. - 31 - 4.2.1 Excitation of a specific mode and minimisation of the edge effect .............................. - 32 - 4.2.2 Impact of sand on wave propagation through an aluminium plate .............................. - 34 - 4.2.3 Generation of only one mode through the plate .......................................................... - 36 - 4.2.4 Defect detection ........................................................................................................... - 38 - 4.2.5 Conclusion ................................................................................................................... - 40 -

4.3 Bended Pipe investigation ......................................................................... - 41 - 4.3.1 Description of the sample ............................................................................................ - 41 - 4.3.2 The Time Reversal Method ......................................................................................... - 41 - 4.3.3 Program Time Reversal ............................................................................................... - 42 - 4.3.4 Defect detection with the Time Reversal method ........................................................ - 42 - 4.3.5 comparison Time Reversal and focusing .................................................................... - 45 - 4.3.6 Conclusion ................................................................................................................... - 48 -

Conclusion ........................................ .................................................................. - 49 - References ........................................ .................................................................. - 50 - Appendix .......................................... ................................................................... - 51 -



Introduction

Guided wave inspection of pipelines is now used all around the world routinely. Indeed, the technique offers the possibility of rapid screening of long lengths of pipe for corrosion and other defects. A test range of 60m or more (30m in each direction) is commonly obtained from a single transducer position. Long range guided wave inspection techniques are also in development for several other applications, including the detection of corrosion in large areas of plates, the detection of cracking in railway lines, and the detection of corrosion in heat exchanger tubing.

During my mechanical engineering degree at UTC, I have done my TN09 in a

world centre for materials joining technology named The Welding Institute (TWI) in Cambridge, UK. During this placement, I was greeted by the Non-Destructive Testing department (NDT), in the Long Range Ultrasonic section (LRU). As mentioned below, Long Range Ultrasonic Testing (LRUT) also called guided wave inspection, is about to expend its use to new assets. My placement subject was thus: the Development of long range ultrasonic techniques for different engineering assets. It is part of a €4.3m European project called: Long range Ultrasonic Condition Monitoring of Engineering Assets (LRUCM), aiming to develop new technologies for the maintenance and inspection of European engineering assets. This placement is also a good opportunity to have an overview of the NDT techniques, one of the famous areas covered by the materials science, in order to settle my choice in the “MIT filière”.

This report presents the work addressed during my placement at TWI. After an

overview of the company and the NDT department, it gives a detailed presentation of the work carried out during my student placement as well as of the state of art of NDT. The three most important application investigated during this placement are then explained. And fianalyy the conclusion and the discussion of the results are presented.



1. Overview of the TWI Group

1.1 TWI Ltd

TWI Ltd is a subsidiary of the Welding Institute Group (a world centre for materials joining technology). It is membership-based company (opposed to the usual shareholder-based company) counting, at present, approximately 3500 members from 60 countries all around the world. TWI Ltd is also different in sense of it is a non-profit distributing company. Based at Great Abington near Cambridge since 1946, TWI is now one of the world’s foremost independent research and technology organisations. It is the only single source of expertise in every aspect of joining technology for engineering materials, known as metals, plastics, ceramics and composites.

Employing more than 550 skilled staff, TWI provides industry with engineering

solutions in welding, joining and associated technologies through: • Information; • Advice and technology transfer; • Consultancy and project support; • Contract R&D; • Training and qualification; • Personal membership.

TWI laboratories and offices are mainly established in five locations : TWI Granta

Park (described below), TWI South East Asia (Training and Certification Center), TWI Technology Centre (North East), TWI Technology Centre (Wales)(NDT Technology), TWI Technology Centre Ltd (Yorkshire)(Welding Technology).

TWI provides its services all around the world. Therefore, it also has to deal with

competition. Every research organisation, such as colleges, national research centres, can be seen as a competitor. However, only few of them have the same “membership-based company” feature. For example, TWI American competitor can be found in the Edison Welding Institute (EWI) which is the largest and primary welding technical organization in the US. This institute covers all areas of welding available and has exactly the same structure as TWI. Indeed, it was founded in 1984 with the help of TWI, which transferred membership of 85 U.S customers to EWI. In other words, since TWI doesn’t sell any product, we can talk about competition. In most of the cases, lots of academic and industrial partners share their skills always to go further in innovation and improvement.

1.2 TWI Granta Park

TWI Granta Park (see Figure 1) is the main spot of TWI group. It was formed in March 1968 by the amalgamation of the Institute of Welding and the British Welding Research Association (BWRA). On site, TWI Ltd, Plant Integrity Ltd, The Test House (Cambridge) Ltd, TWI Certification Ltd, Abington Hall Conference Centre and Mary's Ltd are gathered. (Figure 44, in appendix, provides a map of TWI Granta Park).



TWI Granta Park thus has all the engineering knowledge available in the group; which covers for example know-how around:

• Materials properties and applications (metals, polymers, ceramics, advanced

composites, structural integrity, fracture, design and NDT etc), • Joining and fabrication technology (Welding and cutting processes, surface

engineering, brazing, soldering, adhesive bonding), • Manufacturing (Project management, decision support, manufacturing

systems, health and safety, quality assurance).

Figure 1 TWI Granta Park

1.3 The NDT department

TWI always has the interest in NDT technology and its research since early 1960’s. NDT department is one of the most important departments in TWI. Gathered in NDT are lots of Non-destructive testing techniques which aim to detect flaws in structures before any catastrophic failure takes a place. NDT is a testing method which does not destroy the testing object during the test. NDT have a large marketing potential in different industries as aerospace, automotive, defence, pipelines, power generation, preventative maintenance, pulp and paper, refinery, and shipbuilding and others. At the moment, the TWI NDT department is participating in research programs worth £30 million as part of the European collaborative projects. Besides, NDT department is also helped by a number of academic and industrial partners, all sharing their skills and resources to find solutions and bring the science to a step forward. The NDT department in TWI is divided into four groups:

• NDT technology group : NDT is one of the fastest growing groups in TWI. The group is running by professional expertise to provide their consultant in



choice of technique, method optimisation, detection limitations, assessment of fitness for purpose etc. The group is heavily involved in European and core projects as well as group sponsor projects.

• Long Range Ultrasonic technologies group (LRUT grou p): Conventional

ultrasonic testing techniques and methods have been around for centuries. However, at present, a new emerging technology, the so-called “long range ultrasonic technology” is under the process of expanding its knowledge, developments, industrial utilisation and equipments. Based on the demand and prospects of this new emerging technology, NDT department in TWI have thus established a new group called “Long Range Ultrasonic technologies group”. This group aimed to utilise the usage of a relatively low frequency (from 20kHz to 1MHz) ultrasonic wave to detect defects, flows, and imperfections distance away from ultrasonic wave source. The advantage of this technology is the ability to detect metal loss in pipes when the pipe is; insulated, buried under the roads also a significant length of the pipe can be inspected by accessing a small location.

• Plant Integrity Ltd (P i): Plant Integrity Ltd (Pi) is another subsidiary of TWI

group, work along with LRUT group under the NDT department. Plant Integrity Ltd (Pi) was setup in 1997. The aim of this subsidiary is to commercialise LRUT technology. Pi have developed and produced a product called Teletest®. The functionality of Teletest® is to perform a pipeline scanning using LRUT techniques.

• Training and Examination Services : TWI also provides a professional

training and examination for their customers for different kind of NDT methods under the Certification Scheme for Welding and Inspection Personnel (CSWIP) in the 1970s.



2. Placement subject

My placement subject was to develop long range ultrasonic techniques (LRUT) for different engineering assets such as rails, sheet piles, heat exchanger tubes or lamp posts. This subject is actually a part of one of the biggest projects won by TWI NDT. This European project is called LRUCM (Long Range ultrasonic Condition Monitoring of Engineering Assets) and worth approximately €4.3m (see description below). Besides this project, which represents 85% of my work, I have also been able to work on a smaller onecalled RISERTEST (Development of a Guided Long Range Ultrasonic Inspection System for the examination of offshore subsea Risers, Steel Catenary Risers (SCRs) and Flowlines; worth €2m). More specifically, it is to develop the world’s first technology to continuously monitor deepwater steel catenary risers in service, to develop a novel LRUT technology to detect corrosion in sub-sea flowlines and fatigue cracks and corrosion in sub-sea risers.

During my placement, my work was to carry out the experiments of the LRUCM

project (and RISERTEST, occasionally) as well as to write a report gathering and interpreting the data collected. I thus worked with the senior project leader Dr Chiraz Ennaceur in her project LRUCM and Menno Hoekstra in his project RISERTEST.. The NDT department counts technicians, EngD students, senior or project leaders, consultants, managers among its members. My status and role was to help and assist Chiraz Ennaceur for the experimental work and data analysis. Indeed, every member of the staff usually shares his time between different projects whereas I mainly worked on one. Nevertheless it wasn’t a bad thing, because I could thus work closer to one specific engineer and follow every step of the project.

In this part, a brief overview of the LRUCM project is provided as well as a quick

description of the assets I worked on.

2.1 LRUCM

The project aims to develop new technologies for the maintenance and inspection

of European engineering assets which are now ageing, thus posing a considerable risk of structural failure due to degradation mechanisms such as corrosion or various forms of environmentally assisted cracking (fatigue, corrosion fatigue, stress corrosion, etc.).

To illustrate these threats, the following table gives examples of assets with their

incurred risks.



Asset Risks

Oil and Gas Pipes

Europe has 200,000km of cross-country pipeline carrying natural gas, crude oil and various refined products, as well as approximately 10 million-km of pipework in refineries, etc. carrying hazardous fluids.

Spillages from transmission pipelines in Western Europe over the last 25 years have averaged 14 per year. These incidents involved a total of 1,000 fatalities and serious injuries.

Rails

There are 512,000km of rail in Europe

Two thousand six hundred rail fractures occur every year causing rail delays and derailments. One such failure (Hatfield, UK) led to 4 fatalities, 30 casualties and costs to the track owner of €800 million.

Cable Stayed/Suspension Bridges

Europe has more than 300 cables stayed and suspension bridges.

Most of the cable stayed and suspension bridges around the world are in serious danger because corrosion is attacking their cables.

Sheet-Piled River Wall and Sea Defences

There are thousands of kilometres of such defences throughout Europe.

Sheet piling is corroding endangering their integrity and risking high value real estate due to the consequential flooding. This is likely to be aggravated as global warming leads to higher sea levels and increased loading on the structures.

Heat Exchanger Tubing

There are many tens of thousands of heat exchangers operating in Europe’s petrochemical and process plant industries.

Current inspection methods require heat exchangers to be withdrawn from service and thoroughly cleaned prior to inspection and NDT. LRUT gives the ability to inspect from one of the tube only with no requirement for tube cleaning other than the point of contact for the transducer on the internal diameter.

Table 1 Examples of engineering assets and their respective risks

The annual European maintenance and inspection budget for the above engineering assets is approximately €225 milliards. However, despite this budget, structural failures of engineering assets still kill people.



The LRUCM project thus aims to develop a variety of novel applications of long range guided wave techniques, to help the continuing condition monitoring of these assets. This includes:

• Establishing that long range ultrasonic using guided waves is a viable method of long term monitoring of the structural integrity of engineering structures and components.

• Developing a way to inspect small diameter heat exchanger tubes (less than 50mm diameter). This requires tests to be performed at much higher frequencies than used for large diameter pipes. Besides, the transducer must be placed inside the tube, requiring a completely new approach to transducer array design.

• Studying the behaviour of guided waves in corrugated plates for the examination of sheet piling. Both theoretical and practical developments are required for this type of component, to determine the inspectability, the sensitivity to defects and means of attaching arrays on to flat surfaces.

• Designing an effective long range inspection process for rails, which means studying: the geometry of the railroad rails, studying the types of wave which may be generated; the distribution of energy (between the rail head, web and foot for each wave mode), the sensitivity to defects in each area of the rail; and developing methods of generation and reception of the required modes.

• Designing an effective long range inspection process for both stranded cables and solid tendons used in bridges.(see point above)

The LRUCM project has thus different objectives in terms of technology (develop sensors and systems for finding defects and corrosion in a wide range of engineering assets), economy, society and environment.

2.2 Assets addressed during my placement

The LRUCM project has already been running for two years and engineers and researcher working on it all around the world. My role wasn’t thus to provide a complete solution for one of the asset, but to carry out experiments validating of some modelling results or to carry out experiments providing investigating a discussed idea or solution of a specific problem. The following table gives a quick description of the assets I work on.



Assets Description

Rail

My Objectives

- Validate the

modelling results of Head, the Web and the Foot of the rail

- Detect defects

Heat exchanger tube

My Objectives

- Test the new prototype on different samples

- Detect defects

Sheet pile

My Objectives

- obtain a good pulse echo signal

- Study the scattering of the signal

- Study the attenuation in the plate

- detect defects - Validate

modelling results



Pipework

My Objectives

- Detect defects - Investigate the

capabilities of the Time Reversal method

- Compare the Time Reversal method with the time delay Focusing method

Bridge cable

My Objectives

- Investigate the guided wave in complex geometry

Table 2 LRUCM assets

The above assets are only the most important works that I had to do. In the following parts, I will however only present the the resultys related to the three applications listed below:

• Rails • Sheet piles • Pipeworks

A work planning is not given in this report because it wasn’t defined since the

beginning of my placement. Indeed, my objective was to work on as many assets as possible. My placement being pure research, an accurate schedule couldn’t be made over a short period of 24 weeks since that according to the results, studying an asset could last for 2 weeks or 2months. Besides, assets, equipment and resources were not available all the time. Therefore, when all the conditions were gathered to work on one specific application, the latter had the priority.

The following part provides a description of NDT techniques focusing on the

ultrasonic testing and especially LRUT, as well as a presentation of the Teletest equipment which has been used to carry out most of the experiments.



3. State of Art

3.1 NDT techniques

The Nondestructive Testing (NDT) (also called NDE, NonDestructive Evaluation) has known a real boom since the end of the Second World War. Its main aim is to assure that structural components and systems perform their function in a reliable and cost effective fashion. Using such techniques can characterise materials conditions or detect flaws that might lead to planes crashes, reactors failure, pipelines bursts, etc.. Being non-destructive, NDT doesn’t either affect the future usefulness of the object and also allows the inspection to be made while the structure is in service. NDT therefore provides an excellent balance between quality control and cost-effectiveness.

NDT counts nowadays a wide amount of different methods that can be used for inspection. Researchers actually continue to find new technologies to reduce drawbacks and enhance performances. Nevertheless, there are six NDT methods that are used most often. They are briefly presented in the table below:

• Visual and Optical Testing (VT) Visual inspection involves using an inspector's eyes to look for defects. The

inspector may also use special tools such as magnifying glasses, mirrors, or borescopes to gain access and more closely inspect the subject area. The inspectors follow procedures which can be simple to very complex.

• Penetrant Testing (PT)

The Penetrant Testing (see Figure 46 in appendix) is based on the capillary feature of defects. It means that first a penetrant solution is applied to the surface of a precleaned component. After the liquid has gone into the surface-breaking defects, the excess of penetrant is carefully cleaned from the surface and a developer is applied to pull the trapped penetrant back to the surface, where it forms a defect indication. The indication is much easier to see than the actual defect.

• Magnetic Particle Testing (MT)

This NDE method is accomplished by inducing a magnetic field in a ferromagnetic material (see Figure 47 in appendix). The magnetic lines of force travel through the material and exit and re-enter the material at the poles. Defects such as cracks or voids cannot support as much flux so part of it is forced to go out. Magnetic particles distributed over the component will then be attracted by the flux leakage and produce a visible indication.

• Electromagnetic Testing (ET) or Eddy Current Testin g

The scientific principle of Eddy Current Testing (Figure 48 in appendix) is as follow. An Alternating electrical current is passed through a coil producing a magnetic field. When the coil is placed near a conductive material, the changing magnetic field induces current flow in 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 and used to find flaws and characterize conductivity, permeability, and dimensional features.

• Radiography (RT)

Radiography (see Figure 49 in appendix) involves the use of penetrating gamma or X-radiation to produce images of objects using film or other detector that is sensitive to radiation. The sample is placed between the radiation source and detector. The thickness and the density of the material, that X-rays must penetrate, affect the amount of radiation reaching the detector. This variation in radiation produces an image on the detector, symbolising internal features (cracks or others) of the sample.

• 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 sound waves travel through the material and are received by the same transducer (pulse-echo technique, see Figure 50 in appendix) or a second transducer. The amount of energy transmitted or received and the arrival time of the energy are analysed to determine the presence of defects. Changes in material thickness and changes in material properties can also be measured.

These conventional methods for inspecting engineering assets have been in use for approximately 50 years. However, they have the following major drawbacks:

• From a given position, only a very small area can be inspected as their maximum range is measured in tens of millimetres. Thus they require many man months of effort to inspect large structures e.g. pipelines and pipework, bridge cables, long sections of rail etc..

• They require direct access to the entire structure, i.e. all insulation and coatings must be removed and buried components must be exposed by excavation. Access cost typically exceeds inspection cost by a factor of 5-10 and overall cost is prohibitively high.

• They are so time consuming and costly that the amount of inspection actually carried out is far less than is required to ensure long-term structural integrity.

Knowing that the above presented techniques have some very inconvenient

drawbacks, scientists have developed new technologies to overcome them. One of these technologies is known as the Long Range Ultrasonic Testing (LRUT), and the technology that I used during my placement. The following part introduces its principles.



3.2 LRUT: Long Range Ultrasonic Testing

3.2.1 Ultrasonic Testing (UT) background

Ultrasonic testing is based on time-varying deformations or vibrations in materials. All materials substances contained atoms, atoms which can be moved from their equilibrium positions into a vibrational motion. UT focuses on particles that contain many atoms that move in unison in order to produce a mechanical wave. If particles are displaced from their equilibrium positions, the internal restoration forces between particles, combined with inertia of the particles will then lead to oscillatory motions.

In solids, there are four principle modes (=kind of wave propagation) for the propagation of sound waves. These modes are described below:

• Longitudinal waves

The oscillations occur in the direction of wave propagation (longitudinal direction). Longitudinal waves are also called compression, pressure, or density waves. They can be generated in solids and liquids as they propagate thought the atomic structure by a series of compression and expansion movements.

Figure 2 Longitudinal waves

• Shear waves

Sheer waves are week waves with respect to the longitudinal waves as they are generated by using the longitudinal energy from the longitudinal waves .Sheer waves are also known as transverse waves, since the particles oscillate at a right angle to the direction of propagation. They can only propagate through solids.

Figure 3 Shear waves

• Plate waves (in thin materials)

Plate waves propagate only in very thin materials. The most commonly used plate waves are Lamb waves which can travel through the entire thickness of the material.



However, their propagation depends on the density and the elastic material properties of the sample. With Lamb waves, a number of modes of particle vibration are possible, but the two most common are symmetrical and asymmetrical.

Symmetrical mode Asymmetrical mode

Symmetrical Lamb waves move in a symmetrical fashion about the median plane of the plate, which means that the longitudinal components are equals and the transversal ones are opposed.

Asymmetrical mode is also called flexural mode because the vibrated particles are mostly in the normal direction to the plate and also few particles vibrate in the parallel direction. In this mode, the longitudinal components are opposed and the transversal ones are equals.

Figure 4 Plate waves

• Surface waves (in thick materials)

Surface waves are also called Rayleigh waves, as the particles penetrate the materials in an elliptical orbit. Surface waves usually penetrate thick solid materials with a depth of one wavelength. Surface waves are useful in detecting surface defects for their high sensitivity.

Figure 5 Surface waves

Longitudinal and shear waves are the two modes of propagation the most widely used in ultrasonic testing.

Even though these wave modes are all different, they also share common properties. The most important of them is that the wavelength is directly proportional to the velocity of the wave and inversely proportional to the frequency of the wave. This relationship is shown by the following equation:

)(

)()(

ffrequency

cVelocityWavelength =λ



For a specific material, a wave mode velocity is fixed. The above equation thus shows that, for instance, the higher the frequency, the shorter the wavelength. A change in the frequency will therefore result in a change of the wavelength. With experience, it has been agreed that a discontinuity must be larger than one-half the wavelength to have a reasonable chance of being detected. The significant length of the wavelength is thus very important, as it is directly related to the probability of detecting a discontinuity within the materials. However, varying wavelength will have a change on the sensitivity and resolution, as they are function of frequency. Sensitivity is defined like the ability to locate small discontinuities. Sensitivity generally increases with higher frequency (shorter wavelengths). Resolution is the ability of the system to locate discontinuities that are close together within the material or located near the part surface. Resolution also generally increases as the frequency increases. We should also keep in mind that high frequencies (which have an impact on other physical variables like penetration power, shape and the divergence of ultrasonic beam) as well as the backing material, the transducer material, the property of the crystals, the input voltage, the pulse length and other factors will also affect the ability of ultrasound to detect defects.

We saw that sound travels at different speeds in different materials. This is

because the density and the elastic constants of different materials are different. The general relationship between the speed of sound in a solid and its density and elastic constants is given by the following equation:

ρC

V = V : speed of sound ρ : material density C : elastic constant

Where V is the speed of sound, C is the elastic constant, and p is the material

density. Other sound properties have to be taken into account when devices, tools,

softwares, etc. are designed or signals analysed, for UT inspection. These main sound properties are: attenuation, acoustic impedance, reflection, refraction, signal to noise ratio and wave interference.

Sound attenuation is the diminution of the sound intensity when the sound travels

through a material. It is the combined effect of scattering and absorption of the sound. Scattering is defined as the reflection of the sound in directions other than its original direction of propagation and absorption is the conversion of the sound energy to other forms of energy. The attenuation phenomenon can be described by the following equation:

ZeAA α−= 0

A0 : amplitude of the propagated wave at some point A : reduced amplitude after a Z distance α : Attenuation coefficient in the Z-direction Z : distance from the initial location

Therefore, by knowing the attenuation that an ultrasound beam experiences

travelling through a material, the input signal amplitude can be adjusted to compensate any undesired loss of energy.



Sound travels through materials under the influence of sound pressure. In fact in a solid, the excess pressure results in the propagation of waves, as molecules or atoms of this solid are bound elastically to one another. The acoustic impedance is then important in the determination of acoustic transmission and reflection at the boundary of two materials having different acoustic impedances, the design of ultrasonic transducers and assessing absorption of sound in a material. The acoustic impedance (Z) of a material is defined as the product of its density (ρ) and acoustic velocity (V).

VZ ρ=

Ultrasonic waves are reflected at boundaries where there is a difference in acoustic impedances of the materials on each side of the boundary. This difference in Z is commonly referred to as the impedance mismatch. The greater the impedance mismatch, the greater the percentage of energy that will be reflected at the interface or boundary between one material and another. When the acoustic impedances of the materials on both sides of the boundary are known, the fraction of the incident wave intensity that is reflected (R) can be calculated with the equations below:

2

12

12

+−=

ZZ

ZZR

In addition to reflection, waves also get refracted. Refraction is the resultant of

different velocities of the acoustic waves within the two materials. This usually takes place when the waves are beamed at an oblique angle. This phenomenon can be described physically by the Snell’s Law.

Not only the frequency and wavelength have an effect on defect detectability.

Indeed, the amount of sound that reflects from a defect is also dependent on the acoustic impedance mismatch between the flaw and the surrounding material. A void is generally a better reflector than a metallic inclusion because the impedance mismatch is greater between air and metal than between two metals. Besides, lots of unwanted small reflections come from the surrounding material, unwanted modes, etc. Therefore, a good measure of detectability of a flaw is its signal-to-noise ratio (S/N). The signal-to-noise ratio is a measure of how the signal from the defect compares to other background reflections (categorized as "noise"). The value of the signal-to-noise ratio depends on a very huge number of factors.

It is obvious that not only one wave is propagating through a material during an

inspection. This means that during an inspection, many waves interact or interfere with each other. These interactions can be suitable in order to enhance a mode or to cancel another one. The following figure is an illustration of wave interferences: the red waves are the “input waves” and the blue one is the result.



Figure 6 Waves interferences

This figure shows that wave interferences is an important phenomenon, which is

however still not fully understood but already in use for tool design.

As discussed earlier, the conversion of electrical pulses to mechanical 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 (see Figure 7). 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. 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. These piezoelectric ceramics 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.

Figure 7 Phenomenon of electrostriction



3.2.2 LRUT principles

LRUT is a new technology and has only been commercialised for about 10 years. It is clear that much research remains to be undertaken before the full potential of the technique can be realised. Until now, it has been applied almost exclusively to pipes, mainly in the oil, gas and chemical industries. The principles of LRUT for inspecting pipe are different from conventional NDT. Indeed, where the effective test range of conventional ultrasonics NDT in steel is normally restricted to the foot-print of the search device, long range ultrasonics sets out to detect discontinuities typically tens of metres away from the test location. This is accomplished by using low frequencies (usually between 10 and 150kHz) that attenuate less strongly and specific ultrasound wave modes. Figure 8 offers a comparison of the conventional and long range ultrasonics, regarding to the inspection coverage.

Figure 8 (a) conventional NDT inspection coverage – (b) LRUT inspection coverage

((a) Couverture des NDT traditionelles – (b) Couverture des inpections LRUT)

Conventional ultrasonics propagates a pulse of waves along a narrow beam. These are bulk waves, shear and compressional being the most common. Long range ultrasonics on the other hand uses the parallel boundaries of the materials through which the ultrasound is propagating to guide the pulses. The waves produced in this way are called Lamb waves after the nineteenth century scientist who discovered them, or more specifically plate waves when found in plates or guided waves when found in pipes or bars.

A very important characteristic of Lamb waves is that they are dispersive, that is to say, their velocity depends upon the product of their frequency and the thickness of the medium.

TfV ×∝

V is the velocity ƒ is the frequency T is the cross-sectional thickness

Being dispersive also means that the group velocity and the phase velocity of a wave mode are different. The group velocity is defined as the velocity of a group of waves (forming only one full signal) and the phase velocity is defined as the velocity of one internal wave. The group velocity is used to work out the time of flight of pulses with the equation:



VelocityGroup

ceDistime

tan=

The phase velocity is used to work out the wavelength of a mode at a specific frequency with the following equation:

frequency

VelocityPhasewavelength =)(λ

Since ultrasonic testing methods depend on measuring the time of flight of pulses

between the transducers and the reflector, if the velocity of the ultrasound is not fixed for the material being tested, the range of the reflector cannot be determined.

The complexity of long range ultrasonic waves is such that before they could be successfully implemented in NDT; mathematical models had to be constructed to find the velocity of each wave mode present in a defined wall thickness and at specific test frequency. These models allowed dispersion curves to be plotted for appropriate wave modes in each pipe schedule. These are used to set the test frequency range and calibrate the time base of the A-scan for distance.

Some typical dispersion curves are shown in Figure 9. The L waves are the axi-symmetric waves, the F waves are flexural and the T wave torsional.

Figure 9 Typical dispersion curves

As seen before, conventional ultrasonics relies on the high miss-match in acoustic

impedance between the metal and air at the boundary of a discontinuity. With guided waves, this is also a factor, but in addition, acoustic impedance changes can also be expected wherever the distance between the parallel boundaries of the guided wave increases or decreases. Such a condition exists at a girth weld in a pipe, the cap and root contributing to an increase in wall thickness. The resulting acoustic miss-match between weld and pipe causes a reflection.



In pipes, girth welds are good reflectors of guided waves. In practice they are found to reflect about 20% of the incident energy. This compares with 5% for a circumferential notch of half the pipe wall thickness in depth and half the pipe diameter in length (Figure 10). Because they are good reflectors, girth welds make ideal reference targets for setting the data analysis software according to the pipe geometry. They are also useful for checking the range calibration.

Girth weld

Notch Ø/2 long

T/25%20%

Ø

Figure 10 Reflectivity of long range ultrasound

Like a girth weld, an area of corrosion is a change in wall thickness. It is a decrease in section rather than an increase, but the consequent change in acoustic impedance in the path of the guided wave will cause a proportion of the incident pulse to reflect. The proportion of sound reflected depends on the depth and circumferential extent of the corrosion. A short deep area of corrosion may reflect no more ultrasound than a long shallow area of corrosion despite being the more significant from a pipe integrity point-of-view. Fortunately, the reflectivity increases more rapidly with depth than with circumferential length.

At present, the LRUT technique thus has the advantage of full volume coverage and the ability to test long lengths of structure from a single point. It can also be used to inspect inaccessible regions of a structure from an accessible location. The position of a feature along the pipe can be established with an accuracy better than ±100mm and defects as small as 3% of the cross sectional area of an asset can usually be seen at the limit of detectability. However, the reliable reporting level is about 10% of cross sectional area. (The aim is to improve sensitivity to 1% of cross section).LRUT unfortunately also has limitations that prevent it to be world-recognised and used. These major limitations are:

• Range is reduced to 5m or less with visco-elastic coating or a complex geometry.

• Inability to position the flaw around the pipe. • Inability to distinguish between a wide shallow flaw and a deep narrow one

with the same cross sectional area. • Individual sensor elements are ‘dry coupled’ and are forced against the

component with a load of about 20kg. As considerable numbers of elements are required to make up the array, it becomes difficult to spread these forces unless a clamping device can surround the component, as is the case for pipe. Application to flat plate components is difficult.



• The individual sensor elements are expensive and required in large numbers (e.g. 280 for a 600mm-diameter pipe).

• Inability to inspect flat components (such as sheet-piles) or large diameter structures (>2m) (such as heat exchangers, pressure vessels, etc.) in an efficient way yet.

A comparison between Long Range Ultrasonics and Conventional Ultrasonics for pipe inspection is given in Appendix (Table 3 and Table 4).

3.2.3 Teletest© equipment

The Teletest equipment, marketed by Pi Ltd, was the first commercially available LRUT system in the world. This technology had been developed in a multi-client collaborative project, managed by TWI, with additional funding provided by the European Commission. Teletest equipment has been used primarily for inspecting pipes but is to be used for wider applications like rails, cables, offshore platforms, etc.. The Teletest equipment is composed by :

• a Tool (see), which is made by modules and a collar. The multi-mode modules have five transducer elements, three to excite the longitudinal mode and two to generate torsional waves. The collar is made out of carbon fibre reinforced Kevlar and is clamped around the pipe with a rapid latch mechanism

Multi-mode module

• a unit (currently the Mark 3 Unit, see): which has an integral battery with 12 hour operating capability before recharge and incorporates an 'on board' air pump to inflate the transducer collars. The unit counts 24 channels, corresponding to three rings containing 8 octants each.

• a software (currently “teletest software version2”) which contains all the tools required to carry out an inspection in the best conditions.

The teletest equipment can generate any of the three main wave types used in guided wave technology, longitudinal, torsional and flexural. It has been designed to be used as a phased array. This makes it possible to focus ultrasound at any point both along and around the pipe, thus improving defect detection capability. It also means that it is possible to determine the approximate circumferential extent and depth of any flaw that has been identified using the equipment in its original screening mode. In general, an inspection over a distance of 60m (30m in each direction) can be carried out from a single position. More information about Teletest can be found by following this link: http://www.plantintegrity.com/



4. The development of LRUT in different assets

This part presents the work that I carried out during my placement. The assets addressed below are the rail, the sheet pile, and the pipework. These assets represent over half of my work at TWI.

4.1 Rail investigation

The final objective of this rail survey is to develop modules which can be fitted together to form arrays of transducers able to generate guided waves in complex rail structures and allow a long range inspection of the rail. Therefore, a large amount of previous work has been done on this asset, which includes developing the dispersion curves of a rail, creating some analytic models, studying the ultrasonic physics related to a rail application, etc. My objectives were to validate the modelling results by running experiments on the head, the web and the foot of the rail.

4.1.1 First problem

Initially the plan was to use specific transducers (NEXUS transducers) to inspect the rail but unfortunately they have been broken while being testing on a plate.

The experiments were to be carried out with a Teletest Mark 3 unit. However, the Teletest Tool lead is designed to be plugged into multi-modes modules. Besides, being at an experimental state, no tool exists to carry out rail inspection. The problem was thus raised, how can the rail be connected to the Teletest unit without the NEXUS transducers? Pi Ltd has developed its own piezoelectric transducers for its LRUT tool. The idea was thus to use the shear wave mode piezoelectric ceramics from these transducers and then, to connect these piezo-elements to the Teletest unit, a “box” has been built.

This box aims to split the 8 connectors from the Teletest tool lead into 24 single channels. Indeed, a multi-mode module has 3 longitudinal transducers and 2 torsional ones. Each connector from the Teletest tool lead thus has 3 or 2 available channels. In order to get a maximum number of channels, the box has been built using the longitudinal pins. Figure 11 shows the so-called box (more pictures are gathered in appendix, Figure 51 and Figure 52).

Figure 11 The box

8 connectors from the Teletest Tool Lead

24 single channels



The 24 channels are actually 24 bare wires. The piezo-elements have then been

glued on the rail and soldered to these wires. The different fabrication steps of this box are not described here because they appear to be irrelevant. The construction of this box is however a week of work.

4.1.2 Experimental results of the rail web

The objective of these experiments was to generate a specific mode (the T2 mode) in the web and to verify the modelling results. Using the box, 4 piezo-elements were directly glued on the web on a matrix of 2 X 2.(before gluing the piezo-elements, the surface is rubbed down and cleaned up, a very good accuracy is also required when positioning the elements). My work was mainly to find the optimum setup to generate the selected mode. This involve the tyope of transucers, their number and the distance between them The sleected setup was glued in both side of the web (side A and B). Channels ABCD on the side A and channels EFGH on the side B.

After a couple of tests to find out the best frequency, the final test was carried out at 70kHz. At this frequency, the Phase velocity for T2 is 2584m/s, so the wavelength λ is 37mm, and the Group velocity is 3000m/s. It is also important to notice that the length of the rail is 6.64m and it contains 2 defects as shown in Figure 54 in appendix:

- Defect 1: at 6 m from the position of the piezo-element array - Defect 2: a hole at 6.22 m from the position of the piezo-element array

The following figure shows the pulse echo signal collected from the side B

Figure 12 results of the T2 mode on the web, side B, channels EFGH

A zoom in the results of Channel E (Figure 13) shows that we can detect not only

the rail end but we may also be able to detect the 2 defects next to it. These results also show that the signal to noise ratio is importantand the objective of generating a pure T2 mode focused on the web of the rail is achieved.



Figure 13 Zoom on the wave reflected by the rails end and the 2 defects (Channel E)

The theoretical arrival times for the defect 1, 2 and the rail end are respectively 4000µs and 4146µs. In Figure 13, these arrival times are 3890µs and 4132µs. These peaks thus arrive at the right time, and the defects are clearly represented on this A scan.

Rail end

Defect 1

Defect 2



4.1.3 Experimental results of the rail head

Head without defect

The aim of these experiments is to generate the F3 mode into the head of the rail. To achieve this aim, a 2x2 matrix of piezo-elements has been u sed on the top of the rail head.

After a frequency sweep to find out the best frequency, the final test was carried

out at 70kHz. At this frequency, the Phase velocity for F2 is 2830m /s, so the wavelength λ is 40.43mm, and the Group velocity is 3100m/s. The results in Figure 14 show that there is important reflection from the rail end.

Figure 14 results of the mode F3 on the railhead

The A scan shows that the head end reflects a high amplitude signal with an

important signal to noise ration. The results show that using this setup it is possible to generate a F2 guided wave mode propagating only on the rail head. Head with defect

The F2 mode was successfully generated on the head of the rail without defect, further experiments were thus carried out to study the wave interaction with different sizes of defects on the head. In these experiments, two sizes of defects have been tested. The first defect was a 1mm saw cut which has been increased to a 2mm saw cut defect. This defect is located at 4.46m from the position of the piezo-elements arrays, as shown below in Figure 15. Figure 56 in appendix shows the defect.

Railhead end



Figure 15 Drawing of the head with defect

According to the theory, at 70kHz, the saw cut defect should be represented on

the A scan by a peak at 2877m/s. With a 1mm saw cut defect, the defect signal amplitude is 0.9157mV and the peak arrives at 2831m/s. The following (Figure 16) is the result obtained with a 2mm saw cut defect.

Railhead endDefect

Figure 16 Results obtained with a 2mm saw cut defect on the railhead

The signal amplitude of the defect is 1.176mV and the peak arrives at 2834m/s.

The signal amplitude from the defect has thus been slightly increased when increasing the depth of the defect.

To conclude, the setup used for these experiments is able to generate a pure F2 mode on the head of the rail and it has also been proven that this mode is sensitive to the detection of a saw cut of 2mm deep. Moreover, a defect with a depth of 1mm or 1.5mm seems to be the minimal detectable defect.

4.1.4 Experiental results of the rail Foot

Foot without defect

HL x

x

d d

E

G

F H

6.64 m

4.46 m Saw cut defect



The objective of these experiments is to generate t he F2 mode into the foot of the rail. For this purpose, 2x2 arrays of piezo-elements have been use d on the top of the foot.

The frequency chosen for the foot of the rail was again 70kHz. the final test was carried out at 70kHz. At this frequency, the Phase velocity for T2 is 2584m/s, so the wavelength λ is 37mm, and the Group velocity is 3000m/s. In this first test, the foot of the rail doesn’t contain any defect. Figure 17 presents the pulse echo signal collected from the side A.

Figure 17 Results for the rail foot without defect, side A

The figure above shows that the F2 mode is successfully generated and focused on the foot of the rail. The signal received has an important signal to noise ration and the end of the foot rail is easily identified.

Foot with defect

Once again the results are very good; therefore the aim of the following tests is to investigate whether the F2 mode is able to detect different sizes of defects in the foot. In these experiments, two sizes of defect have been tested; a 2mm saw cut defect, which has then been increased to a 5mm saw cut defect. These defects are situated in the edge of the side A of the foot. These defects are located at 4m from the position of the piezo-element arrays, as shown below in (also see Figure 58 in appendix). According to the theory, the saw cut defect should be represented on the A scan with a peak at 2643m/s. The figure below is the result obtained with a 5mm saw cut defect (Figure 18).

Foot end



Figure 18 Results obtained with a 5mm saw cut defect on the foot of the rail, side A

The defect is hardly detectable under a depth of 5mm since it is undetected at 2mm. The following (Figure 19) is a zoom on the results of channel B clearly showing the presence of the defect.

Figure 19 Zoom on the wave reflected by the defect and the rail end, channel B

4.1.5 Conclusion

These experiments showed that the guided wave could be successfully used for a rail inspection. The main conclusions are:

• the use of a transducer array greatly reduces the noise in the generated mode.

• The most appropriate mode for inspection of the web of the rail is T2. The experiments have shown that T2 is able to detect defects in the web of the rail.

Defect

Reflexion of the defect by F2



• The most appropriate mode for inspection of the head of the rail is the mode F3. The results have confirmed that this mode is sensitive to defects in the head. During the experiments a saw cut of 2mm depth has been detected.

• For the foot of the rail, the mode F2 is used for the long range ultrasonic testing and it has been shown that is sensitive to small defects in the foot (5mm deep)



4.2 Sheet Pile investigation

The objective of this survey is to provide an experimental work to support the design of a sheet pile inspection device. The results of these experiments have been taken into account to produce a low weight device, able to apply the pressure required for the transducers, with enough flexibility in its design to able the transducers to be moved in order to generate the selected modes and to minimise the edge effect.

The plate used for these experiments is an aluminium plate with 158cm length, 125cm width and 1cm thickness (see Figure 20).

Figure 20 Plate used for the experiments

In an early state of this survey, the guided waves have been generated both by shear piezo-elements (connected the same way as in the rail inspection survey) and by actuators (which generate compressive waves and also connected via the “box”). A frequency sweep has shown that the optimum frequency for this plate is at 70kHz. The dispersive curves have been drawn for a 10mm thickness aluminium plate. They show that at 70 kHz, 2 modes could be generated: A0 and S0. The characteristics of these 2 modes are presented in Table 1.

A0 Frequency kHz Phase velocity

(m/s) Group Velocity

(m/s) Wave length �

(cm)

70

2110

3065

3.01

S0 Frequency kHz Phase velocity

(m/s) Group Velocity

(m/s) Wave length �

(cm)

70

5397

5310

7.71 Table 1 Characteristic of the modes A0 and So at 70 kHz on the aluminium plate with

10mm thick.

In general, the access for the inspection of the sheet piles is limited to only one side of the plate. For this reason it has been decided that the shear piezo-elements

158 cm

125 cm



and the compressive actuators will be used to generate and propagate a pure A0 mode (Asymmetric mode due to an asymmetric excitation). Different configurations of arrays have been investigated to answer the questions below: - How could we excite the specific mode A0 from one side of the structure? - How could we cancel or minimise the S0 mode, if generated? - How could we cancel or minimise the edge effect? -Could we detect defects of different sizes and positions on the plate?

4.2.1 Excitation of a specific mode and minimisatio n of the edge

effect

To achieve this aim, different types and sizes of arrays have been experimentally tested. The settings tested are (also see Figure 59 in appendix):

• Linear arrays: horizontal arrays with 1 up to 8 piezo-elements and actuators Vertical arrays with 1 up to 6 piezo-elements and actuators

• Matrix arrays: 2x2, 2x3 and 3x2 matrices of piezo-elements and actuators

A complete analysis of the results provided by the different arrays with their comparison to the theoretical models cannot be presented in this report. Therefore, this part only provides the comparison of the best results.

Aiming to generate the A0 mode, There was no spacing between the elements and the near edge of the plate. To investigate the effect of the number of active transducers per array, different sizes of arrays (1 to 8 transducers) have been tested. The first tests showed that the arrays composed by 1 or 2 transducers were not giving good results because the reflected signal from the plate end was very dispersive. However we have noticed a significant improvement in the reflected signal by using an array with more than 2 piezo-elements. The details of the results of pulse echo test using the 3x2 matrix are presented by the figure below:

Figure 21 Zoom of the pulse echo signal recorded by the channel H of the matrix 3x2 The analysis of this figure shows that the A scan is composed by 4 reflections:

First unknown

Second unknown A0 S0



• Reflection 1: it arrives at 415µs with amplitude of 13mV • Reflection 2: it arrives at 600 µµµµs with an amplitude of 20.5mV, this

reflection corresponds to the mode S0 • Reflection 3: it arrives at 820 µs with an amplitude of 23.5mV • Reflection 4: it arrives at 998 µµµµs with an amplitude of 208 mV, this

reflection corresponds to A0

More investigations were required to understand the source of the reflections 1 and 3. However, it is clear that both the S0 and A0 modes are generating through the plate. To identify the two unknown reflections, the signals recorded at the edges and at the far end of the plate have been investigated by gluing piezo-elements at both whereabouts (see Figure 60 in appendix). A Theoretical model has also been generated. The results are shown in the figure below:

Figure 22 The directivity pattern of the transducer array 2x4. The spacing along both

axes is equal to wavelength of A0 mode λA0: blue – A0, red – S0.

With the further experiments, it has been agreed that the unknown peaks come from the reflection of S0 and A0 modes at the edges of the plates (regarding the time of flight). Indeed, the model shows that A0 is propagated in both x and y directions. The second strong unknown peak can thus come from the edge. We can also notice than S0 is much weaker than S0. These experiments also showed that the accurate alignment of the transducers in the arrays had an important effect on the quality of the results. The Figure 23 gives a comparison of the best settings tested with piezo-elements and actuators.

0.2

0.4

0.6

0.8

1

30

210

60

240

90

270

120

300

150

330

180 0

x

y



Comparison actuators and piezo-elements pulse echo results

-80

-60

-40

-20

0

20

40

60

80

-113 387 887 1387 1887

time (us)

Am

plitu

de (m

V) 6 horiz trans

mat 3x2

6 horiz actu

6 vert actu

Figure 23 Comparison of the pulse echo signals

This comparison shows that the piezo-elements, which generate shear waves,

are much efficient for generating the selected mode. These results are due to the fact that the piezo-element gives directionality to the guided waves. So the important energy is focalised in the direction of the wave propagation and only a minimum amount is propagated in other direction (e.g. the plate edges). The matrix arrays of piezo-elements 3x2 has given the best results for the plate application in term of generating A0 with important amplitude and no dispersion. However, further theoretical models have shown that varying the spacing in the x-direction could enhance the generated signal by decreasing the amount of waves travelling to the edges of the plate.

4.2.2 Impact of sand on wave propagation through an aluminium plate

The previous part has shown than both modes were propagated through the plate. Besides, studying the wave attenuation is very important since sheet piles are generally embedded. These experiments thus aim to see whether a layer of sand on the surface of the plate has any effect on the wave propagation and like this aims to find out which mode is the more suitable to use in the future. For this purpose, a 2.5cm layer of sand was spread over the plate. This layer of sand was successively dry, wet and saturated with water (very wet). The tests were then run at 70kHz using a 7 piezo-elements horizontal array with spacing λA0. They were also run at 100kHz with a spacing of λS0/2. These setups respectively aim to privilege the generation of A0 and S0 (see Figure 24).



Figure 24 Setup of the experiments

The following figure is an example of attenuation curves. It shows the attenuation

curves of the A0 mode at 70kHz generating A0 for the different setups (without sand, dry sand, wet sand and very wet sand).

Generation of A0- Results of the pulse echo A0 at 70k Hz

0

50

100

150

200

250

300

0 2 4 6 8 10 12 14

Distance (m)

Am

plitu

de (

mV

)

clean sandwet sandvery wet

Figure 25 Attenuation curves of A0 at 70kHz in the pulse echo scan, generating A0

The first conclusion which can be worked out from the attenuation curves is that

sand does increase the wave attenuation. All the curves follow the same trajectory (there are similar), however, in presence of sand, the amplitudes start at a lower level. According to the attenuation curves and taking into account the experimental error, we can assume presence of water in the sand doesn’t make a significant difference in the attenuation.

Sand

λA0 or λS0/2

1.58m

1.25m



Figure 26 shows the ratio curve of the data from the clean plate over the data from the dry sand layer plate collected at 100kHz, generating S0 (=spacing of λS0/2). For each reflection, the ratio of the peak-to-peak values has been calculated and plotted.

Exciting S0 - ratio clean plate data over dry sand layer data pulse echo at 100kHz

0

5

10

15

20

25

30

3.16 6.32 9.48 12.64

Distance (m)

ratio A0

S0

Figure 26 Ratio of the data from the clean plate over the data from the dry sand layer

plate, in the pulse echo scans at 100kHz, generating S0

The A0 mode is much more attenuated than the S0 mode when a layer of sand is spread over the plate. It can be explained by the physical nature of the waves. Indeed, the A0 mode is an asymmetric mode; it travels at the surface of the plate whereas the S0 mode is a symmetric mode which has an inner propagation. Therefore, the A0 mode would have much more attenuation if a layer of some materials is laying at the surface of the sample. To conclude, it is better to generate the S0 mode. The next part thus aims to develop a technique generating only one mode through the plate, in our case, the S0 mode.

4.2.3 Generation of only one mode through the plate

The dispersion curves of a 10mm thickness aluminium plate show that the two modes A0 and S0 can be generated at LRUT frequencies. The theory shows that the A0 mode can be cancelled using two arrays of piezo-elements. Therefore, in order to cancel the A0 mode,. The excitation signal of one was also inverted with respect to the other. The outgoing A0 signal in each direction was thus zero since the A0 mode from each array was cancelled (see Figure 27).

Ring 1 Ring 2



Figure 27 Cancellation of the A0 mode

Besides, inside the arrays, the piezo-elements were 2.7cm away from each other,. This setting aims to reduce the scattering of waves to the lateral edges of the plate. A 4x2 matrix of Teletest piezo-elements was used to run the experiment as shown in Figure 28.

Figure 28 Setting of the experiment to cancel the A0 mode

Figure 29 and Figure 30 show the efficiency of this setting.

Figure 29 Pitch catch signal at 100kHz of a normal matrix (2 arrays in the same way)

and a semi inverted matrix cancelling A0 mode (one array inverted with respect to the other)

x]

Wave propagation

d

Inverted array

Pitch catch matrix 4x2 100kHz

-60

-40

-20

0

20

40

60

0 200 400 600 800 1000 1200 1400 1600 1800 2000

time(us)

Am

plitu

de (m

V)

matrix 4x2 invertedmatrix 4x2

Arrival time S0

Arrival time A0



Figure 30 Pulse echo signal at 100kHz of a normal matrix (2 arrays in the same way)

and a semi inverted matrix cancelling A0 mode (one array inverted with respect to the other)

These figures show that the S0 mode generation (seen at 600µs on Figure 30) is

widely enhanced whereas the A0 peak-to-peak signal drops from 80mV to 10mV (seen at 1000µs on Figure 30). The cancellation of a specific mode is thus a real success. The objective is fully achieved.

4.2.4 Defect detection

The last experiment was to find out whether a defect into the plate could be detected using the previous setting. The defects were little trenches in the plate. Two sizes of defect have been studied, a 30x12x6mm defect and a 60x12x6mm defect. Figure 31 shows the setting.

Figure 31 Setting of the experiment to cancel the A0 mode with defect

x]

Wave propagation

d

Inverted array

Defect 60x12x6mm

1 m

1.58 m

Pulse echo matrix 4x2 100kHz

-60

-40

-20

0

20

40

60

0 200 400 600 800 1000 1200 1400 1600 1800 2000

time(us)

Am

plitu

de (m

V)

matrix 4x2 invertedmatrix 4x2

Arrival time S0 Arrival time A0



Figure 32 shows the results at 100kHz with the different defects.

Figure 32 Zoom in the pulse echo signal at 100kHz of a 4x2 matrix cancelling A0

mode Figure 32 shows the defect arrives at 385µs. The theory gives an arrival time of

sv

dt µ375

5310

21 =×== .

Table 2 gathers the arriving times and peak-to-peak amplitudes of A0, S0 and the defect from the pulse echo signal in the different settings.

Pulse echo signal data

non inverted Inverted without

defect Inverted with a defect of 30mm

Inverted with a defect of 60mm

arrival

time (µs) Amplitude

(mV) arrival


(mV) arrival


(mV) arrival


(mV) A0 986 65.276 1033 20.489 1025 13.763 1021 13.413 S0 611 9.836 605 52.951 610 49.647 607 45.331

defect - - - - 390 6.609 388 9.681 Table 2 Pulse echo signal data of A0, S0 and the defect for the different settings

This table shows that the longer the defect, the lowest the intensity of the mode

reflection. In other words, the bigger the defect, the more energy it will reflect. It can also be noticed that the bigger the defect, the bigger its amplitude in the pulse echo signal. Both the reflections from the A0 and S0 mode decrease when the size of the defect increases.

Pulse echo matrix 4x2 100kHz generating S0 (one array inverted)

-30

-20

-10

0

10

20

30

0 100 200 300 400 500 600 700 800

time(us)

Am

plitu

de (

mV

)

defect 6cm

defect 3cm

without defect

Arrival time defect Arrival time S0

Defect



4.2.5 Conclusion

This survey shows that the guided wave can be successfully used for a sheet pile inspection. The main conclusions of the research carried out are the following:

• Almost in all cases, A0 and S0 modes are generated and also in both directions.

• The most promising arrays in directivity patterns are the following: • A layer of sand on the plate increases the wave attenuation. Therefore, with

an embedded sheet pile it is recommended to generate S0 mode instead of A0 mode because the latter is more attenuated.

• It is possible to cancel/reduce the amplitude of one mode by using at least 2 arrays of transducers.

• Defects in sheet piles can be detected by using LRU techniques.



4.3 Bended Pipe investigation

This survey aims to provide a solution for inspecting bended pipeworks of small diameters. Indeed, current technologies are not able to focus on a defect if the pipe is not straight. This survey aims to find out a way of inspecting challenging bended pipe. Below, two methods have been tried and compared: the Time Reversal method and the traditional Focusing method. This section also looks whether it is possible to detect different kinds of defects as saw cuts or notches, and tries to give optimum settings for manipulations.

4.3.1 Description of the sample

The experiments have been carried out on a 2 inch, (51mm) outer diameter, pipe which total length is 8.75m and wall thickness 4mm. The pipe was manufactured by welding a 2-inch OD bended pipe and a 2.5-inch OD bended one together. The pipe also contains four defects: two 9% saw cuts and two burr grindings (notches), located as shown below. Figure 33 provides a sketch of the sample, pictures are gathered in appendix.

Figure 33 Sketch of the sample

The waves were generated using a Teletest Mark 3 Unit and a 2-inch Pi torsional tool. The tool is composed by three arrays having 8 torsional transducers each. The spacing between the rings is 30mm.

4.3.2 The Time Reversal Method

The Time reversal method of ultrasonic waves represents a way of focusing through an inhomogeneous material on a reflective target that behaves as an acoustic source. This technique is based on a time-reversal mirror, in other words on arrays of transmit-receive transducers that respond linearly and allow the incident acoustic waves to be recorded. Basically the pulse echo signal is recorded and

9% Saw cut 1 9% Saw cut 2

Weld

Notch 1

Notch 2

2” pipe

2.5” pipe

Pipe end 2

Pipe end 1



analysed to work out the location of potential defects. The waveform of the defect is then time-reversed and re-emitted. Time-reversal focusing technique thus provides the optimal inputs to the transducer elements by maximizing the acoustic pressure at the target location.

4.3.3 Program Time Reversal

In order to re-emit a signal via the Teletest software, the files of the waveforms must match with a specific shape and format. Figure 34 describes the file shape.

Ring 1 Test name of the experiment 4 number of channels 1 way of reading 0 x-offset 63.35 maximum absolute value in the waveforms 0.80 time (us) between each point 376 number of points -0.5867510 -2.4087660 -2.0690680 0.1544080 -0.4632246 -2.0381868 -0.7102768 1.3155568 … … … …

Data Figure 34 Example of Teletest input waveform file

The first 8 lines are the header of the file and change for each test as well as the waveforms and one file has to be created for each ring (3 rings). Therefore, the creation of these files manually is a huge amount of work. Indeed, the waveforms have first to be extracted as ascii files, then load into excel, the selected part of the waveform extracted and time-reversed. Besides, the ascii files created by the Teletest software are at a frequency of 1MHz, whereas the input waveforms have to be at a frequency of 1.25MHz. The waveform thus have to be resampled using Matlab. The creation of the input waveform files, manually, takes about 2h. After a couple of tests, a little Matlab program has been written to create these files. This program in its final version is a stand-alone application with a graphical user interface. It has as main features:

• works with octants or quadrants (8 or 4 waveforms per rings) • 2 ways of loading the ascii files, an automatic way for traditional uses of the

Time Reversal Method or a waveform-by-waveform way. • extracts the selected part of the signal, time-reverses and resamples it (at a

selected frequency) • can plot the waveforms at any time.

Screenshots of the program are given in appendix (Figure 61 to Figure 63).

4.3.4 Defect detection with the Time Reversal metho d

The first aim of this survey was to see whether the Time Reversal method can be used to focus on a defect on a bended pipe. The following figures are the description of the setting. Table 3 gathers the distances of all the defects from the tool and Figure 35 provides a sketch of the experiment. The tool was set up to study the forward direction. Using torsional waves, the frequency was chosen as low as possible, in our case, according to the design of the tool, 36kHz seemed to be the best frequency.



Defect Distance (cm) Notch 1 174

Saw cut 1 194 Pipe end 1 310

Table 3 Defects distances from the tool

Figure 35 Position of the tool for the comparison of TR and focusing

Figure 36 to Figure 38 show the results of the time reversal method focusing on different parts of the signal received with a normal Teletest acquisition

Figure 36 Data processing of the normal 10 pulse signal

9% Saw cut 1 Notch 1

Notch 2

Teletest© Tool

194 cm 174 cm

310 cm

Forward Pipe end 1

Notch Saw cut



Figure 37 Data processing of the TR method with the notch

Figure 38 Data processing of the TR method with the saw cut

Comparing Figure 36 with Figure 37, and Figure 36 with Figure 38, it can be

concluded that the Time Reversal method applied on a specific defect does improve its reflection. Regarding the results, we can wonder how many times we can use the Time Reversal Method for the same defect before decreasing the quality of the output. Figure 39 shows the result of the Time Reversal method used with the saw cut reflection for the second time.

The signal from the notch has been improved

The signal from the saw cut has been improved



Figure 39 Data processing of the TR method with the saw cut for the second time

Using the Time Reversal method twice with the same defect doesn’t improve the result. It even seems to make it messier.

4.3.5 comparison Time Reversal and focusing

The focusing method needs the interaction between different modes to work properly; therefore the more modes the better. At 36kHz, only 2 modes of the T(0,1) family are present, this method thus cannot be used. Nevertheless, this kind of torsional tool, with 3mm spacing between the transducers, possesses another optimum frequency at 72kHz (where more modes are present). Hence, the following experiments have been carried out at 72kHz, instead of 36kHz.

• Results Time Reversal method

This part presents the results obtained with the Time Reversal method.

Figure 40 Pulse echo signal at 72kHz before the Time Reversal

U tube 72kHz TR tool at the top forwards direction

-80

-60

-40

-20

0

20

40

60

80

0 500 1000 1500 2000 2500

time (us)

Am

plitu

de (

mV

)

pulse echo

Notch 1060 to 1200 us

Saw cut 1200 to 1370 us




-80

-60

-40

-20

0

20

40

60

80

0 500 1000 1500 2000 2500

time (us)

Am

plitu

de (

mV

)

pulse echo1060-1200

Figure 41 Result of the Time Reversal method applied between 1060µs and 1200µs

(Notch 1)


-80

-60

-40

-20

0

20

40

60

80

0 500 1000 1500 2000 2500

time (us)

Am

plitu

de (

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)

pulse echo1200-1370

Figure 42 Result of the Time Reversal method applied between 1200µs and 1370µs

(Saw cut 1)

Figure 40 to Figure 42 show the results of the Time Reversal method at 72kHz. They clearly show that this method can be very effective to detect defects. These results are even better than the previous ones at 36kHz, which means that the Time



Reversal method can be used at different frequencies and also that these frequencies have to be chosen carefully. In Figure 41, the peak arriving at 1090µs has been only slightly increased however, in Figure 42,the peak arriving at 1215µs has been highly improved and the first peak reduced. The Time Reversal method thus seems to work perfectly even after a bend on a pipe. However, according to these results, we can wonder if the kind of defect has an impact on the technique efficiency. Further works have been done in this direction. It turned out that the signal input rather than the kind of defect was relevant in the efficiency of the Time Reversal Method. Moreover, regarding the length of the signal input, it also turned out that using a long input signal, containing the signals of the two defects, results in having a signal unclear and noisy. Besides, the method only focused on the first defect.

• Results of the focusing method

Time-delay focusing techniques are also based on the use of transducer arrays. The pressure signals coming from an acoustic source are recorded by each transducer element and then digitised (pulse echo signal). A cross-correlation algorithm is next used to estimate the time-delay between signals from neighbouring array elements. These delays determine the optimal time-delay characteristic required to focus on the source. A pressure signal is resent by taking into account the time-delay between the array elements and the waves should then focus on the acoustic source. Time-delay focusing method is the technology currently use by the Teletest equipment to focus on a defect. However, this technology suffers from the fact it can’t be applied on a bended pipe. The following experiments attest to this fact.

Figure 43 compares the basic pulse echo signal with the results of the focusing method on the bended pipe at two different focusing points: 1.74m and 1.94m which are the distances of the two defects.

Results of the focusing method at different focusin g lengths at 72kHz

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0 500 1000 1500 2000 2500 3000 3500

Time (us)

Am

plitu

de (

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)

pulse echo315d 1.74m315d 1.94m

Figure 43 Result of the focusing method different focal distances, 1.74m and 1.94m



As expected, the focusing method can only be used on a straight pipe and is totally inappropriate to use on a bended one. Indeed, the reflections occurring at 2450µs and 2620µs, don’t match with anything known on the pipe. Moreover, there is no reflection at 1.74m (1067µs) or 1.94m (1190µs) on the A-scan. The results obtained with this method on a bended pipe are thus simply wrong because of the wave interactions occurring at the bend.

4.3.6 Conclusion

This work shows that the Time Reversal Method can be successfully used for bended pipe inspection. The main conclusions of the research carried out are the following:

- The Time Reversal Method can be used for a bended pipe inspection whereas the traditional Time-delay Focusing technique is irrelevant.

- The efficiency of the Time Reversal Method is dependant to the signal input, a too long input signal leads to a messy pulse echo signal, a too short input signal leads to a lack of information for the method and thus a decrease in the optimum efficiency.

- Resending the signal of two defects in once leads to a messy pulse echo signal and only the waves only focus on the first defect

- Using the Time Reversal method twice on the same defect doesn’t seem to increase the signal amplitude from the defect

- Any kind of defect seems to be detectable, at least welds, saw cuts and notches.

- The Time Reversal method can be used at different frequencies



Conclusion

After 6 months of placement in one of the most famous world centre for materials joining technology, surrounded by challenges, discoveries and people as kind and brilliant as we could expect in such a place, it is hard to conclude in few lines my experience in a professional or personal point of view.

First of all, regarding my work, LRUT appears to consolidate its position as a

technology . During my placement, most of the experiments have given very promising results, and will lead to the development of new tools and devices, allowing a wider amount of structure to be tested (for instance: rail, plates, bended pipe).

This placement was also the opportunity for me to discover and learn a lot about

a widely used engineering technology such as NDT techniques. Being non-destructive these techniques are to be developed and still used for a long time so having this knowledge, knowing the possibilities of this technology is a real advantage for the materials science engineer that I am to be in few months.

Being interested in R&D for my future career, working in TWI for 6 months was

my chance to observe this environment before being graduated. I have been fortunate to live the life of a “researcher” by myself, dealing with their problems, their responsibilities and their motivations.

During this placement, I have also learnt a lot about myself. I developed my

autonomy at work, my report writing skill and work management skill. I also learnt how to use new technologies and softwares.

In the end, this placement taught me a lot about human relationships in a

company. TWI is probably one of the most international companies in the UK in terms of employees, full of nice people during this experience I learnt also how to deal with daily problems . However, this placement couldn’t have been so rewarding without the dynamic and hearty atmosphere in the NDT department.



References

WebPages: http://www.twi.co.uk/ http://www.ndt-ed.org/ http://www.techno-science.net/?onglet=glossaire&definition=6756 http://www.mathpages.com/home/kmath210/kmath210.htm Articles: European Rail Travel 2004 - Brochure Lyons D: ‘Western European cross-country oil pipelines, 25yr performance statistics, CONCAWE Oil Pipelines Management Group. Teletest® Interpreter Level 2, Training Course Notes, Pi, Mars 2007 Long Range Guided Wave Inspection Usage – Current Commercial Capabilities and Research Directions, M.J.S Lowe and P. Cawley, March 2006 SIXTH FRAMEWORK PROGRAMME Horizontal Research Activities involving SMEs Collective Research, LRUCM, June 2005 2BWPG-Theory of Long Range Ultrasonic, notes, Graham Edwards, June 2007



Appendix

Figure 44 Map of TWI site, Cambridge (Plan du site de TWI Cambridge)



Figure 45 Organisation chart of the NDT Technology Group on the 7th of June 2007 (Organigramme du department NDT au 07/06/07)



Figure 46 Penetrant Testing

Figure 47 Magnetic Particle Testing

Figure 48 Eddy Current Testing



Figure 49 Radiography

Figure 50 Ultrasonic Testing

Figure 51 Box, front view



Figure 52 Box under construction

Figure 53 Setup of the rail web (side A)

Figure 54 Defects 1 and 2 in the rail web

λ λ

Defect 1 Defect 2



Figure 55 Setup of the rail head

Figure 56 2mm depth defect on the railhead

Figure 57 Setup of the foot

λ λ

Defect

Arrays side A



Figure 58 Zoom in the defect in the foot

Figure 59 The different arrays of piezo-elements used in these experiments

Vertical linear array

Horizontal linear array

Matrix 2x3

Matrix 2x2

Matrix 3x2



Figure 60 The set-up of the 3X2 piezo-element matrix

Figure 61 Program Time Reversal first page

λλλλ

D C

H F G

B

A

80 cm

E

λλλλ λλλλ



Figure 62 Program Time Reversal bis

Figure 63 Program Time Reversal ter



Conventional Ultrasonics Long Range Ultrasonics

Ultrasound frequency usually 2-5MHz Ultrasound frequency usually less than 100 KHz

Pulses 5-10cycles long Pulses 5-10cycles long Pulses propagated along a beam Pulses guided by surfaces of test piece Bulk waves Guided waves Compressional, shear, surface and creep wave modes.

Many wave modes including axis-symmetrical, flexural and torsional and their harmonics

Velocity of sound is constant for a given wave mode and medium of propagation

Velocity of sound is dependant on frequency as well as wave mode and medium propagation.

Attenuation caused by probe coupling and material

Attenuation caused by probe coupling, material, surface roughness and material on internal and external boundaries.

Sound reflected at medium/air boundaries

Sound reflected at medium/air boundaries

Sensitive to changes in section across the sound direction

Sensitive to changes in section parallel with the sound direction.

Location of defects by time of flight measurement

Location of defects by time of flight measurement

Transducers vibrate in compression Transducers vibrate in shear. Transducers heavily dampened to give short pulse lengths.

Pulse length controlled by ‘tone bursts’ of excitation voltage.

Requires a liquid couplant between transducer and test surface.

Does not require a couplant between the transducer and the test surface.

Single test probe with one transducer, except in some automated test systems

Multiple probes arrays

Transducers scanned over the surface to get coverage

Transducers in a fixed location

Signals presented in an A-scan display Signals presented in an A-scan display

Amplitude of signals referenced to a calibration echo from a machined reflector in a reference block.

Amplitude of signals referenced to a girth weld in the test pipe.

Time base calibrated for distance with velocity for wave mode at any sound frequency

Time base calibrated for distance with velocity for particular frequency as well as wave mode.

Velocity found from look-up tables or by using reflectors at known distance in a calibration block.

Velocity found from Dispersion curves.

Signal interpretation through echo-dynamic pattern in response to probe movement.

Signal interpretation through signal position, amplitude pattern and wave modes

Amplitude of reflected ultrasound can be used in qualitative assessment of reflector size

Amplitude of reflected ultrasound can only be used to set threshold levels.



Test range of a few centimetres with shear waves or a few metres with compressional waves.

Test range of several tens of metres

Capable of detecting corrosion on internal pipe surface.

Capable of detecting corrosion on internal and external surfaces of pipe.

Quantitative defect sizing capability Qualitative defect sizing capability An assessment tool A screening tool Table 3 Comparison between Long Range Ultrasonics and Conventional Ultrasonics

for pipe inspection No

access problem

Insulated

Buried Buried in road crossin

g

Buried &

insulated

Elevated Elevated &

insulated

Visual 7 70 150 250 220 30 90

Manual UT 13 75 160 255 220 35 100

Mechanised UT 110 170 260 350 320 130 190

Profile radiography

70 70 220 310 220 90 90

Pulsed eddy current

80 80 230 320 230 105 105

MFL 80 150 240 320 290 105 170

LRUT 15 15 20 33 21 16 18

Table 4 Estimated costs in Euro for inspecting 1m of 12 inch diameter pipe

Documents

TN 09 Development of long range ultrasonic techniques for ... · TN09 Placement report Development of long range ultrasonic technique (LRUT) for different engineering assets Camille-Paul