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on Zhang/ Charlie Chong

My Reading on Acoustic Emission Testing2016-03 For my ASNT Level III Examination on

coming 2016 August.27 th June 2016

P r e

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on Zhang/ Charlie Chong

Acoustic Emission Testing

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harlie Chong/ Fion Zhang


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Fion Zhang

22nd June 20


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SME- Subject Matthttp://cn.bing.com/videos/search

https://www.youtube.com/channe

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http://www.yumpu.com/zh/browse/user/charliechong

http://issuu.com/charlieccchong

http://independent.academia.edu/CharlieChong1


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The Magical Book of Tank Inspection ICP

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ASNT Certification Guide

NDT Level III / PdM Level IIIAE - Acoustic Emission TestingLength: 4 hours Questions: 135

1 Principles and Theory

• Characteristics of acoustic emission testing

• Materials and deformation

• Sources of acoustic emission• Wave propagation

• Attenuation

• Kaiser and Felicity effects, and Felicity ratio

• Terminology (refer to acoustic emission glossary, ASTM


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• Signal condition

• Signal detection

• Signal processin

• Source location• Advanced signa

• Acoustic emissio

• Accessory mate

• Factors affecting

selection

2 Equipment and Materials

• Transducing processes

• Sensors

• Sensor attachments• Sensor utilization

• Simulated acoustic emission sources

• Cables


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4 Interpretation a

• Data interpretat

• Data evaluation

• Reports

5 Procedures

6 Safety and Hea

7 Applications

• Laboratory stud

characterization

• Structural applic

3 Techniques

• Equipment calibration and set up for

test

• Establishing loading procedures• Precautions against noise

• Special test procedures

• Data displays


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References & Catalog Numbers

NDT Handbook, Second Edition: Volume 5, Acoustic E

Catalog Number 130

Acoustic Emission: Techniques and Applications Catal


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Reference Standards: ASTM:E 569 – Acoustic Emission Monitoring of Structures Durin

StimulationE 650 – Guide for Mounting Piezoelectric Acoustic Emiss

E 750 – Practice for Characterizing Acoustic Emission Ins

ASTM E 749-96 is a standard practice of AE monitoring

of continuous welding.

ASTM E 1932 for the AE examination of small parts

ASTM E1419-00 for the method of examining seamless, gvessels.


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API:RP 575 – Guidelines and Methods for Inspection of Existi

Low- Pressure Storage Tank.

ST 307 – An Engineering Assessment of Acoustic Method

in Aboveground Storage tanks.

ST 322 – An Engineering Evaluation of Acoustic Methods

in Aboveground storage Tank.

ST 325 – An evaluation of a Methodology for the detection

Aboveground Storage Tank.


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Reading#1


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Acoustic Emission Non-DestrucTesting of Structures using Sou

Location Techniques Alan G. Beattie (2013)

Prepared by Sandia National Laboratories Albuquerque, New Mexico 87185 and Livermore, California 94550

Sandia National Laboratories is a multi-program laboratory managed and operated by Sandia Corporation, a wholly owned subs

the U.S. Department of Energy's National Nuclear Security Administration under contract DE-AC04-94AL85000.


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AbstractThe technology of acoustic emission (AE) testing has bee

used at Sandia for the past 40 years. AE has been used o

including pressure vessels, fire bottles, wind turbines, gas

weapons, and solar collectors.

This monograph begins with background topics in acousti

instrumentation and then focuses on current acoustic emi

covers the overall design and system setups for a test, wi

blade as the object. Test analysis is discussed with an em

location. Three test examples are presented, two on expeturbine blades and one on aircraft fire extinguisher bottles

for a FORTRAN source location program is given as an e

analysis program. Throughout the document, the stress is

real structures, not on laboratory experiments.


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Nuclear Silo


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Talented Youngster


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Foreword Acoustic emission techniques in both Non-Destructive Te

Research and Development were first investigated by Kai

ago. The ability to triangulate to the source of an emission

same sound wave at several different sensors was recogn

In 1966, Green reported successful use of triangulation in

on a hydrotest of a Saturn S-II propellant tank. Since then

emission source location has rapidly expanded, pushed in

technological advances in the field of personal computers

simple analog systems to sophisticated digital systems. Aplaced not only on acquiring data, but also on using unive

included in the commercial systems, to analyze the data.

programs are quite versatile and significantly advanced ov

programs included in the early source location systems, th

of the possible types of analysis in the field of acoustic em


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Saturn S-II propellant tank


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Saturn S-II propellant tank


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For the last forty years, the author has been using acoust

techniques at Sandia National Laboratories in widely vary

many of which required the development of specialized te

analysis techniques. Obviously, as in any new field, not a

techniques worked, but a general approach emerged. Thidetail the testing and analysis techniques developed in tw

Energy and Small Pressure Vessels. These applications u

to perform acoustic emission source location and involve

digital data. Because early commercial programs were no

for analysis, custom programs in FORTRAN were written.evolved, many of the custom techniques have been incorp

software furnished with the commercial systems. Howeve

commercial software are hidden in executable files. A few

developed FORTRAN programs are included here to illus

and assumptions used in acoustic emission programs. (FO

the easier languages in which to follow the workings of a cdespite the rise, decline, and death of many other comput

These working programs can be used with necessary min

an acoustic emission source location test.


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This treatise was written to describe some acoustic emiss

procedures which have worked well for the author. Neithe

rigorous scientific paper, this publication assumes that the

some familiarity with acoustic emission (AE). References

because upon my retirement 15 years ago, I gave my collpapers to a university library. For readers who want either

particular AE applications, or just a wide survey of the field

second edition of the Nondestructive Testing Handbook, v

Emission Testing, edited by Ronnie Miller. For a source o

papers on acoustic emission, I recommend the Journal ofpublished and edited by professor Kanji Ono. The Journa

internet and the entire 28 years of publication can be purc


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Following a general discussion of acoustic emission and a

sources, this monograph then offers a general discussion

elastic waves in materials. To understand what is occurrin

either Ultrasonics or Acoustic Emission, one needs this ba

is included both to aid readers who are new to the field of and to serve as reference material for those with some pri


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Briefly covered are elastic waves in solids including differe

reflection and refraction of waves, attenuation, superposit

of waves, resonance effects, and other phenomena which

encountered in an acoustic emission test. Also discussed

couplants, preamplifiers and other topics involved in perfoemission test.

Two brief sections cover acoustic emission source mecha

parameters of acoustic emission waveforms which are us

commercial systems. All of this material was covered in th

review article and makes no pretense of being new or diffethat the most useful parts of this monograph will be the dis

design, conduction, and analysis of several types of acous

No two tests are ever identical but the general methodolo

applicable.


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1.0 CHAPTER 11.1. Introduction to Acoustic Emission

Acoustic Emission (AE) is the study and practical use of e

generated by a material subjected to an external stress. T

was recognized by early miners who exited a mine when t

supporting timbers started groaning. Tin cry, the sound pr

bar is bent, was known soon after the production of metal

C.S. Barrett mapped a low temperature phase transition in

Magnesium alloys by sticking a phonograph needle into th

recording the output as the temperature was changed. J. the signals produced by samples undergoing tensile testin

the Kaiser effect, i.e. that no signals were generated by a

second loading until the previous maximum load was exce

Kaiser’s thesis was published in 1950, several groups inv

phenomena for possible use in testing structures. In the e

Green and a group at Aerojet Corporation started using ASaturn Rocket propellant tanks. They used a form of trian

the arrival times of the acoustic pulse at several acoustic

This was the direct precursor of the work that is described


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Despite its longstanding use, the definition of “acoustic em

source of controversy for the last fifty years. Strictly, “acou

pressure waves detected by one’s ear. However, elastic w

not limited to pressure waves, and all types of vibrational

generated by acoustic emission sources. Even so, the teremission” has become almost universally used for the phe

waves generated by an internal event in a media.

In this monograph, “acoustic” will refer to any elastic wave

acoustic emission source. Acoustic emission, then, is the

elastic wave by the rapid change in the stress state of sommaterial. This change is usually caused by the application

stimulus to the material. The material can be a solid, liquid

and the external stress (?) can be applied mechanically, t

magnetically, etc.


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The stress change must be rapid enough to transmit some

surrounding material and dissipates as an elastic wave.

On a macroscopic scale this definition includes earthquak

while on a microscopic scale it includes the fracture of cryMartensitic phase transformations. The occurrence of the

completely determined by the local conditions, the local st

physical state of the region. As a result, neither the exact

energy burst occurs nor the exact details of the generated

determined beforehand.

In general, the event generating the emission is irreversib

ruptured geologic fault nor a fractured crystallite in a meta

spontaneously.


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Once the elastic wave is generated, it then travels through

and can be detected at considerable distances from its po

Traveling from its source to the point of detection, the wav

the characteristics and variations of its acoustic path. Its e

by (1) geometric spreading and (2) scattering by both micmacroscopic variations in the material’s structure. Other k

may also be present.

The wave’s frequency content is generated by the source

travels the acoustic path.

The primary information carried by the wave is the (1) time

(2) the elastic energy detected at each sensor on the stru

of a sensor indicates that something happened in the spe

time, while the amplitude indicates the level of the disturba


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The apparent location of the source and all other characte

detected signal are modified by the characteristics of the w

sensor as well as the characteristics the sensor. As a resu

controlled laboratory experiment, a reasonable estimate c

characteristics of the emission source; however, in a test structure, we are limited to what are basically statistical es

multiple emissions to tell us what is happening in the spec

analysis is not generally used in the study of acoustic emi

of the specimen and the location coordinates of the sourc

averages of calculated values from multiple emissions.


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Elastic energy is ubiquitous. It travels through all materials

interaction between atoms. Only a hard vacuum does not

energy. The distance from its source over which an acous

detected depends on its initial amplitude and the acoustic

the material through which it is traveling. For many structuacoustic emission signals can be detected from almost an

structure. This allows an AE test to cover an entire structu

small region. However, many acoustic signals which have

the test can be present and detected. Acoustic isolation o

highly desirable. Most acoustic emission testing filters outfrequencies below 20 KHz, thus ignoring background nois

communication in the test area.


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An acoustic emission test occurs in real time. The test doe

preexisting defects, but detects flaw growth as it occurs. If

application simulates the conditions that the test structure

experience in actual operation, serious acoustic emission

pathological flaws that could lead to failure of part or all ofamount of emission detected and the locations of the emi

depend both on the design of the structure and the materi

fabrication. For example, compare the bending of a strip o

of FRP (Fiber Reinforced Plastic) with identical dimension

will likely give a single high-amplitude burst of emission juthe FRP will show a period of low level emissions followed

level emissions and then one or more high amplitude emi

starts to fail and then tears apart or snaps. Interpreting de

the test engineer’s job. The value of the information obtain

determined first by the design of the test and second, by t

detected emission. For complex structures, both the test dinterpretation are seldom simple. It is the purpose of this m

the test engineer in both areas.


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1.2. Sources of Acoustic Emission

Acoustic emissions are acoustic waves generated by a ra

stress state of a region in a material.

Acoustic waves are one of two (?, UT? VA?) non-electromtransmitting energy through a material; the other method i

A difference between the two is that thermal diffusion invo

transfer between individual atoms, while an acoustic wave

by a cooperative motion of many atoms. An acoustic wave

as a pressure pulse in a gas or liquid, or as complicated a

of transmission in a bounded solid. The generation of an ainvariably involves a large region of atoms.


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The most common acoustic waves are sound waves in ai

are produced by vibration of a surface such as the vocal a

animal, a leaf in the wind, or the diaphragm of a speaker.

quasi-continuous, involving a modulated vibration of an in

generating region and the transmitting medium. Another tywave is generated by a sudden disturbance in or on the m

transient event that produces a transient acoustic wave. S

wave is what we define as acoustic emission. It may have

damped wave with complex frequency content or it may a

small transient events which sum into quasi-continuous nocharacteristics are that it is generated in the medium whic

is transient in nature. Any sudden movement of a group o

near the sound velocity in a material can produce a transi

The apparent quasi-continuous signals which are often se

generated by the overlap of many transient events instead

vibration of a surface.


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Figure 1-1 shows waveforms both from a transient event a

superposition of many transient events. The long decay o

is produced by reflections of the original wave in the comp


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Figure 1-1.

Examples of burst

emission and

continuous

emission from ahigh strength

aluminum alloy


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The size of the region generating an emission can vary fro

row or plate of atoms moving simultaneously into a new c

during a Martensitic phase transition to a fracture in a sub

between two continental plates. The energy released in a

emission event will be roughly proportional to the volumeEnergy from these waves ranges from smaller than an ele

energy contained in a thunder clap or that of a magnitude

Wave frequencies are generally related inversely to the vo

generating region, ranging from thousandths of a Hertz fo

several MHz in fine grain metals. An important characterisemission in solids is that the fracture of the region occurs

stress vector exceeds the strength of the region to withsta

other words, the exact time when the emission occurs stri

local conditions. In a metal, for example, the precise envir

crystallite differs, and the fracture of one crystallite or of th

between two crystallites will make small changes in the lothe other crystallites.


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This complete dependence on local conditions means tha

predict the exact time of any one emission or the time inte

two emissions. Acoustic emissions occur unpredictably in

response not to the applied external stress but to the loca

produced at each position throughout the material. The padetected emission depend not only on the characteristics

source, but also on the characteristics and geometry of th

the source and sensor, and on the characteristics of the s

couplant between sensor and medium.


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The signal characteristics detected by individual identical

various positions on a specimen will often vary greatly for

The more complex the medium in which the emission is g

transmitted, the more likely there will be large differences

signals from the same emission at different sensors. The of origination and the wide variety of waveforms in separa

fundamental characteristics of acoustic emission. As such

a profound effect on the type of analysis used on the dete


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In structural metals, both inter-granular and trans-granula

crystallites can generate low level acoustic emission. Emi

generated at inclusions both by fracture of the inclusion a

the bonds between the inclusion and metal. Crack growth

several crystallites are involved can generate emission of amplitudes. The presence of corrosion on a metal surface

the fracture of brittle corrosion byproducts while active cor

emission from bubble formation. In high stress environme

of crystalline distortion) can occur in some metals and this

emission. Room temperature creep in metals may involvedislocations in the metal. Laboratory experiments have cla

detect very low amplitude emission from creep, but the au

its use in structural flaw detection. The creep rate in struct

normal usage is usually far too slow to generate acoustic

for flaw detection.


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Source mechanisms in FRP include matrix cracking, debo

matrix and fibers, fiber fracture, and crack propagation. Th

mechanisms can all be activated by the application of an

Based on the idea that the emission amplitude is related t

source, matrix cracking should produce the lowest amplitufollowed by matrix-fiber debonding. Fiber breakage would

partly due to the amount of energy released by the fractur

fiber. Finally, crack propagation, which includes all three o

mechanisms, would produce the highest amplitude emiss

generally to be the case, but trying to quantify it as a rule very well, probably because FRP is usually laid up by han

close to the structural uniformity of a well annealed metal.

an FRP structure will generate acoustic emission at loads

strength of the structure.


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The most likely cause of this emission is the relief, by min

FRP during the first loading, of high residual stresses whic

local regions during the curing process. The fractures hav

strength of the material, and the emission usually stops be

loads are reached. This emission is seldom seen upon thesubsequent loadings (Kaiser’s effect?) .

Another source of emission in FRP structures is often see

loading. This emission appears in the middle ranges of th

the rate of change in applied stress is highest, instead of a

appears to be caused by friction between small regions inare not bonded. This type of emission can be present dur

load test; however, it does not correlate with structural da

object. Structural damage is associated with emission tha

peak loads, especially during the rising load.


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As discussed above, most acoustic emission mechanisms

permanent change in the micro structure of the material. O

fracture occurs, it will not happen again unless there is so

mechanism. Therefore, acoustic emission appears irrever

effect, where the re-stressing of a specimen will not generemission until the previous load level has been exceeded

irreversibility.

The Kaiser effect holds very well for the immediate re-stre

specimen, but less well for composites. The problem is th

of an external stress does not necessarily take the specimmicro-stress path. One often sees emission on subsequen

lower than previously reached. The ratio of the load value

starts on subsequent loadings, to the maximum load value

previous loading, known as the Felicity ratio, indicates po

induced by the previous loading. Many NDT tests of FRP

test load in a series of steps, returning to zero between eaappearance of Felicity ratios much less than 1.0 is a good

significant damage occurred in previous loadings.


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2.0 CHAPTER 22.1. Acoustic Waves in Solids

Acoustic emissions are acoustic waves generated by a m

response to a change in stress. Once generated, these w

under the same rules as any other acoustic wave. They a

motion in a collection of atoms. Acoustic wave motion is a

movement of the atoms in a material extending over a hug

This collective motion implies that the wavelength is long

distance between the atoms. Wavelength is inversely prop

frequency of a wave; therefore, an acoustic wave is usualrelatively low frequency. For example, a frequency as high

Aluminum would still imply a wave extending over 1.5 x 1

simplest type of acoustic wave is a pressure wave which o

of material around a region is suddenly compressed by a

region. This compression can be either positive or negativ

exploding or imploding.


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The material in this shell experiences a change in its dens

change is then passed to the next shell by coupling betwe

density in the first shell then rebounds, usually going past

value to a smaller density change in the opposite direction

the density is transmitted to the next shell, and so on, throThe strength of the coupling between the material’s atoms

the density of the material determines the speed with whic

propagates . The resulting wave is known as a compressi

occurs in all materials, solids, liquids, gases, and plasmas


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In a compressional wave, the average motion of the atom

wave’s direction of travel. As materials become more rigid

forces between atoms become far more complex and dep

geometrical positions of the atoms. More modes of wave

and the averaged atomic motions are no longer restrictedpropagation direction . The introduction of boundaries in a

introduce further complications As a wave passes through

another material, the difference in physical propertied betw

will produce reflections and refractions as well as changes

propagation. A wave traveling along a surface will have far different pa

one propagating in the bulk.


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2.2. Characteristics of Acoustic Waves

Acoustic emission signals generate complexity as the wav

the medium. Understanding acoustic emission signals req

the wave characteristics, starting with the properties of the

which the wave travels. All materials are collections of atoattractive forces while simultaneously prevented, by short

forces, from approaching each other too closely. The supe

forces results in an equilibrium position for the atom at its

material’s most stable configuration. In crystals, for examp

forces between the atoms result in defined locations for thpositions. The result is a crystal structure. In a liquid the s

amorphous, but an approximate distance between atoms

density. In a liquid, only the density is defined. There are n

the atoms as there are in a crystal lattice.


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The ability of the atoms to change position is measured by

liquid, which can range from very low to extremely high. A

are a gas and a solid. A gas is a liquid with very low visco

liquid with very extremely high viscosity. The density of a

by its total mass and the volume of its container. The acoumaterial depend on its density and the long-range couplin

between its atoms.

The long-range correlations in an acoustic wave result in

small region being displaced in the same direction from thpositions. This displacement is a local dynamic strain in th

strain’s direction and magnitude are constantly changing a

When the atomic motion is pseudo-oscillatory, so is the st

wave is an oscillating strain moving through a material. Be

strain are always directly related in a material, there is als

stress field. Therefore, an acoustic wave can be describeddynamic stress or strain field in a material.


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2.3. Wave Motion

The most familiar depiction of a wave is a sinusoidal curve

Figure 2-1a. The amplitude oscillates between positive an

a fixed rate, known as the frequency, and the curve exten

curve can be plotted equally well as a function of space oa wave has both a spatial and a time component. An equa

curve is:

Where:

A is the amplitude;

ω is 2π times the frequency, υ; andk, the wave number, is 2π over the wave length, λ.


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The frequency, wave length, and wave velocity, v are rela

2.1b. The wave described in equation 2.1a propagates in In three dimensions, the wave front, which is a surface of

the wave, is a plane perpendicular to the X axis. Such a w

plane wave. Most waves originating at a point in an exten

have a spherical wave front. However, at some distance f

origin, the spherical surface will approximate a plane oversimplicity, we will assume plane waves for the rest of this


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Figure 2-1. (a) simple sine wave, (b) sum of two sine wav

wave, (d) spectrum of transient wave


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If two waves exist in a medium simultaneously, their ampl

algebraically. Fig.2-2 shows the sum of two waves:

where only the time component is plotted for clarity. Thus

waves can represent a complex wave form. It has been lo

arbitrary transient function which does not contain a disco

represented by an infinite sum of sinusoidal curves knownOne form of such a series can be written as


(2.2)

(2.3)

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where Ao and An are the amplitudes of the sine curves, the

frequencies, and the αn are the phases.

The Fourier series does not dictate that the curve or wave

A transient wave such as that shown in Fig. 2-1(c) can beFourier series. A useful method of analyzing a wave is to

components. A plot of the square of the amplitudes of the

components, An in equation 2.3, against the frequency, υfrequency spectrum of the wave. Fig. 2-1(d) shows the sp

shown in Fig. 2-1(c).


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2.4. Acoustic Media

An acoustic wave exists only in a material, whether a solid

plasma. The material’s characteristics determine the wave

stronger the force between neighboring atoms, the more c

be their motion. On the other hand, the larger the mass ofmore force must be applied for the same acceleration. Be

synchronized movement of a large number of atoms, it is

of the material, ρ, rather than the mass of the individual atwave motion. Thus the wave velocity should be directly pr

atomic restoring force between the atoms or molecules, aproportional to the density. The actual relationship is:


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where Vi is the velocity for the particular type of wave and

elastic constant for that type of wave. The elastic constan

strength of the coupling between atoms for that particular

Different relative motions of the atoms will have different v

constant. Another property of the material is the characterdefined by equation 2.5, as follows:


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The reflection and transmission of acoustic waves at an in

two materials depend on the characteristic acoustic imped

materials. This dependence is given in equation 2.10. Aco

acoustic impedances, and densities for some materials of

emission tests are given in Table 1.

Table 1. Acoustic velocities and impedances for longi

Raleigh waves for several materials


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2.5. Types of Acoustic Waves

The path traced out by a particle under the influence of an

generally be represented by an ellipse with one of its axes

direction of the wave’s travel. The type of wave in bulk ma

determined by the relationship between the average particdirection of travel of the wave. For materials with boundar

of the wave and particle motion will be determined by the

the physical geometry of the sample, and the frequency o

Waves traveling through an extended medium (one whosmuch larger than the acoustic wave length) are called bul

types of pure bulk waves are longitudinal (compressional)

(transverse) waves. In both these waves, the minor axis o

atomic paths collapses toward zero, resulting in an approx

oscillatory motion.


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For longitudinal waves, the average motion of the atomic

the material is parallel to the direction of the wave propag

have the average of this motion perpendicular to the direc

(The motion of atoms around their equilibrium position is v

with much higher frequency components than acoustic wawaves are averages over a very large number of atoms in

average acoustic motion will be ellipses around the lattice

is no requirement that the axes of the ellipse correspond t

directions in the lattice).


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Particle motion in longitudinal and shear waves are illustra

and 2-2(b).

Since the relative motions of the particles in these two wa

the elastic constants and the wave velocities, therefore, athe shear velocity is slightly greater than one half of the lo

Waves often have both shear and longitudinal component

component traveling at its own velocity. In a non-attenuati

medium, a transient wave—sampled at some distance fro

origin—may appear to be two separate waves, one longitu

shear, as illustrated in Fig. 2-3.


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Figure 2-2. Particle

displacement for Bulk

acoustic waves, (a)

compressional wave,

(b) shear wave


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Figure 2-3. Waveform with

compression and shear wave (a) at

origin, (b) some distance from

origin


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compression and shear wave (a)

at origin, (b) some distance from

origin


shear

shear velocity is slightly greaterthan one half of the longitudinal

velocity. (Vshear = ½ Vcompression )

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The anisotropy of the coupling forces between atoms at th

bounded solid will produce additional types of waves. A su

maximum amplitude at the surface of the solid; its amplitu

distance from the surface. The plane of the particle motion

either parallel (Love waves) or perpendicular (Rayleigh waHowever, because most acoustic emission sensors detec

perpendicular to the surface, the parallel component is se

velocity of Rayleigh waves is slightly lower than the shear

is bounded by two surfaces so that it is a plate, and the th

is on the order of a few acoustic wave lengths or less, plawaves) can occur. A plate wave is essentially two surface

synchronized either symmetrically or antisymmetrically. P

Rayleigh waves and plate waves are illustrated in Fig. 2-4

Bulk waves, surface waves, and plate waves are the mos

waves seen in the field of acoustic emission. However, th

types of waves found in solids. In general, bounded solids

symmetrical geometry can support unique types of waves


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Figure 2-4. Particle displacements for

acoustic waves: (a) Rayleigh Wave, (b)

Plate wave, first symmetric mode (c)

Plate wave, first antisymmetric mode


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R a y l e i g

h W a v e

P l a t e w

a v e

s y m m e t r i c m o d e

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2.6. Dispersion and Group Velocity

The velocity defined in 2-4 is the phase velocity.

For unbounded media and surface waves on a single surf

velocity is independent of frequency.


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By contrast, all waves traveling in bounded media (where

dimensions are within an order of magnitude of the acous

dispersive, that is, the phase velocity is a function of frequ

Fig. 2-5 illustrates this principle, showing the frequency develocities for symmetric and anti-symmetric plate waves. D

have little effect on continuous waves. However, acoustic

packets of waves which can be thought of as a superposit

waves, as shown in equation 2.3. If each wave train, mak

travels at a different velocity, the wave packet will change

through the medium. As a result, the same acoustic emissdifferent when detected by the same sensor at different po

wave packet travels not at the phase velocity, but at the g

phase velocity can be defined by rewriting equation 2.4 as


(2

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Figure 2-5. Phase velocities for

different plate wave modes

plotted against the ratio of plate

thickness to acoustic

wavelength. Plotted for steelwith a Poisson’s ratio of 0.28.

The longitudinal, extensional,

shear, and Raleigh wave

velocities are shown.


Keys:Lamb, S0 symmetric mode

Lamb, A0 asymmetric mode

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while the group velocity is defined as:

In the absence of dispersion, these are the same velocity,

bounded solids the group velocity will be less than the pha

frequency dependent velocity can have real effects in acoone is attempting source location by measuring the differe

times at two or more sensors.

Keywords:

group velocity

phase velocity


(2.7)

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2.7. Attenuation

A wave packet is generated with a well-defined energy. A

propagates away from its source, the energy content will r

the absence of any dissipative mechanisms.

However, if the wave front of the packet is expanding, the

area on the wave front must decrease to conserve the tota

wave front. The rate of this decrease will depend on the g

medium.

In three dimensions, the energy per unit area will decreas

the distance from the source, while in two dimensions the

area will decrease linearly with this distance. If the packet

dimension, as in propagation down a rod, the energy per

independent of the distance from the source.


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DiscussionSubject: “In three dimensions, the energy per unit area w

square of the distance from the source, while in two dimen

per unit area will decrease linearly with this distance. If theto one dimension, as in propagation down a rod, the ener

be independent of the distance from the source.”


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If the packet is confined to o

as in propagation down a ro

per unit area will be indepen

distance from the source

in two dimensions thearea will decrease line

distanceIn three dimensions, the energy

per unit area will decrease as the

square of the distance from the

source

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Normally, in the context of acoustic waves, one assumes

traveling in only one dimension. Therefore this geometrica

packets’ energy is ignored. However, in an acoustic emiss

neither the location of the source nor the geometry of the

the investigator’s control, this geometrical effect should beattempt to measure the energy of the generated wave pac

attenuation of a plane wave arises from dissipative mecha

as the wave propagates. In a homogeneous medium, thes

occur as a fixed percentage of the wave packet energy pe

travel.


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Mathematically, this is an exponential decrease in the wav

distance that can be expressed as:

where α is an attenuation constant per unit length, and βconstant per unit time. The two constants are related by th

as shown by:


(2.9)

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Both forms of the attenuation constant are seen in the lite

Most of the many types of acoustic attenuation mechanism

of frequency dependence. Fortunately, in the normal acou

frequency range of 50 kHz to 1.0 MHz, both the frequency

the magnitude of many of these attenuation mechanisms structural materials. However, in composites, geological m

concrete the attenuation can be a severely limiting factor

tests, often restricting the useable frequency range to 100


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2.8. Interfaces, Reflection, Transmission, and Mode C

If a plane wave strikes a surface between two materials w

impedances, part of the wave will be reflected and part tra

intensities of the reflected and transmitted components ar

where the Zi are the acoustic impedances of the materialsare symmetrical, i.e. it does not matter in which medium th

when it hits the interface. The differences in acoustic impe

in large differences in the acoustic intensities transmitted

For example, the transmitted intensity of longitudinal wave

steel-aluminum, 12% for a steel-water, and 0.004% for a s


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When a plane wave strikes the interface, the angles of the

transmitted waves are governed by Snell's law

where θ1 is the angle of incidence, θ2 is the angle of reflerefraction, and the Vi (V1,2?) are the velocities in the mate

2.11, a transmitted velocity is positive and a reflected oneparticle motion anywhere on a wave front of a plane wave

wants to remain the same even when the wave passes an

at an interface, the direction of propagation will change ev

particle motion does not.

For a wave perpendicular to the surface (θ1 = 0°), this reschange of 180° in the relative motion of the particle to the without changing the character of the wave.


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Wave Perpendicular To The Surface (

1 = 0

)


For a wave

the surface

results in a180° in thethe particle

direction w

character o

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For non-perpendicular angles of incidence, the reflected a

waves will have both longitudinal and shear components

motion is parallel to the interface) because of the change

between the particle motion and the propagation direction

in Fig. 2-6. The process of generating both modes of bulkreflection or refraction is known as mode conversion.

In acoustic emission, where there is no control of the wav

inevitably the wave reaching the sensor is composed of b

shear components, no matter what its original polarizationsituations, surface waves are also present. Since mode co

almost every reflection, it is an almost continuous process

propagates in a bounded medium. Because of this continu

between modes traveling at different wave velocities, the

will lengthen in time as it travels instead of dividing into se

and shear components as shown in Fig. 2-3.


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compression and shear wave (a) at

origin, (b) some distance from

origin


Because of this continuous

transformation between modes

traveling at different wave

velocities, the transient wave form

will lengthen in time as it travelsinstead of dividing into separate

longitudinal and shear

components as shown in Fig. 2-3.

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Figure 2-6. Reflected and transmitted waves across an in

incident wave is a longitudinal wave with an angle of incid

double arrows show the direction of particle motion assoc

wave.


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Figure 2-7. Reflected and transmittedwaves inside a plate immersed in water.

(a) Successive reflections have been

displaced for clarity.

(b) Strain in a plate one half wavelength

thick.

(c) Amplitude as a function of frequency

in the plate for high and low Q

materials.

(d) Strain in a plate one wavelength and

one and one half wavelength thick.


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If there are a great many reflections, the peak strain can rlevel. If there are only a few reflections, the amplitude of tmuch less, and the peak will spread over a wider frequencfrequency of this wave need not be exactly that of a half w

some reinforcement; however, the greater the number of rnarrower will be the allowed frequency range at maximumlarger this maximum strain will be. This is illustrated in Fhigh amplitude peak is said to have a high Q, where Q is thenergy stored to energy dissipated. This increase of the str

a material at a half-wave thickness is known as a resonancfrequency. From Fig. 2-7b, we see that at resonance, the athroughout the plate is a maximum. Resonances can occugeometry allows acoustic waves to reflect in such a way thseveral reflections of the wave are superimposed. In piezo very high Q resonances allow precise generation of single


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Figure 2-7. Reflected and transmitted waves inside a plateimmersed in water. (a) Successive reflections have been

displaced for clarity. (b) Strain in a plate one half wavelength

thick. (c) Amplitude as a function of frequency in the plate for

high and low Q materials. (d) Strain in a plate one wavelength

and one and one half wavelength thick.

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Longitudinal waves- Plane pressure pulse waveLongitudinal waves, Longitudinal "l waves", are waves in which the displacement o

direction as, or the opposite direction to, the direction of travel of the wave. Mechan

also called compressional waves or compression waves, because they produce co

when traveling through a medium. The other main type of wave is the transverse w

displacements of the medium are at right angles to the direction of propagation. Somechanical, meaning that the wave needs a medium to travel through. Transverse

called "t-waves" or "shear waves".


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Longitudinal waves- Representation of the propagation

omnidirectional pulse wave on a 2d grid (empirical shape)Longitudinal waves, Longitudinal "l waves", are waves in which the displacement o

direction as, or the opposite direction to, the direction of travel of the wave. Mechan

also called compressional waves or compression waves, because they produce co

when traveling through a medium. The other main type of wave is the transverse w

displacements of the medium are at right angles to the direction of propagation. So

mechanical, meaning that the wave needs a medium to travel through. Transverse

called "t-waves" or "shear waves".


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Shear Wave– Plane shear waveS-waves, secondary waves, or shear waves (sometimes called an elastic S-wave)

and are one of the two main types of elastic body waves, so named because they

object, unlike surface waves. The S-wave moves as a shear or transverse wave, s

the direction of wave propagation. The wave moves through elastic media, and the

from shear effects. These waves do not diverge, and they obey the continuity equamedia.


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Shear Wave– Propagation of a spherical S-wave in a 2

model) S-waves, secondary waves, or shear waves (sometimes called an elaselastic wave, and are one of the two main types of elastic body waves, so named b

the body of an object, unlike surface waves. The S-wave moves as a shear or tran

perpendicular to the direction of wave propagation. The wave moves through elast

restoring force comes from shear effects. These waves do not diverge, and they o

for incompressible media.


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Longitudinal WavesIn a longitudinal wave the particle displacement is parallel to the direction of wave

at right shows a one-dimensional longitudinal plane wave propagating down a tube

down the tube with the wave; they simply oscillate back and forth about their individ

Pick a single particle and watch its motion. The wave is seen as the motion of the

pressure wave), which moves from left to right. The second animation at right showoscillatory motion of individual particles and the propagation of the wave through th

also identifies the regions of compression and rarefaction.

The P waves (Primary waves) in an earthquake are examples of Longitudinal wave

the fastest velocity and are the first to arrive.

harlie Chong/ Fion Zhang http://www.acs

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Transverse WavesIn a transverse wave the particle displacement is perpendicular to the direction of w

animation below shows a one-dimensional transverse plane wave propagating from

do not move along with the wave; they simply oscillate up and down about their ind

as the wave passes by. Pick a single particle and watch its motion.

The S waves (Secondary waves) in an earthquake are examples of Transverse waa velocity slower than P waves, arriving several seconds later.


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Water WavesWater waves are an example of waves that involve a combination of both longitudi

As a wave travels through the waver, the particles travel in clockwise circles. The r

decreases as the depth into the water increases. The movie below shows a water

right in a region where the depth of the water is greater than the wavelength of the

particles in yellow to show that each particle indeed travels in a clockwise circle as


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More Reading on“Phase velocities for different p

modes”


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Condition monitoring of large oil anstorage tanks using guided waves

AbstractLarge storage tanks containing hazardous liquids such as

products and food processing liquids are common through

Corrosion in the tank floor is a serious environmental and

order to monitor the condition of the tanks and prevent lea

must be inspected at regular intervals. Currently before ancarried out, the tank must be emptied and cleaned. This is

dangerous process due to weeks of lost production, trans

to temporary storage tanks and exposure of the workers to

inspection and cleaning.

harlie Chong/ Fion Zhang http://www.twi-global.com/technical-knowledge/published-papers

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TWI is managing a European CRAFT project called TANK

overcome the drawbacks of current inspection practices. B

wave sensors outside the tank and using reconstructive to

techniques there is potential for carrying out an inspection

instantly and without the requirement of emptying and cleaoperator entry inside the tank. TWI has been working with

Lithuania, using numerical modelling to study the potentia

technique. The effects of lap joints in the tank floors and a

liquid contents of the tank have been considered using bo

finite element modelling methods.


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1. IntroductionIn producing a system for the inspection of tank floors the

technical challenges to overcome. Guided waves, by natu

prismatic structures and therefore any changes to the cros

lap joint can cause undesirable reflections and noise, mak

signal difficult to interpret. In addition to this, the sound en

by materials surrounding the tank such as the liquid conte

sand or concrete underneath the tank. Modeling has been

study such effects in isolation and therefore gain a better

these issues so that practical solutions can be found.


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2. Global matrix model An initial global matrix model (a set of analytical formulae

waves in a multi-layer structure) was set up to consider th

varying connectivity possible at a lap joint. The bonding m

region where the steel plates overlap and these variations

the Lamb waves velocities and attenuation. The bond stre

layers has been considered as three different states: 'perf

The 'perfect' bond state means that all displacements are

the interface. In this case, the bonding layer properties ha

be the same as the steel plate properties and the bondingrelatively small. The 'poor' bond state means that displace

boundary are reduced compared with the 'perfect' bonding

interface. This was achieved by reducing the longitudinal

velocities in the bonding layer by a factor of two. The 'slip'

that shear displacements are not transferred through the i

state was reached setting shear velocity to 0m/s. A schem

shown in Fig.1.


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Fig.1. Schematic of lap joint model


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Fig.2. Phase velocity dispersion curves for 6mm steel pla


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Fig.3. Attenuation curves for 6mm steel plate with diesel o


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The phase velocity dispersion curves for a multi-layered z

one side are shown in Fig.4. There is little difference betw

states considered. However, in the 'slip' case, it was found

generated that propagates in the bond interface with zero

a Stoneley wave.


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Fig.4. Phase velocity dispersion curves for steel lap joint w

bonding layer conditions


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3. Finite element model A finite element model was used to analyse the propagati

a lap joint with diesel on one side and a vacuum on the ot

was assumed to exist between the two plates. The plates

be joined by a 45° fillet weld. Figure 5 shows the propagawave at different moments in time.

The first two figures in Fig.5 shows the lamb waves in the

attenuation into the liquid before the waves reach the weld

in Fig.5 shows the interaction of the waves with the joint.

This is shown in more detail in Fig.6. A lot of attenuation i

observed, induced by the fillet weld geometry.

Figure 7 shows the energy still existing in the lower steel p

through the lap joint.


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Fig.5. Lamb wave propagating in steel lap joint at differen


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Fig.5. Lamb wave propagating in steel lap joint at differen


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Fig.7. Finite element modeling results after lamb wave ha

fillet weld


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5. ConclusionsThere is great potential for the use of guided waves to ins

service. The work has shown that modelling can be us

modes to optimise the minimisation of attenuation loss

contents of the tank. The modelling has also shown tha

propagate past fillet welds and therefore there is poten

corrosion anywhere on a typical tank floor with lap join

Acknowledgement

The authors would like to thank the other partners working on this project: Spree E

ISOtest Engineering Srl, Coaxial Power Systems Ltd., Royal Vopak, ST ServicComputer Consultancy Ltd. and the European commission for funding this wo

References

Non-destructive testing Handbook, 2nd edition, Vol. 7. Ultrasonic Testing / A.S. Bir

USA: American Society for Non-destructive Testing, 1991.

Demcenko A., Mazeika L. Calculation of Lamb waves dispersion curves in multi-la

Ultragarsas. 2002. Vol. 3 (44). P. 15 - 17.


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More on Lamb WaveVelocity dispersion inherent in the characteristic equa

Lamb waves exhibit velocity dispersion; that is, their veloc

depends on the frequency (or wavelength), as well as on and density of the material. This phenomenon is central to

understanding of wave behavior in plates. Physically, the

the ratio of plate thickness d to wavelength {\displaystyle \

determines the effective stiffness of the plate and hence th

wave. In technological applications, a more practical para

derived from this is used, namely the product of thickness


since for all waves

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The relationship between velocity and frequency (or wave

in the characteristic equations. In the case of the plate, th

not simple and their solution requires numerical methods.

intractable problem until the advent of the digital compute

Lamb's original work. The publication of computer-generacurves" by Viktorov[3] in the former Soviet Union, Fireston

Worlton in the United States, and eventually many others

theory into the realm of practical applicability. Experiment

observed in plates can be understood by interpretation wi

dispersion curves.


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Dispersions curves of free Lamb waves for two different P

x-axis shows the product of angular frequency and plate t

by the shear wave velocity. The y-axis shows the phase v

wave normalized by the shear wave velocity. For high freq

have the Rayleigh wave velocity, approximate 92 % of thevelocity.


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3.0 CHAPTER 33.1. Detection of Acoustic Emission

3.1.1. Sensors

A sensor is a device which generates an electrical signal w

by an acoustic wave. Acoustic emission (AE) sensors can

several physical principles. The signals can be generated

devices such as phonograph pickups, capacitive micropho

magnetostrictive devices, piezoelectric devices, and by th

interferometers to detect the surface displacement of the s

relationship between the characteristics of the wave and twill depend on both the sensor and the wave. An ideal se

a voltage-time curve identical to the amplitude-time curve

point where the sensor is located. Although no sensor app

for certain types of acoustic waves, laser interferometry w

available sensors operate quite well for specified types of

ranges of parameters.


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Magnetostriction (cf. electrostriction) is a property of ferromagthem to change their shape or dimensions during the process of magnetization. Th

magnetization due to the applied magnetic field changes the magnetostrictive strai

value, λ. The effect was first identified in 1842 by James Joule when observing a s

This effect causes energy loss due to frictional heating in susceptible ferromagnetiresponsible for the low-pitched humming sound that can be heard coming from tran

oscillating AC currents, which produce a changing magnetic field

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Magnetostrictive ultrasonic transducers utilize the principle of m“ferromagnetic” materials which include iron, nickel and cobalt as well as many alloys of these th

waves in a liquid. The magnetostrictive principle was described in some detail in a previous blog

magnetostriction and ferromagnetism can be found in a “reader friendly” paper entitled Magneto

written by Geoffrey P. McKnight of the UCLA Active Materials Lab. Devices to create motion us

magnetostriction were first developed in the 1930’s and perhaps earlier although earlier work wa

1950’s, the technology expanded into the ultrasonic frequency range in response to a need for urobust than those using the (then) fragile crystal compositions that were used in piezoelectric ult

Some of the developmental work on magnetostrictive transducers in the 1950’s was done by Ale

Murdoch Laboratories, a predecessor of Blackstone Ultrasonics and Blackstone-NEY Ultrasonic

ceramic piezoelectric materials and advanced Langevin type piezoelectric ultrasonic transducers

magnetostrictive transducer was, probably, the most reliable and powerful ultrasonic transducer

ultrasonic manufacturers.

A magnetostrictive ultrasonic transducer consists, essentially, of series of laminations of a magn

active material attached directly to a vessel or tank which holds the liquid to be ultrasonically acteither around or in proximity to the laminations of magnetostrictive material to provide the oscilla

cause cause them to, in turn, vibrate. The basic construction is shown below.


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Initially, all piezoelectric devices were made from single c

useful of these were quartz, Rochelle salt, and ammonium

phosphate. Later, a class of materials known as ferroelect

materials which have a polarization even in the absence o

investigated and found useful. Ceramics made of ferroelea uniform direction of polarization similar to that found in a

crystal. It became possible to produce ferroelectric ceram

properties superior to piezoelectric single crystals with the

all acoustic emission sensors today are made from a varie

ceramics.


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3.1.3. Size Effects

An infinitesimal piece of piezoelectric material with many d

electrodes totally embedded in a sample would come clos

sensor. However, when we scale up the piezoelectric to a

placing one set of electrodes on the outside surface of thedepart from that ideal. The physical size of the sensor res

effects, resonance and strain averaging. Both can becom

the physical dimensions of the sensor approach or exceed

the acoustic wave. Since the output of a piezoelectric crys

the strain (and proportional to the average strain for a crysdimensions), the maximum output of a sensor occurs at it

frequencies.


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The fundamental resonant frequency of a plate

occurs when the plate thickness is one-half

wave length as shown in Fig.2-7b. The reflected

wave is in phase with the incident wave at the

surface and the strains add. If the frequency isincreased until there is one full wavelength in

the crystal, there will again be strain re-

enforcement due to the reflected waves.

However, we can see in Fig. 2-7d that while the

strain level may be very great at this frequency,2υo, the average strain over the crystal exactlycancels so that the output of the sensor is zero.

Increasing the frequency to 3υo, we see 1.5wavelengths in the crystal and re-enforcement

again occurring. The average strain over two-

thirds of the crystal now cancels out, but theaverage strain over the last third reaches a

maximum.


Figure 2-7. Rimmersed in water.

clarity. (b) Strain in a

a function of frequen

Strain in a plate onethick.

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The result is that a piezoelectric sensor will have a maxim

whenever the thickness, d, is:

d = (2n - l)λ/2 (3.1)

and no output when

d = nλ (3.2)


Thus a sensor can be operated either

at its fundamental frequency,υo (1/2,1/4λ?), or its harmonic frequencies

nυo where n is odd.

Comments: Why odd?

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Standing Wave FormationThe animation below depicts two waves moving through a medium in opposite directions. The blue wave is moving to the right and the green w

situation in which two waves meet while moving along the same medium, interference occurs. The blue wave and the green wave interfere to fo

The resultant in the animation below is shown in black. The resultant is merely the result of the two individual waves - the blue wave and the gr

principle of superposition. The result of the interference of the two waves above is a new wave pattern known as a standing wave pattern. Stan

identical frequency interfere with one another while traveling opposite directions along the same medium. Standing wave patterns are characte

which undergo no displacement. These points of no displacement are called nodes (nodes can be remembered as points of no desplacement)

animation above. The nodes are always located at the same location along the medium, giving the entire pattern an appearance of standing st

inspection of the above animation will reveal that the nodes are the result of the destructive interference of the two interfering waves. At all timegreen wave interfere to completely destroy each other, thus producing a node.

harlie Chong/ Fion Zhang http://ww

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First Harmonic

Standing Waves on a String A standing wave pattern is a pattern which results from the interference of two or more waves along the same medium. All stan

positions along the medium which are standing still. Such positions are referred to as nodal positions or nodes. Nodes occur a

one wave is displaced upward the same amount that a second wave is displaced downward. This form of interference is know

point of "no displacement." A node is a point of no displacement. Standing wave patterns are also characterized by antinodal p

vibrate back and forth between a maximum upward displacement to a maximum downward displacement. Antinodes are locatetwo interfering waves are always undergoing constructive interference. Standing wave patterns are always characterized by an

There are a variety of patterns which could be produced by vibrations within a string, slinky, or rope. Each pattern corresponds

frequency and is known as a harmonic. The lowest possible frequency at which a string could vibrate to form a standing wave

or the first harmonic. An animation of a string vibrating with the first harmonic is shown below.The frequency associated with e

which waves move through the medium and the wavelength of the medium. The speed at which waves move through a mediu

medium (tension of the string, thickness of the string, material composition of the string, etc.). The wavelength of the harmonic

the harmonic number (first, second, third, etc.). Variations in either the properties of the medium or the length of the medium wthe string will vibrate.

harlie Chong/ Fion Zhang http://www

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There are a variety of other low energy vibrational pattern

established in the string. For guitar strings, each pattern i

some basic traits:

There is an alternating pattern of nodes and antinodes

There are either a half-number or a whole number of w

pattern established on the string.

Nodal positions (points of no displacement) are establ

the string where the string is clamped down in a fixed

One pattern is related to the next pattern by the additio

one or more nodes (and antinodes).

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Harmonic:

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Fundamental and HarmonicsThe lowest resonant frequency of a vibrating object is called its fundamental freque

have more than one resonant frequency and those used in musical instruments typ

the fundamental. A harmonic is defined as an integer (whole number) multiple of th

Vibrating strings, open cylindrical air columns, and conical air columns will vibrate

fundamental. Cylinders with one end closed will vibrate with only odd harmonics ofmembranes typically produce vibrations at harmonics, but also have some resonan

harmonics. It is for this class of vibrators that the term overtone becomes useful - t

non-harmonic overtones.

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Fundamental and Harmonics with one EnCylinders with one end closed will vibrate with only odd harmoni

Vibrating membranes typically produce vibrations at harmonics,


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Fundamental and Harmonics with one EnCylinders with one end closed will vibrate with only odd harmoni

Vibrating membranes typically produce vibrations at harmonics,

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Open End Standing Wave Patterns

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Non-Harmonic


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Harmonic: If both ends of the string are fixed the pulses t

left will interfere, left below. If, instead of pulses, we imag

of the same amplitude, frequency and wavelength, but tra

directions we obtain the standing wave pattern, below righ


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The Q of the transducer depends only on the number of re

sensor; therefore, the Q is independent of the harmonic a

operating, as long as the material of the sensor does not s

dependent attenuation. Also, the sensor will always have

frequencies below the fundamental frequency, υo. At frequabout 3υo/4, the resonance will have no effect and the outessentially independent of frequency. In reality, a materia

in one dimension without producing strains in other directi

Fig. 3-1a. Many acoustic emission sensors use this to get

response, using a piezoelectric element in the shape of a wave with a vertical displacement on the cylinder face wil

resonance. This resonance will then give a large output s

sensor is a very sensitive detector of acoustic emission. H

not be used to measure the frequency spectrum nor the a

the acoustic wave since this cross coupling of vibration m

distorted representation of the wave


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Figure 3-1. (a) Deformation of a material

showing multiaxial strain resulting from

uniaxial force. (b) spectral response of an

acoustic emission sensor


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In addition to resonance effects, there is another importan

averaging by a sensor. Fig.3-2a shows a block with a sen

the sensor is excited with a compressional wave moving p

surface, the entire sensor face will move in phase. Exclud

average strain in the sensor will be independent of frequea Rayleigh wave which is traveling parallel to the sensor f

particle motion perpendicular to the sensor face. In this ca

distribution in the transducer will vary as a function of dista

wave. Fig.3-2b shows the strain variation where the diame

less thanλ

/2. Here the output is still proportional to the awave. In Fig. 3-2c, the diameter of the sensor is larger tha

In this case, for every complete wave-length under the se

averages to zero. Only the extra fraction of the wavelengt

contributes to its output.


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Figure 3-2. (a) Sample block with a sensor mounted on o

compressional wave is shown traveling perpendicular to t

a Rayleigh wave travelling parallel to the face. (b) Instanta

block’s surface from Rayleigh wave with wavelength much

sensor diameter. (c) Strain on surface from Rayleigh waveshorter than sensor diameter. (d) Sensor output as a func

Rayleigh waves with equal amplitudes.


λ

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This averaging essentially reduces the effective area of th

higher the frequency, the greater the reduction. Additiona

frequencies, depending on the shape of the sensor and th

of the sample, the total strain averages to zero. These effe

Fig. 3-2d where the response for this type of surface wavesensor with a flat frequency response to compressional w

to its face. The high frequency response of such a sensor

drastically on the angle of incidence with which the wave s

This averaging effect depends on the acoustic wave-lengt

Therefore, the sensor response not only is going to vary wangle of incidence, but also it is going to vary when used w

materials. The best answer to this problem of averaging s

of sound over the surface of the sensor is to make the sen

For steel, a 3mm diameter sensor should work reasonably

kHz. The inevitable tradeoff is that the smaller sensor has

capacitance and thus, as will be discussed later, a reducesensitivity.


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3.1.4. Couplants

We have assumed to this point that the sensor has simply

surface of the material containing the acoustic wave. Whe

found that the sensor produces a very weak signal. If a th

placed between the sensor and the surface, a much largeThis fluid acts a couplant that ensures good contact betwe

a microscopic level. The use of some type of couplant is a

the detection of low level acoustic signals. Physically, this

by looking at the acoustic wave as a pressure wave trans

surfaces in contact. On a microscopic scale, the surfaces the material are quite rough; they actually touch in only a

are in contact. Stress is force per unit area and the actual

force is very small. If the microscopic gaps are filled with a

will be uniformly transferred between the surfaces. For a s

variable strain component parallel to the surfaces, very litt

transferred between the surfaces because of the few poinIn this case, filling the gaps with a low viscosity liquid will

such a liquid will not support a shear stress. However, a h

or a solid will help transmit the parallel strain between sur


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Although the terms bond and couplant have been used in

many AE reports (including the author’s), their meanings s

Strictly, a couplant is any material which aids the transmit

waves between two surfaces, while a bond is a couplant w

holds the sensor to the surface. For example, water is a cepoxy resin is a bond. Many problems have come about f

an inapplicable way. If a rigid bond is used to attach a sen

which elastically deforms during the test, the normal resul

and poor or no sensitivity to the acoustic wave.


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Similarly, in an experiment where the temperature is chan

the use of a rigid bonding material can lead to broken bon

differential thermal expansion between the sensor and the

agents, then, must be chosen carefully, considering the co

materials under the test conditions. Usually, if the bond won, it will be an adequate couplant. For a compressional w

act as a couplant. A highly viscous fluid will transfer some

the boundary which may or may not be an advantage. In o

author tested a large number of couplants with compressi

all couplants showed an increase in the signal strength ov30+2 dB. The variation was little more than the uncertaint

measurement. Practically, a couplant can be a thin layer o

that wets both surfaces. The sensor should be held again

some pressure furnished by magnets, springs, tape, rubbe

secret is to use as thin a layer as possible. If a rigid bond

be minimal differential expansion between the two surface


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A flexible bond can also be used. Over the years, the auth

excellent results with GE Silicone II sealant. It is available

household glue which will set up within about 12 hours wh

one inch diameter sensor and a metal or plastic surface. I

glue, a perpendicular sinusoidal force of about 100 G prodfailures. It is quite flexible, allowing for sensor removal fro

knife blade or wood chisel slid between the sensor and th

2, a few commonly used couplants are listed along with th

range in which they can be used.


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Table 2. Some Common Acoustic Emission Couplants

Approximate Tempera-ture Ranges


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3.1.5. Temperature Effects

The temperature dependence of piezoelectricity is compli

discussion of its effect on sensors is beyond the scope of

However, certain effects can lead to problems when a sen

different temperatures. First, ferroelectric materials, such ceramics, have a Curie temperature above which the mat

another, and usually non-ferroelectric, phase. Taking a fe

through the Curie temperature will remove the polarization

piezoelectric properties of the sensor, and may shatter the

Ferroelectric sensors will usually work well up to temperatthe Curie temperature, if the other materials in the sensor

temperature. The Curie temperatures of PZT ceramics lie

and 400°C.


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Ferroelectric ceramics are poly-crystalline. Each crystallite

or more ferroelectric domains, i.e. regions where the spon

polarization is all in one direction. This polarization exists

directions in the crystal structure. When the ceramic is po

are aligned, as closely as the crystal orientation allows, topolarization. Because of the random orientation of the cry

number will have several possible orientations approxima

polarization in the ceramic. Small strains may be enough

to change orientation. Such a flip of a domain may cause

in the polarization of the sensor. However, this change is magnitude as the change caused by a small acoustic wav

impossible to distinguish an electric signal caused by a fli

one caused by acoustic emission.


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3.1.6. Sensor Sensitivity - Effects of Cables

The sensitivity of a sensor is governed by the intrinsic sen

piezoelectric material, the dimensions of the piezoelectric

design and materials used in its case. Practically, one rec

manufacturer a measured response curve to a standard scapacitance of the sensor. This curve is often presented a

independent of the measurement technique. However, the

sensor will always depend, in part, on the equipment with


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The open circuit voltage produced by a sensor is a proper

piezoelectric element and is：

Vo(S) = Q(S)/Co (3.3)

where Q is the charge produced by a strain S, and Co is tthe sensor. When connected to a preamplifier, the actual

input resistor of the preamplifier (given large input resistan

V(S) = Q(S)/(Co + Cc + CI)

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