Muravin physical principals of acoustic emission

Seminar onFundamentals of Acoustic Emission

לשכת המהנדסים, האדריכלים והאקדמאים

במקצועות הטכנולוגיים בישראלאגודת מהנדסי מכונות - ענף בדיקות לא

הורסות

Dr. Boris MuravinChairman of Israeli Acoustic Emission Group

Feb 24, 2011

יום עיון השנתי בנושא פליטה אקוסטית

This presentation for acoustic emission education purposes onlyMore in www.muravin.com

סיכום פעילות התא לפליטה אקוסטית 2010בשנת .

יום יעון בפליטה אקוסטית.•

.2010כנס מכונות •

.2011כנס בל"ה •

.EN473 ו ASNTאירגון קורס בייסיק לפי •

.ASNT לפי IIIאירגון מבחני הסמכה לרמה •

ליווי פרואקטים ויעוץ.•

More in www.muravin.com

Seminar Outline

9:15 – 10:30 History of AE, physical principals of acoustic emission, AE waves, AE source location

10:30-10:45 Break

10:45 – 12:30 Apparatus, data acquisition and analysis

12:30-13:15 Lunch break

13:15 – 14:30 Apparatus, data acquisition and analysis – Cont.

14:30-14:45 Break

14:45 – 16:00 Applications and standards.

16:00-16:15 Break

16:15 – 17:30 Applications and standards – Cont.


History of AE


Who was the First?They were the First who used AE

as an alarm systemHe was the First who used AE as a

forecasting tool


Early History of AE

ירמיהו נא,נדקול זעקה מבבל ושבר גדול מארץ כשדים “ The sound of a cry from Babylon and the sound of great fracture

<comes> from the land of the Chaldeans.” Jeremiah 51:54 • One of the first sources that associates sound with fracture can

be found in the Bible.• Probably the first practical use of AE was by pottery makers,

thousands of years before recorded history, to asses the quality of there products.

• Probably the first observation of AE in metal was during twinning of pure tin as early as 3700 B.C.

• The first documented observation of AE in Middle Ages was made by an Arabian alchemist, Geber, in the eighth century. Geber described the “harsh sound or crashing noise” emitted from tin. He also describes iron as “sounding much” during forging.


History of First AE Experiments

• In 1920, Abram Joffe (Russia) observed the noise generated by deformation process of Salt and Zinc crystals.“ The Physics of Crystals” , 1928.

• In 1936, Friedrich Forster and Erich Scheil (Germany) conducted experiments that measured small voltage and resistance variations caused by sudden strain movements caused by martensitic transformations.

• In 1948, Warren P. Mason, Herbert J. McSkimin and William Shockley (United States) suggested measuring AE to observe the moving dislocations by means of the stress waves they generated.

• In 1950, D.J Millard (United Kingdom) performed twinning experiments on single crystal wires of cadmium. The twinning was detected using a rochelle salt transducer.


History of First AE Experiments

• In 1950, Josef Kaiser (Germany) used tensile tests to determine the characteristics of AE in engineering materials. The result from his investigation was the observation of the irreversibility phenomenon that now bears his name, the Kaiser Effect.

• The first extensive research after Kaiser was done in the United States by Bradford H. Schofield in 1954. Schofield investigated the application of AE in the field of materials engineering and the source of AE. He concluded that AE is mainly a volume effect and not a surface effect.

• In 1957, Clement A. Tatro, after performing extensive laboratory studies, suggested to use AE as a method to study the problems of behavior of engineering metals. He also foresaw the use of AE as an NDT method.


Start of Industrial Application of AE• The first AE test in USA was conducted in the Aerospace industry to verify

the integrity of the Polaris rocket motor for the U.S Navy (1961). After noticing audible sounds during hydrostatic testing it was decided to test the rocket using contact microphones, a tape recorder and sound level analysis equipment.

• In 1963, Dunegan suggested the use of AE for examination of high pressure vessels.

• In early 1965, at the National Reactor Testing Station, researchers were looking for a NDT method for detecting the loss of coolant in a nuclear reactor. Acoustic Emission was applied successfully.

• In 1969, Dunegan founded the first company that specializes in the production of AE equipment.

• Today, AE Non-Destructive Testing used practically in all industries around the world for different types of structures and materials.


Physical Principals of Acoustic Emission


Definition of Acoustic Emission Phenomenon

Acoustic Emission is a phenomenon of sound and ultrasound wave radiation in materials undergo deformation and fracture processes.

ASTM E1316-2010 definition:

Acoustic Emission (AE)—the class of phenomena whereby transient elastic waves are generated by the rapid release of energy from localized sources within a material, or the transient waves so generated. Acoustic emission is the recommended term for general use. Other terms that have been used in AE literature include (1) stress wave emission, (2) microseismic activity, and (3) emission or acoustic emission with other qualifying modifiers.


Classification of AE


Classes and Mechanisms of Acoustic Emission

Mechanical acoustic emission - acoustic emission generated by a leakage, friction, impact or other sources of mechanical origin.

Material acoustic emission - acoustic emission generated by a local dynamic change in a material structure due to fracture development and/or deformation processes.


Source Mechanisms of AE in Metals

……..interactionmotionformation

……..interactionmotionformation

Phase

changes

Possible combinations

AE SOURCES10 6.9236

Twining

Slip

……branchingdevelopmentnucleation

……branchingdevelopmentnucleation

crackformation

fracturing

bond connectionfracturing

crackformation

fracturing

bond connectionfracturing

Inclusions

interactiongrowthnucleation

interactiongrowthnucleation

Micro-crack

Voids

movement

annihilation

interactionmigration

generationnucleation

movement

annihilation

interactionmigration

generationnucleation

Dislocations

Recrystalli-

zation

More then 80% of energy expended on fracture in common industrial metals goes to development of plastic deformation.

More then 80% of energy expended on fracture in common industrial metals goes to development of plastic deformation.


Source Mechanisms in Composites

• Matrix cracking.• Fiber fracture.• Delamination.• Fiber pullout.• Friction.


Primary vs. Secondary AE

Primary AESecondary AE

Crack jumpCrack surface friction

Plastic deformationInclusion breakage in the process zones

Crack growthCorrosion layer fracture in corrosion fatigue cases


AE Types: Burst and Continuous AE Signals


Some Mechanisms of Burt and Cont. AE


TreeTree Falls FallsTreeTree Falls Falls

PaperPaper teartear

ICE CRACKICE CRACK GlassGlassIce Crack

BrakeBrake

Identification of distinctive flaw-type-material specific AE characteristics of different flaws in high energy equipment is a necessary condition for reliable diagnostics.

GlassGlass BrakeBrake

Acoustic Emission is Flaw-Type-Material Specific


AE in Metals


Metals


Factors that Tend to Increase or Decrease the Amplitude of AE

Nondestructive Testing Handbook, Volume 6 “Acoustic Emission Testing”, Third Edition, ASNT.


“Friendly” Metals for Traditional AE Inspections

• “Friendly” (for newcomers) metals can be considered those producing high amplitude (high Signal-to-Noise ratio) emissions.

• Examples are various carbon steels, cast irons, low ductility steels, high inclusion content steels, large grain size steels.


Detectability by AE Depends on Failure Mechanism

• Flaws due to different failure mechanisms have different propagation characteristics: intergranular or transgranular, cleavage or ductile fracture, small scale or large yielding, etc.

• For example high cycle and low cycle mechanical fatigue may different detectability (if we define detectability based on level of AE amplitudes).


Plastic Deformation as a Major Source of AE


Plastic Zone at the Crack Tip• Flaws in metals can be revealed by detection of indications of plastic

deformation development around them.• Cracks, inclusions, and other discontinuities in materials concentrate stresses.• At the crack tip stresses can exceed yield stress level causing plastic

deformation development.• The size of a plastic zone can be evaluated using the stress intensity factor K,

which is the measure of stress magnitude at the crack tip. The critical value of stress intensity factor, KIC is the material property called fracture toughness.

2

1

2

plastic zone size in elastic material

Iy

ys

y

Kr

r

Fracture Mechanics Fundamentals and Applications, Second Edition, T.L. Anderson.


Comparison of Plastic Deformation around Crack Tips Mode I and II

1

2

3

Mode II

0.5 mm

1

Mode I

Plastic Strain

Plastic Stress

Mode I Mode II

1. The size of plastic deformation around crack mode II is significantly larger (up to 3-5 times, see /1-3/) than the plastic deformation zone around crack mode I under the same load/max load ratio.

2. According to our experimental results, the estimated relation between plastic deformation zone for Mode II and Mode I crack is approximately 3.

Our experimental results

J. F. Kalthoff, Failure Methodology of Mode-II Loaded Cracks, Mechanics, Automatic Control and Robotics Vol.3, No 13, 2003, pp. 533 - 552More in www.muravin.com

Ellipses of Dispersion Energy vs. Average Frequency of Single Fatigue Crack under Mode I and Mode II Loading (1200kgf)

3_

_

MIPlastic

MIIPlastic

E

E

1 –Plastic Deformation 2 – Micro Cracking

Flaw TypeAmount of Data

Released Energy, r.u.

Plastic Deformation

990.33

Micro-Cracks160.5

Flaw TypeAmount of Data

Released Energy, r.u.

Plastic Deformation

2281

Micro-Cracks231

2__

__

MICracksMicro

MIICracksMicro

E

E


Models of AE in MetalsPlastic Deformation Model

• Plastic deformation model relates AE and the stress intensity factor ( ).• AE is proportional to the size of the plastic deformation zone.• Several assumptions are made in this model: (1) The material gives the highest rate of AE

when it is loaded to the yield strain. (2) The size and shape of the plastic zone ahead of the crack are determined from linear elastic fracture mechanics concepts.

1K

2

11

2 or 6 (plain stress or plain strain)

yys

Kr

(3) Strains at the crack tip vary at where r is the radial distance from the crack tip. (4)

AE count rate

volume strained between (yield strain) and (uniform strain)

p

p y u

N V

N

V

0.5r

2 22 2 4 42 2 4

4

4

1 1

2 2 4 4

plate thickness

V

u yp y u

y u y u

p

K K BV r r B B K

E E E

B

K

N K

The assumptions lead to development of the following equations for the model ( ) 2


ASTM E1316: 2010Kaiser effect—the absence of detectable acoustic emission at a fixed sensitivity level, until previously applied stress levels are exceeded. Discussion—Whether or not the effect is observed is material specific. The effect usually is not observed in materials containing developing flaws.

AE Effects• Kaiser effect is the absence of detectable AE at a fixed sensitivity level, until

previously applied stress levels are exceeded.• Dunegan corollary states that if AE is observed prior to a previous maximum

load, some type of new damage has occurred. The dunegan corollary is used in proof testing of pressure vessels.

• Felicity effect is the presence of AE, detectable at a fixed predetermined sensitivity level at stress levels below those previously applied. The felicity effect is used in the testing of fiberglass vessels and storage tanks.

stress at onset of AEfelicity ratio

previous maximum stress

Kaiser effect (BCB)

Felicity effect (DEF)More in www.muravin.com

Kaiser Effect• The immediately irreversible characteristic of AE resulting from an applied

stress at a fixed sensitivity level.• If the effect is present, there is an absence of detectable AE until previously

applied stress levels are exceeded.

Example of the Kaiser Effect in a cyclically loaded concrete specimen. Thick black lines represents AE activity, thin lines the loads and dashed lines the Kaiser Effect.

http://www.ndt.net/ndtaz/content.php?id=476


AE Waves


Types of Acoustic Emission Waves

Type of AE waves generated depend on material properties, its mechanical behavior and level of stresses at the source. AE waves can be:

• Elastic.• Non-linear elastic.• Elastic-plastic.• Elastic-viscoplastic and other.

Inelastic waves attenuate at short distances and therefore elastic waves are mostly detected and analyzed in acoustic emission testing.


Modes of Elastic Waves Propagation

• Longitudinal (dilatational, P-) wave is the wave in which the oscillations occurring in the direction of the wave propagation.

• Shear (or transverse, or distortional, or equivolumal, or S-) wave is the wave in which the oscillations occurring perpendicular to the direction of the wave propagation.

• Rayleigh (or surface) wave is the wave with elliptic particle motion in planes normal to the surface and parallel to the direction of the wave propagation.

• Lamb (or plate) wave is the wave with particles motion in perpendicular to the plate.

• Stoneley (or interfacial) wave is the wave at interface between two semi-infinite media.

• Love wave is the wave in a layered media, parallel to the plane layer and perpendicular to the wave propagation direction.

• Creeping wave is the wave that is diffracted around the shadowed surface of a smooth obstacle.


Longitudinal, Shear, Rayleigh and Love Waves

Reference:http://web.ics.purdue.edu/~braile/edumod/slinky/slinky.htmMore in www.muravin.com

Wave Modes in Different Geometries• In infinite media there are only two

types of waves: dilatational (P) and distortional (S).

• Semi-infinite media there are also Rayleigh and Lateral (Head) waves. Head waves produced by interaction of longitudinal wave with free surface.

• In double bounded media like plates there are also Lamb waves.

t = 10 mm

t = 5 mm

In thinnest plates only Lamb wave arrivals are visible .

Symmetric

Antisymmetric


Example of AE Signal


Wave Speed in Different Materials

2

2

2

1

1

2

1

14.1862.0

2

CC

CC

C

C

P

R

Wave speeds derivation:

λ and μ – Lame constantsν – Poisson’s ratioρ – material density


Properties of Elastic Waves in Semi-Infinite Media

• Rayleigh waves carry 67% of total energy (for ν=0.25).• Shear 26%.• Longitudinal 7%.

• Longitudinal and shear waves decay at a rate 1/r in the region away of the free surfaces.

• Along the surface they decay faster, at a rate 1/r2.• Rayleigh waves decays much slower, at a rate of

1/sqrt(r).

Reference: “Dynamic Behavior of Materials” by M. MeyersMore in www.muravin.com

Wave Propagation EffectsThe following phenomena take place as AE waves propagate along the structure: Attenuation: The gradual decrease in AE amplitude due to energy loss

mechanisms, from dispersion, diffraction or scattering. Dispersion: A phenomenon caused by the frequency dependence of speed for

waves. Sound waves are composed of different frequencies hence the speed of the wave differs for different frequency spectrums.

Diffraction: The spreading or bending of waves passing through an aperture or around the edge of a barrier.

Scattering: The dispersion, deflection of waves encountering a discontinuity in the material such as holes, sharp edges, cracks inclusions etc….

Attenuation tests have to be performed on Attenuation tests have to be performed on actual structures during their inspection.actual structures during their inspection.

The attenuation curves allow to estimate The attenuation curves allow to estimate amplitude or energy of a signal at a given amplitude or energy of a signal at a given distance from a sensor. distance from a sensor.


Group and Phase VelocityLord Rayleigh: “It have often been remarked that when a group of waves advances

into still water, the velocity of the group is less than that of the individual waves of which it is composed; the waves appear to advance through the group, dying away as they approach its interior limit” (1945, Vol. I, p. 475).

• Group velocity is the velocity of propagation of a group of waves of similar frequency.

• Phase velocity is the velocity at which the phase of the wave propagates in the media.

Reference:http://www.owrc.com/waves/waveSpeed/waveSpeed.html


Dispersion Curves

Example calculated for steel 347 plate (10mm thick)

Triple pointNon-dispersive part of A0 mode


Use of Dispersion CurvesDispersion curves can be effectively used for accurate location

and characterization of AE sources. Examples:• Filtering AE waveforms at frequency of the triple point (200

kHz), one can improve location accuracy. This is because all modes at this frequency have similar speed and the threshold will be triggered by the same wave mode at all sensors.

• Filtering AE waveforms over non-dispersive range of A0 mode (80-180 kHz) can improve location accuracy even further. In this technique a wider frequency range of the original signal remain after filtration while the frequency content of the mode remain unchanged over the distance.


AE Source Location


Principals of Acoustic Emission Source Location• Time difference based on Time of Arrival

locations.• Cross-correlation time difference location.• Zone location.• Attenuation based locations.• Geodesic location.


Time of Arrival Evaluation• Most of existing location procedures require

evaluation of time of arrival (TOA) of AE waves to sensors.

• TOA can detected as the first threshold crossing by AE signal, or as a time of peak of AE signal or as a time of first motion. TOA can be evaluated for each wave mode separately.


Effective Velocity• Another parameter necessary for time difference location method is effective

velocity.• Effective velocity can be established experimentally with or without considering

different wave propagation modes.• When propagation modes are not separated, the error in evaluation of AE source

location can be significant. For example, in linear location it can be about 10% of sensors spacing.

• Detection of different wave modes arrival times separately and evaluation of their velocities can significantly improve location accuracy. Nevertheless, detection and separation of different wave modes is computationally expensive and inaccurate in case of complex geometries or under high background noise conditions.

• Another parameter necessary for time difference location method is effective velocity.

• Effective velocity can be established experimentally with or without considering different wave propagation modes.

• When propagation modes are not separated, the error in evaluation of AE source location can be significant. For example, in linear location it can be about 10% of sensors spacing.

• Detection of different wave modes arrival times separately and evaluation of their velocities can significantly improve location accuracy. Nevertheless, detection and separation of different wave modes is computationally expensive and inaccurate in case of complex geometries or under high and variable background noise conditions.


Linear Location• Linear location is a time difference method commonly used to locate AE

source on linear structures such as pipes, tubes or rods. It is based on evaluation of time difference between arrival of AE waves to at least two sensors.

• Source location is calculated based on time difference and effective wave velocity in the examined structure. Wave velocity usually experimentally evaluated by generating artificially AE at know distances from sensors.

1

2distance from first hit sensor

D = distance between sensors

wave velocity

d D T V

d

V


One Sensor Linear Location• It is possible to use one sensor to evaluate the distance

from AE source (but not direction).• The principal of this location is based on phenomenon of

different velocity of propagation of different wave modes.• Such location method can be used on short rods, tubes or

pipes, when mode detection and separation can be effectively performed.


Two Dimensional Source Location

1,2 1 2

2

2 2 21 2

2 2 2 22 1 2

2 2 22 1

1 1,2 2

2 2 21,2

21,2

sin

( )

sin ( cos )

2 cos

1

2 cos

t V R R

Z R

Z R D R

R R D R

R R D D

R t V R

D t VR

t V D

Sensor 1

Sensor 2

Sensor 1

1

2

1,2

2

distance between sensor 1 and 2

distance between sensor 1 and source

distance between sensor 2 and source

time differance between sensor 1 and 2

angle between lines and

line perpend

D

R

R

t

R D

Z

icular to D

Z D

R3R2

R1

R1

R2R3

Sensor 2

Sensor 3

For location of AE sources on a plane minimum three sensors are used. The source is situated on intersection of two hyperbolas calculated for the first and the second sensors detected AE signal and the first and the third sensor.


Over-determined Source Location• Generally, it is necessary 2 sensors for linear, 3

sensors for 2D and 4 sensors for 3D locations.• When more sensors detect AE wave from a

source than necessary it is possible to use this information to improve location accuracy by error minimization optimization methods.

2 2, ,( )i obs i calct t

2 2 2 2, 1 1

,

,

1( ) ( ) ( ) ( )

The calculated time difference between the sensor and the first hit sensor, where and are the unknown coordinates of the source.

T

i calc i s i s s s

i calc s s

i obs

t x x y y x x y yV

t i x y

t

he observed time difference

Chi Squared error function that minimized in over-determined source location.


-0.1 0 0.1-0.1

0

0.1

0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

3

6

9

X [m]

4

7

1

2

5

8

Y [m]

Z [

m]

VyyxxyyxxttT iiii2

012

012

02

01 )()()()(

N

imeasuredi TT

1

22

),( 00 yx

Minimization on χ2 :

(xo,yo)– location of source(xi,yi)– location of sensor iti – arrival time to sensor it1- arrival time to sensor 1

The time delay between the signal arrival to two sensors:

•At least 3 sensors are required for location.•However, more sensors increase the accuracy of the source location

),( 00 yx

2D Location on Cylinder


Source

((((*))))

x),( 11 Ex ),( 00 Ex

I

),( 22 Ex ),( 33 Ex

0302

0201

3

2

2

1 lnlnxxxx

xxxx

EE

EE

)(0

0xxi

ieEE

Xo – location of sourceXi – location of sensor iEo – energy at sourceEi – energy at sensor iβ - the decay constant

Energy attenuation in line:

* 3 sensors are required for location for unknown β(for known β 2 sensors are required for location)

Energy Attenuation Location


Location in Anisotropic Materials• In anisotropic materials, the velocity of wave propagation is different in different

direction.• In order to achieve appropriate results in source location it is necessary to evaluate

velocity profile as a function of propagation direction and incorporate this into the calculation of time differences as done in the example of the composite plate.

Velocity vs. Angle

0

1000

2000

3000

4000

5000

6000

7000

0 5 10 15 20 25 30 35 40 45 50 55 60 65 70 75 80 85 90

Angle [Degrees]

Vel

oci

ty [

m/s

]

R=0.9m

R=0.45m2 2 2 2

1 1,

, ,1

,

( ) ( ) ( ) ( )

The time difference recorded by the sensor relative to the first hit sensor

i s i s s si calc

i

i calc

x x y y x x y yt

v v

t i


Ch 1

Ch 2

Δt

Normalized cross-correlation function

Δt)}(max{

21 )()()(

tC

ChCh

tt

dttSStC

Cross-correlation function

Cross-correlation based Location

Cross-correlation method is typically applied for location of continuous AE signals.

Cross-correlation is another method for location of AE sources based on estimation of time shifts between AE signals detected by different sensors. It is usually applied for continuous AE signals when it is impractical to estimate the time of wave arrival but possible to estimate time shifts between sensors.


Zone Location• Zone location is based on the principle that the sensor with the highest

amplitude or energy output will be closest to the source. • Zone location aims to trace the waves to a specific zone or region around

a sensor.• Zones can be lengths, areas or volumes depending on the dimensions of

the array. • With additional sensors added, a sequence of signals can be detected

providing a more accurate result.


Geodesic Location• This time-difference location method is based on calculation of the

shortest wave path over the mesh of the object by the principle of minimum energy.

• The method allows to solve location problems in complex geometries but computationally expensive.

Reference:G. PRASANNA, M. R. BHAT and C. R. L. MURTHY, “ACOUSTIC EMISSION SOURCE LOCATION ON AN ARBITRARYSURFACE BY GEODESIC CURVE EVOLUTION”, Advances in Acoustic Emission - 2007


Other Location Methods

• FFT and wavelet transforms are be used to improve location by evaluation of modal arrival times.

• Cross-correlation between signals envelopes.• There are works proposing use of neural

network methods for location of continuous AE.


The End


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