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6/3/2014 1Hareesha N G, Dept of Aero Engg,DSCE


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Outline

1. Acoustic Emission phenomena.2. History of Acoustic Emission from Stone Age to these days.

3. AE instrumentation:

1. Sensors, preamplifiers, cables (types, specific applications).

2. Data Acquisition systems (analog and digital, signal digitation,filtration).

4. Principals of AE data measurement and analysis.

5. Source location. Attenuation, dispersion, diffraction and scatteringphenomena.

6. AE in metals.

7. Relationship between AE and fracture mechanics parameters and effects of

AE.8. AE applications.

9. International AE standards.

10. Conclusions.

6/3/2014 Hareesha N G, Dept of Aero Engg,DSCE

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Definition of Acoustic Emission Phenomenon

Acoustic Emission is a phenomenon ofsound and ultrasound wave radiation inmaterials undergo deformation and

fracture processes.


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Acoustic Emission InstrumentationTypical AE apparatus consist of the following components:

Sensorsused to detect AE events. Preamplifiersamplifies initial signal.

Cablestransfer signals on distances up to 200m to AE devices. Cables aretypically of coaxial type.

Data acquisition device performs filtration, signals parameters evaluation,data analysis and charting.

SensorsMain amplifiers

with filtersMeasurement CircuitryPreamplifiers

with filtersComputer

Acquisition

softwareData

storage

Data presentation


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AE Sensors

Purpose of AE sensors is to detect stress waves motion that cause a local

dynamic material displacement and convert this displacement to anelectrical signal.

AE sensors are typically piezoelectric sensors with elements maid ofspecial ceramic elements like lead zirconate titanate (PZT). Mechanicalstrain of a piezo element generates an electric signals.

Sensors may have internally installed preamplifier (integral sensors).

Other types of sensors include capacitive transducers, laserinterferometers.

Regular piezoelectric sensor Integral piezoelectric sensorPreamplifier 60 dB


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Sensors Characteristics Typical frequency range in AE applications varies between 20 kHz and 1 MHz.

Selection of a specific sensor depends on the application and type of flaws to berevealed.

There are two qualitative type of sensor according to their frequency responds:resonant and wideband sensors.

Thickness of piezoelectric element defines the resonance frequency of sensor.

Diameter defines the area over which the sensor averages surface motion.

Another important property of AE sensors includes Curie Point, the temperatureunder which piezoelectric element loses permanently its piezoelectric properties.Curie temperature varies for different ceramics from 120 to 400C0. There areceramics with over 1200C0Curie temperature.

AE signal of lead break and corresponding Power spectrum.6/3/2014 Hareesha N G, Dept of Aero Engg,DSCE 6


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Installation of Sensors on StructureType of installation and choice of couplant material is defined by a specifics of

application. Glue (superglue type) is commonly used for piping inspections.

Magnets usually used to hold sensors on metal pressure vessels. Grease and oilthen used as a couplant.

Bands used for mechanical attachment of sensors in long term applications.

Waveguides (welded or mechanically attached) used in high temperature

applications. Rolling sensors are used for inspection rotating structures.

Special Pb blankets used to protect sensors in nuclear industry.

Sensor attachedwith magnet

Pb blanket in nuclearapplications

Waveguide Rolling sensorproduces by

PAC6/3/2014 Hareesha N G, Dept of Aero Engg,DSCE

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AE Data Acquisition Devices

Example of AE device parameters:

16 bit, 10 MHz A/D converter.

Maximum signal amplitude 100 dBAE.

4 High Pass filters for each channel

with a range from 10 KHz to 200 KHz(under software control).

4 Low Pass filters for each channelwith a range from 100 KHz to 2.1MHz (under software control).

32 bit Digital Signal Processor.

1 Mbyte DSP and Waveform buffer.


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Principals of AE Data Measurement and

Analysis


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Threshold and Hit Definition Time (HDT)

Hit 1

Hit 1

Hit 2

Short HDT

Long HDT

Time

Voltag

eThreshold

Long HDT

Short HDT

Threshold and HDT are parameters that used for detection AE signals in traditional AE

devices. HDT: Enables the system to determine the end of a hit, close out the

measurement

process and store the measured attributes of the signal.


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Burst and Continuous AE Signals

Burst AEis a qualitative description of the discrete signal's related to

individual emission events occurring within the material.

Continuous AEis a qualitative description of the sustained signal

produced by time-overlapping signals.

AE Testing Fundamentals, Equipment, Applications , H. Vallen


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AE Parameters

Peak amplitude - The maximum of AE signal.

dB=20log10(Vmax/1volt)-preamlifier gain

EnergyIntegral of the rectified voltage signal over the duration of the AE hit.

DurationThe time from the first threshold crossing to the end of the last thresholdcrossing.

Counts

The number of AE signal exceeds threshold. Average FrequencyDetermines the average frequency in kHz over the entire AE hit.

Rise time - The time from the first threshold crossing to the maximum amplitude.

Count rate - Number of counts per time unit.

. [ ]AE counts

A F kHzDuration


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Background NoiseBackground Noise: Signals produced by causes other than acoustic emission and are not relevant to the purpose of the test

Types of noise:

Hydraulic noiseCavitations, turbulent flows, boiling of fluids and leaks.

Mechanical noiseMovement of mechanical parts in contact with the structure e.g. fretting of pressure vessels against their

supports caused by elastic expansion under pressure.

Cyclic noiseRepetitive noise such as that from reciprocating or rotating machinery.

Electro-magnetic noise.

Control of noise sources:

Rise Time DiscriminatorThere is significant difference between rise time of mechanical noise and acoustic emission.

Frequency DiscriminatorThe frequency of mechanical noise is usually lower than an acoustic emission burst from cracks.

Floating Threshold or Smart Threshold

Varies with time as a function of noise output. Used to distinguish between thebackground noise and acoustic emission events under conditions of high, varying background noise.

Time

Amp

litude

Floatingthreshold

MasterSlave TechniqueMaster sensor are mounted near the area of interest and are surrounded by slave or guard sensors.

The guard sensors eliminate noise that are generated from outside the area of interest.


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Attenuation, Dispersion, Diffraction and

Scattering PhenomenaThe following phenomena take place as AE wave propagate along the structure:

Attenuation: The decrease in AE amplitude as a stress wave propagate along a structure dueto 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

the actual structures during their inspection. The attenuation curves allows to estimate

amplitude or energy of a signal at the at the

given the distance from the sensor.


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Source Location


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Source Location Concepts

Time difference based on threshold crossing.

Cross-correlation time difference.

Zone location.


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

on linear structures such as pipes. It is based on the arrival time difference

between two sensors for known velocity. Sound velocity evaluated by generating signals at know distances.

1

2

distance from first hit sensorD = distance between sensors

wave velocity

d D T V

d

V

Material Effectivevelocity in athin rod [m/s]

Shear[m/s] Longitudinal[m/s]

Brass 3480 2029 4280

Steel 347 5000 3089 5739

Aluminum 5000 3129 63196/3/2014 Hareesha N G, Dept of Aero Engg,DSCE 18


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Ch 1

Ch 2

t

Normalized cross-correlation function

t )}(max{

21 )()()(

tC

ChCh

tt

dttSStC

Cross-correlation function

Cross-correlation based Location

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


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Acoustic Emission in Metals


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Sources of AE in Metals

Microscopic sources

includes dislocation movement, interaction, annihilation, slip formation, voids nucleation,growth and interaction and many other.

Major macroscopic sources

of AE in metals are: crack jumps, plastic deformation development, fracturing and de-bonding

of hard inclusions.

..

interaction

motion

formation

..

interaction

motion

formation

Phase

changes

Possible combinations

AE SOURCES

6.9 10236

Twining

Slip

branching

development

nucleation

branching

development

nucleation

crack

formation

fracturing

bond

connection

fracturing

crack

formation

fracturing

bond

connection

fracturing

Inclusions

interaction

growth

nucleation

interaction

growth

nucleation

Micro-crack

Voids

movement

annihilation

interaction

migration

generation

nucleation

movement

annihilation

interaction

migration

generation

nucleation

Dislocations

Recrystalli-

zation

More then 80% of energy

expended on fracture in

common industrial metals

goes to development of

plastic deformation.


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Plastic Deformation Plastic deformation development is accompanied by the motion of a large numbers of dislocations.

The process by which plastic deformation is produced by dislocation motions is called slip. Thecrystallographic plane along which the dislocation line moves is called the slip plane and thedirection of movement is called the slip direction. The combination of the two is termed the slipsystem.(1)

The motion of a single vacancy and a single dislocation emits a signal of about 0.01-0.05eV. The best sensitivity of modern AE devices equals 50-100eV.

Edge dislocation Screw dislocation Mixed dislocation

Edgedislocationmotion

1 2 3 4 5

Edge and screw are the two fundamental types of dislocation.

(1)Materials Science and Engineering an

Introduction, William D. Callister, Jr.

Physical

Process

Activation

Energy (eV)

Dislocation

glide

1.2

Formation ofdislocation

8-10


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F t th t T d t I D


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Factors that Tend to Increase or Decrease

the Amplitude of AE

Nondestructive Testing Handbook, volume 6Acoustic Emission Testing, Third Edition, ASNT.


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Relationship between AE and

Fracture Mechanics Parametersand AE Effects


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M d l f AE i M t l


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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 AEwhen 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)

y

ys

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 4

2 2 4

4

4

1 1

2 2 4 4

plate thickness

V

u y

p 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


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Fatigue Crack Model Several models were developed to relate AE count rate with crack

propagation rate.

2' '

2

'

'

' ' '

AE count rate due to plastic deformation

AE count rate due to fracture

(1 )(1 )

mm

p p c s m

p

c

p c

K KN C K N C

RRN

N

N N N

'

'

(Eq.1) The relation between AE count rate and stress intensity factor

(Eq.2) Paris law for crack propo

AE count rate per cycle

Stress intensity factor

, constants

m

n

dadN

N A K

N

K

A n

C K

gation in fatigue

The combined contribution of both plastic deformation andfracture mechanism is as follows for plastic yielding:


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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 maximumload, some type of new damage has occurred. The dunegan corollaryis usedin proof testing of pressure vessels.

Felicity effect is the presence of AE, detectable at a fixed predeterminedsensitivity level at stress levels below those previously applied. The felicityeffect 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)6/3/2014 Hareesha N G, Dept of Aero Engg,

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Applications


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AE I ti f P V l


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AE Inspection of Pressure Vessels


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AE T ti f P V l


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AE Testing of Pressure Vessels

(1)Nondestructive Testing Handbook, volume 6Acoustic Emission Testing, Third Edition, ASNT.

Pressure Policy for a New Vessel(1)

Example of Transducers Distribution on Vessel's Surface(1) Typical Results Representation of Acoustic Emission Testing (1)


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Example of Pressure Vessel Evaluation

Historic index is a ratio of average

signal strength of the last 20% or200, whichever is less, of events toaverage signal strength of all events.

N

i

i

N

Kt

i

S

S

KNNtH

1

0

1

0

)(

10

1

010

1 i

i

iav SS

(1)Nondestructive Testing Handbook, volume 6Acoustic Emission Testing, Third Edition, ASNT.

The numbers on plot correspond tosensors numbers.(1)

Nnumber of hits, S0ithe signal strength ofthe i-th event, Jspecific number of eventsK=0.8J for JN1000 and K=N-200 for N>1000

Severity is the average of tenevents having the largestnumerical value of signalstrength.


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AE Standards


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AE S d d


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AE StandardsASNT - American Society for Nondestructive Testing

ANSI/ASNT CP-189, ASNT Standard for Qualification and Certification of Nondestructive Testing Personnel.

CARP Recommended Practice for Acoustic Emission Testing of Pressurized Highway Tankers Made ofFiberglass reinforced with Balsa Cores.

Recommended Practice No. SNT-TC-1A.

Association of American Railroads

Procedure for Acoustic Emission Evaluation of Tank Cars and IM-101 tanks, Issue 1, and Annex Z thereto, Test Methods to Meet FRA Request for Draft Sill Inspection program, docket T79.20-90 (BRW) ,Preliminary 2

Compressed Gas Association

C-1, Methods for Acoustic Emission Requalification of Seamless Steel Compressed Gas Tubes.

European Committee for Standardization

DIN EN 14584, Non-Destructive TestingAcoustic EmissionExamination of Metallic Pressure Equipmentduring Proof Testing; Planar Location of AE Sources.

EN 1330-9, Non-Destructive TestingTerminologyPart 9, Terms Used in Acoustic Emission Testing.

EN 13477-1, Non-Destructive TestingAcoustic EmissionEquipment CharacterizationPart 1,Equipment Description.

EN 13477-2, Non-Destructive TestingAcoustic EmissionEquipment CharacterizationPart 2,Verification of Operating Characteristics.

EN 13554, Non-Destructive TestingAcoustic EmissionGeneral Principles.

Institute of Electrical and Electronics Engineers

IEEE C57.127, Trial-Use guide for the Detection of Acoustic Emission from Partial Discharges in Oil-Immersed Power Transformers.


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AE St d d


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AE StandardsInternational Organization for Standardization

ISO 12713, Non-Destructive Testing - Acoustic Emission InspectionPrimary Calibration ofTransducers.

ISO 12714, Non-Destructive Testing - Acoustic Emission InspectionSecondary Calibration ofAcoustic Emission Sensors.

ISO 12716, Non-Destructive Testing - Acoustic Emission InspectionVocabulary

ISO/DIS 16148, gas CylindersRefillable Seamless Steel gas CylindersAcoustic EmissionExamination (AEE) for Periodic Inspection.

Japanese Institute for Standardization

JIS Z 2342, Methods for Acoustic Testing of Pressure Vessels during Pressure Tests andClassification of Test Results.

Japanese Society for Nondestructive Inspection

NDIS 2106-79, Evaluation of performance Characteristics of Acoustic Emission TestingEquipment.

NDIS 2109-91, Methods for Absolute calibration of Acoustic Emission Transducers byReciprocity Technique.

NDIS 2412-80, Acoustic Emission Testing of Spherical Pressure Vessels of High TensileStrength Steel and Classification of Test Results.


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More educational materials on

Acoustic Emission available atwww.muravin.com

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