NON- DESTRUCTIVE TESTING

B. Tech VI Semester

BY

Mr. Vishnu Pratap Singh, Assistant Professor

DEPARTMENT OF MECHANICAL ENGINEERING

BUDDHA INSTITUTE OF TECHNOLOGY

GIDA GORAKHPUR

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UNIT-V

Outline

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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 scattering phenomena.
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.

Definition of Acoustic Emission Phenomenon

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

Acoustic Emission Instrumentation

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Typical AE apparatus consist of the following components:

• Sensors used to detect AE events.
• Preamplifiers amplifies initial signal.
• Cables transfer signals on distances up to 200m to AE devices. Cables are typically of coaxial type.
• Data acquisition device performs filtration, signals’ parameters evaluation,

data analysis and charting.

Sensors

Main amplifiers with filters

Measurement Circuitry

Preamplifiers with filters

Computer

AE Sensors

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• Purpose of AE sensors is to detect stress waves motion that cause a local dynamic material displacement and convert this displacement to an electrical signal.
• AE sensors are typically piezoelectric sensors with elements maid of special ceramic elements like lead zirconate titanate (PZT). Mechanical strain of a piezo element generates an electric signals.
• Sensors may have internally installed preamplifier (integral sensors).
• Other types of sensors include capacitive transducers, laser interferometers.

Regular piezoelectric sensor

Integral piezoelectric sensor

Preamplifier 60 dB

Sensors Characteristics

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•

•

•

•

• 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 be revealed.

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 temperature under which piezoelectric element loses permanently its piezoelectric properties. Curie temperature varies for different ceramics from 120 to 400C⁰. There are ceramics with over 1200C⁰ Curie temperature.

Installation of Sensors on Structure

Type 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 andoil then 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 attached with magnet

Pb blanket in nuclear Waveguide applications

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Rolling sensor produces by PAC

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Methods of AE Sensors Calibration

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• The calibration of a sensor is the measurement of its voltage output into an established electrical load for a given mechanical input. Calibration results can be expressed either as frequency response or as an impulse response.
• Surface calibration or Rayleigh calibration: The sensor and the source are located on the same plane surface of the test block. The energy at the sensor travels at the Rayleigh speed and the calibration is influenced by the aperture effect.

• Aperture Effect:

A _S

r(x, y) − local sensitivity of the tranducer face

S − region (m² ) of the surface contacted by the sensor

A − area of region S

u(x, y, t) − displacement (m) of the surface

U (t) = ¹ _∫∫u(x, y, t)r(x, y)dxdy

• Through pulse calibration: The sensor and the source are coaxially located on opposite parallel surfaces. All wave motion is free of any aperture effect.

AE Data Acquisition Devices

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Example of AE device parameters:

• 16 bit, 10 MHz A/D converter.
• Maximum signal amplitude 100 dB AE.
• 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 channel with a range from 100 KHz to 2.1 MHz (under software control).
• 32 bit Digital Signal Processor.
• 1 Mbyte DSP and Waveform buffer.

Principals of AE Data Measurement and Analysis

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Threshold and Hit Definition Time (HDT)

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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.

Hit 1

Hit 1

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Short HDT

​

​

Hit 2

Long HDT

Time

Voltage

Burst and Continuous AE Signals

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Burst AE is a qualitative description of the discrete signal's related to individual emission events occurring within the material.

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Continuous AE is a qualitative description of the sustained signal produced by time-overlapping signals.

“AE Testing Fundamentals, Equipment, Applications” , H. Vallen

AE Parameters

Duration

• Rise time - The time from the first threshold crossing to the maximum amplitude.
• Count rate – number of counts per time unit

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• Peak amplitude - The maximum of AE signal.

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

• Energy – Integral of the rectified voltage signal over the duration of the AE hit.
• Duration – The time from the first threshold crossing to the end of the last threshold crossing.
• Counts – The number of AE signal exceeds threshold.
• Average Frequency –Determines the average frequency in kHz over the entire AE hit.

A.F = ^{AE counts} [kHz]

Background Noise

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

• Hydraulic noise –Cavitations, turbulent flows, boiling of fluids and leaks.
• Mechanical noise –Movement of mechanical parts in contact with the structure e.g. fretting of pressure vessels against their supports caused by elastic expansion under pressure.
• Cyclic noise – Repetitive noise such as that from reciprocating or rotating machinery.
• Electro-magnetic noise.

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Control of noise sources:

• Rise Time Discriminator – There is significant difference between rise time of mechanical noise and acoustic emission.
• Frequency Discriminator – The 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 the background noise and acoustic emission events under conditions of high, varying background noise.

Amplitude

Floating threshold

Time

• Master – Slave Technique – Master 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.

Attenuation, Dispersion, Diffraction and Scattering Phenomena

The 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 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….

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• 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.

Source Location

Source Location Concepts

• Time difference based on threshold crossing.
• Cross-correlation time difference.
• Zone location.

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.

d = ¹ ₍D − ΔT ⋅V ₎

2

d = distance from first hitsensor

D = distance between sensors

V = wave velocity

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Two Dimensional Source Location

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• For location of an AE source on a plane two sensors are used. The source is situated on a hyperbola.

Δt_1,2V = R₁ − R₂

2

Z = R sinθ

Z ² = R ² − (D − R )²

1 2

2 2 2

2

2 1 2

2 2 2

2 1

R₁ = Δt_1,2V + R₂

1,2

2

2_V 2

D² −Δt

→ R sin θ = R − (D − R cosθ )

R = R − D + 2D cosθ

1

2 Δt V + D cosθ

⇒ R =

Sensor 2

Sensor 1

D − distance between sensor 1 and 2

R₁ − distance between sensor 1 and source

R₂ − distance between sensor 2 and source

1,2

Δt − time differance between sensor 1 and 2

2

θ − angle between lines R and D

Z − line perpendicular to D

Z

D

R3

R1

​

Sensor 1

R1

R2

R3

Sensor 2

Sensor 3 R2

θ

1,2

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• Three sensors are used to locate a source to a point by intersecting two

hyperbolae using the same technique as two sensors.

Cross-correlation based Location

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Ch 1

Ch 2

Δt

Normalized cross-correlation function

Δt

Δt = t

max{ C (t)}

C(t) = _∫ S_Ch1(τ) ⋅ S_Ch2 (τ + t)dt

Cross-correlation function

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

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.
• Zonal 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 giving a more accurate result using time differences and attenuation characteristics of the wave.

Acoustic Emission in Metals

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

annihilation

interaction

migration

generation

nucleation

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.

movement

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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. The

crystallographic plane along which the dislocation line moves is called the slip plane and the

direction of movement is called the slip direction. The combination of the two is termed the slip system.(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

Edge dislocation motion

1

2

4

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(1) Materials Science and Engineering an Introduction, William D. Callister, Jr.

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• Edge and screw are the two fundamental types of dislocation.

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3

4

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.

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_{1 ⎛ K} ⎞²

r_y − plastic zone size in elastic material

I

⎜ ⎟

⎝

2π σ

ys _⎠

y

r =

DSCE

Factors that Tend to Increase or Decrease the

Amplitude of AE

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

Relationship between AE and Fracture Mechanics Parameters and AE Effects

Models of AE in Metals

α = 2 or 6 (plain stress or plain strain)

^{(3) Strains at the crack tip vary at} r ^{−0.5where r is the radial distance from the crack tip. (4)}

N ∝ V_p

N − AE count rate

V_p − volume strained between ε _y (yield strain) and ε_u (uniformstrain)

• The assumptions lead to development of the following equations for the model (_{α = 2})

y

Plastic Deformation Model

• Plastic deformation model relates AE and the stress intensity factor ( ^K₁).
• 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.

_{1 ⎛ K} ⎞²

r =

1

⎜ ⎟

⎝

απ σ

ys _⎠

( )

4

2 2

B − platethickness

→ V_p ∝ K ₄

⇒ N ∝ K⁴

u y

p

y u

y u

B

V ≅ π r − r B = πB

Eε

⎡

2

_⎞2 _⎤

4

^ε − ε

^⎡ 1 ^⎛ K ^⎡ 1 ^⎛ K

2

_⎞2 _⎤

− ^⎢

⎥

⎢

⎜

_⎟ ^⎥ =

⎜

⎢ _⎟ ⎥

⎢ ^2π

⎥

_⎢ 2π Eε

^4π _⎢ 4π ₍Eε ε _)⎥

⎝ u ⎠

⎝

⎣

⎤

⎥ K ⁴

⎦

⎣

⎦

⎣

y _{⎠ ⎦}

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Fatigue Crack Model

• Several models were developed to relate AE count rate with crack propagation rate.

p

p

c

_N '

= C Δ K^m

N ^' = C

Δ K^m

^s (1− R)^m

ΔK ²

(1− R)²

p

c

_N '

_N '

• AE count rate due to plastic deformation

• AE count rate due to fracture

N ^' = N ^' + N ^'

p c

_N '

N ^' = AΔKⁿ

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

(Eq.2) Paris law for crack propogation in fatigue

• AE count rate per cycle

ΔK −Stress intensity factor

A, n − constants

^da = CΔK ^m

dN

• The combined contribution of both plastic deformation and fracture 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 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 AE

felicity ratio =

previous maximum stress

Kaiser effect (BCB)

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Applications

_{AE I} nspection of PressureVessels

AE Inspection of Pressure Vessels

AE Testing of Pressure Vessels

Pressure Policy for a New Vessel⁽¹⁾

⁽¹⁾Nondestructive Testing Handbook, volume 6 “Acousti^Dc ^SE^Cm^EissionTesting”, Third Edition, ASNT.

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% or 200, whichever is less, of events to average signal strength of all events.

N − K

H (t) =

N

N

t = K +1

∑ ^S0i

_N ∑ ^S0i

10

i=1

av 0i

i=1

N – number of hits, S_0i – the signal strength of

the i-th event, J – sp_ie₌₁c₀ific number of events

^{K=0.8J for J≤}S^N≤=¹⁰¹⁰_∑^0aS^{nd K=N-200 for N>1000}

The numbers on plot correspond to

sensors numbers.⁽¹⁾

• Severity is the average of ten events having the largest numerical value of signal strength.

⁽¹⁾Nondestructive Testing Handbook, volume 6 “Acousti^Dc ^SE^Cm^EissionTesting”, Third Edition, ASNT.

AE Standards

AE Standards

ASME - American Society of Mechanical Engineers

• Acoustic Emission Examination of Fiber-Reinforced Plastic Vessels, Article 11, Subsection A, Section V, Boiler and

Pressure Vessel Code

• Acoustic Emission Examination of Metallic Vessels During Pressure Testing, Article 12, Subsection A, Section V, Boiler and Pressure Vessel Code
• Continuous Acoustic Emission Monitoring, Article 13 Section V ASTM - American Society for Testing and Materials
• E569-97 Standard Practice for Acoustic Emission Monitoring of Structures During Controlled Stimulation
• E650-97 Standard Guide for Mounting Piezoelectric Acoustic Emission Sensors
• E749-96 Standard Practice for Acoustic Emission Monitoring During Continuous Welding
• E750-98 Standard Practice for Characterizing Acoustic Emission Instrumentation
• E976-00 Standard Guide for Determining the Reproducibility of Acoustic Emission Sensor Response
• E1067-96 Standard Practice for Acoustic Emission Examination of Fiberglass Reinforced Plastic Resin (FRP) Tanks/Vessels
• E1106-86(1997) Standard Method for Primary Calibration of Acoustic Emission Sensors
• E1118-95 Standard Practice for Acoustic Emission Examination of Reinforced Thermosetting Resin Pipe (RTRP)
• E1139-97 Standard Practice for Continuous Monitoring of Acoustic Emission from Metal Pressure Boundaries
• E1211-97 Standard Practice for Leak Detection and Location Using Surface-Mounted Acoustic Emission Sensors
• E1316-00 Standard Terminology for Nondestructive Examinations
• E1419-00 Standard Test Method for Examination of Seamless, Gas-Filled, Pressure Vessels Using Acoustic Emission
• E1781-98 Standard Practice for Secondary Calibration of Acoustic Emission Sensors
• E1932-97 Standard Guide for Acoustic Emission Examination of Small Parts
• E1930-97 Standard Test Method for Examination of Liquid Filled Atmospheric and Low Pressure Metal Storage Tanks Using Acoustic Emission
• E2075-00 Standard Practice for Verifying the Consistency of AE-Sensor Response Using an Acrylic Rod
• E2076-00 Standard Test Method for Examination of Fiberglass Reinforced Plastic Fan Blades Using Acoustic Emission

AE Standards

ASNT - 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 of Fiberglass reinforced with Balsa Cores.
• Recommended Practice No. SNT-TC-1A.

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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

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Compressed Gas Association

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

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European Committee for Standardization

• DIN EN 14584, Non-Destructive Testing – Acoustic Emission – Examination of Metallic Pressure Equipment during Proof Testing; Planar Location of AE Sources.
• EN 1330-9, Non-Destructive Testing – Terminology – Part 9, Terms Used in Acoustic Emission Testing.
• EN 13477-1, Non-Destructive Testing – Acoustic Emission – Equipment Characterization – Part 1, Equipment Description.
• EN 13477-2, Non-Destructive Testing – Acoustic Emission – Equipment Characterization – Part 2,

Verification of Operating Characteristics.

• EN 13554, Non-Destructive Testing – Acoustic Emission – General Principles.

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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.

AE Standards

International Organization for Standardization

• ISO 12713, Non-Destructive Testing - Acoustic Emission Inspection – Primary Calibration of Transducers.
• ISO 12714, Non-Destructive Testing - Acoustic Emission Inspection – Secondary Calibration of Acoustic Emission Sensors.
• ISO 12716, Non-Destructive Testing - Acoustic Emission Inspection – Vocabulary
• ISO/DIS 16148, gas Cylinders – Refillable Seamless Steel gas Cylinders – Acoustic Emission Examination (AEE) for Periodic Inspection.

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Japanese Institute for Standardization

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

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Japanese Society for Nondestructive Inspection

• NDIS 2106-79, Evaluation of performance Characteristics of Acoustic Emission Testing Equipment.
• NDIS 2109-91, Methods for Absolute calibration of Acoustic Emission Transducers by Reciprocity Technique.
• NDIS 2412-80, Acoustic Emission Testing of Spherical Pressure Vessels of High Tensile

Strength Steel and Classification of Test Results.

Thank you

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