NON- DESTRUCTIVE TESTING

DEPARTMENT OF MECHANICAL ENGINEERING

BUDDHA INSTITUTE OF TECHNOLOGY

GIDA GORAKHPUR

​

​

B. Tech VI Semester

BY

Mr. Vishnu Pratap Singh, Assistant Professor

1

2

NDT

NON- DESTRUCTIVE TESTING

(KME 061)

UNIT-I

Other terms used in NDT

3

Non-destructive examination (NDE)

Non-destructive inspection (NDI) Non-destructive evaluation (NDE)

OBJECTIVE OF NDT

4

• Material sorting
• Material characterization
• Property monitoring (for process control)
• Thickness measurement
• Defect Detection/ Location and
• Defect characterization.

However the major task of NTD is to detect and identify the range of defects. Defects can include production flaws such as heat treatment cracks, grinding cracks, voids(pores), and fatigue cracks (Generated during service).

NDT(Non-destructive testing )?

5

Non-destructive testing (NDT) is the process of inspecting, testing, or evaluating materials, components or assemblies for discontinuities, or differences in characteristics without destroying the serviceability of the part or system.

In other words, when the inspection or test is completed the part can still be used.

Non-destructive testing

6

“NDT is an examination that is performed on an object of any type, size, shape or material to determine the presence or absence of discontinuities, or to evaluate other material characteristics”

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Types of NDT

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Visual Inspection

9

• Visual inspection is the simplest, fastest and most widely used NDT method.
• Visual inspection is commonly defined as “the examination of a material component, or product for conditions of non conformance using light and eyes, alone or in conjunction with various aids.
• Visual inspection often also involves shaking, listening, feeling ,

and sometimes even smelling the component being inspected.

• Visual inspection is commonly employed to support/ compliment

other NDT methods.

• Digital Detector and computer technology have made it possible to automate some visual inspections. This is known as machine vision inspection.

Characteristics Detected(Applicility)

10

This visual inspection is commonly used:

​

1. To detect surface characteristics such as finish, Scratches, cracks or colour.
2. To check stain in transparent materials.
3. To inspect corrosion.

Principle

11

• Seeing is believing and the art of seeing is the visual inspection technique.
• Visual testing requires adequate illumination of the test surface and proper eye-sight of the tester.
• The test specimen is illuminated and the test surface is observed and examined. Whenever required, the optical aids such as mirrors, magnifying glasses, microscopes, video cameras and computer- vision system can be employed.

Advantages of VT

12

• Simple and easy to use.
• Relatively inexpensive.
• Testing speed is high.
• Testing can be performed on components which are in –service.
• Permanent record are available when latest equipment is used.

Limitation

13

• The test result depend on skill and knowledge of tester.
• Limited to detection of surface flaws.
• Eye resolution is weak.
• Eye fatigue.

Applications

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• Checking of the surface condition of the component.
• Checking of alignment of surfaces.
• Checking of shape of the component.
• Checking for evidence of leaking.
• Checking for internal side defects.

Types of visual testing

15

Unaided or direct visual testing, and

​

Aided visual testing.

Unaided or direct visual testing

16

• As the name

suggest, the unaided visual

testing is carried out with naked eye(and without using any optical aids)

• The most important instrument is visual testing in the human eye.

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MAGNETIC PARTICL TESTING (MT)

• MAGNETIC PARTICL TESTING (MT) is an non- destructive testing to locate surface and subsurface discontinuities in parts made by ferromagnetic materials.

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MAGNETIC LINES OF FLUX:

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• It is the number of magnetic field lines passing through a surface (such as a loop of wire). The magnetic flux through a closed surface is always zero. ...
• The SI unit of magnetic flux is the Weber (Wb)
• The magnetic lines of forces existing in a magnetic field is called magnetic flux.
• The lines of flux ran through the magnets from south to
• north, exiting the north pole and re entering the south pole.
• The lines of flux formed closed loops that never crossed.

20

DIRECT MAGNETIZATION

21

• With direct magnetization, current is passed directly through the component.
• The flow of current causes a circular magnetic field to form in and around the conductor.
• When using the direct magnetization method, care must be taken to ensure that good electrical contact is established and maintained between the test equipment and the test component to avoid damage of the component (due to arcing or overheating at high resistance points).

22

Clamping The Component Between Two Electrical Contacts

23

• One way involves clamping the component between two electrical contacts in a special piece of equipment.
• Current is passed through the component and a circular magnetic field is established in and around the component.

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CLAMPS OR PRODS

25

• A second technique involves using clamps or prods, which are attached or placed in contact with the component.
• Electrical current flows through the component from contact to contact.
• The current sets up a c path of the current.

INDIRECT MAGNETIZATION

26

• Indirect magnetization is accomplished by using a strong external magnetic field to establish a magnetic field within the component. As with direct magnetization, there are several ways that indirect magnetization can be accomplished.

​

• 1.PERMANENT MAGNETS:-
• The use of permanent magnets is a low cost method of

establishing a magnetic field.

• However, their use is limited due to lack of control of the field strength and the difficulty of placing and removing strong permanent magnets from the

27

PERMANENT MAGNET

ELECTRO MAGNET

ELECTROMAGNET

28

• Electromagnets in the form of an adjustable horseshoe magnet (called a yoke) eliminate the problems associated with permanent magnets and are used extensively in industry.

​

• Electromagnets only exhibit a magnetic flux when

electric current is flowing around the soft iron core.

​

• When the magnet is placed on the component, a magnetic field is established between the north and south poles of the magnet.

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ULTRASONIC TESTING

INTRODUCTION

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• Ultrasonic testing(UT) is the one of the popular flaw detection non-destructive testing methods.

​

• In ultrasonic testing high frequency sound energy is used to

identify surface an sub- surface discontinuities.

​

• Ultrasonic testing is completely safe method of NDT and it is extensively used in many basic manufacturing and service industries. Especially in applications of inspecting welds and structural metals.

​

• Because of its high penetration capacity, inspection of extremely

thick sections are possible using Ultrasonic testing.

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Modes of Propagation

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• Longitudinal waves
• Shear waves
• Surface waves (Rayleigh), and
• Lamb waves (plate)

longitudinal waves

33

• In a longitudinal waves, Particle motion in the medium is parallel to the direction of the wave front

Transverse wave

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Transverse wave

35

• A transverse wave is a moving wave that consists of oscillations occurring perpendicular (right angled) to the direction of energy transfer (or the propagation of the wave).

Surface waves

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• Surface waves represent an oscillating motion that travels along the surface of a test piece to a depth of one wavelength.

​

• Surface waves can be used to detect surface breaking cracks in a test piece.

Terminologies used in UT

37

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FREQUENCY

39

• Generally the choice of test frequency depends upon two factors : the minimum size of defect, which is to be detected and the medium in which such a defect is situated.

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• Penetration Depth:

Penetration depth is the maximum depth in a material, the flaws can be located by the ultrasonic waves in testing.

• Scattering:

Scattering is the reflection of sound beam its original direction of propagation.

• Absorption:

Absorption is conversion of sound energy from one form to some another form.

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RADIOGRAPHY TESTING

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RADIOGRAPHY TESTING

47

• The radiation used in radiography testing is a higher energy (shorter wavelength) version of the electromagnetic waves that we see as visible light. The radiation can come from an X-ray generator or a radioactive source.

48

Electrons

-

+

X-ray Generator or Radioactive

Source Creates Radiation

Introduction

49

• This module presents information

on the NDT

method of radiographic inspection or radiography.

• Radiography uses penetrating radiation

that is

directed towards a component.

• The component stops some of the radiation. The amount that is stopped or absorbed is affected by material density and thickness differences.
• These differences in “absorption” can be recorded on film, or electronically.

outline

50

• Electromagnetic Radiation
• General Principles of Radiography
• Sources of Radiation
• Gamma Radiography
• X-ray Radiography
• Imaging Modalities

–Film Radiography

–Computed Radiography

–Real-Time Radiography

–Direct Digital Radiography

–Computed Radiography

• Radiation Safety
• Advantages and Limitations
• Glossary of Terms

Electromagnetic Radiation

51

• The radiation used in Radiography testing is a higher energy (shorter wavelength) version of the electromagnetic waves that we see every day. Visible light is in the same family as x-rays and gamma rays.

General Principles of Radiography

52

• The part is placed between the radiation source and a piece of film. The part will stop some of the radiation. Thicker and more dense area will stop more of the. Radiation.
• The film darkness (density) will vary with the amount of radiation reaching the film through the test object.

Flaw Orientation

53

IDL 2001

Radiography has

sensitivity limitations when detecting cracks.

X-rays “see” a crack as a thickness variation and the larger the variation, the easier the crack is to detect.

When the path of the x-rays is not parallel to a crack, the

thickness variation is less and the crack may not be visible.

Optimum

Angle

easy to d= etect

= not easy

to detect

Flaw Orientation

• Since the angle between the radiation beam and a crack or other linear defect is so critical, the orientation of defect must be well known if radiography is going to be used to perform the inspection.

₀o

10^o

20^o

54

Radiation Sources

• Two of the amost commonly used sources of radiation in industrial radiography are x-ray generators and gamma ray sources. Industrial radiography is often subdivided into “X-ray Radiography” or “Gamma Radiography”, depending on the source of radiation used.

55

Gamma Radiography

• Gamma rays are produced by a radioisotope.
• A radioisotope has an unstable nuclei that does not have enough binding energy to hold the nucleus together.
• The spontaneous breakdown of an atomic nucleus resulting in the release of energy and matter is known as radioactive decay.

56

Gamma Radiography (cont.)

• Most of the radioactive

material used in industrial

radiography is artificially

produced.

• This is done by subjecting

stable material to a source of

neutrons in a special nuclear

reactor.

• This process is called

activation.

57

Gamma Radiography (cont.)

Unlike X-rays, which are produced by a machine, gamma rays cannot be turned off. Radioisotopes used for gamma radiography are encapsulated to prevent leakage of the material.

The radioactive “capsule” is attached to a cable to form what is often called a “pigtail.”

The pigtail has a special connector at the other end that attaches to a drive cable.

58

Gamma Radiography (cont.)

A device called a “camera” is used to store, transport and expose the pigtail containing the radioactive material. The camera contains shielding material which reduces the radiographer’s exposure to radiation during use.

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Gamma Radiography (cont.)

A hose-like device called a guide tube is connected to a threaded hole called an “exit port” in the camera.

​

The radioactive material will leave and return to the camera through this opening when performing an exposure!

60

Gamma Radiography (cont.)

A “drive cable” is connected to the other end of the camera. This cable, controlled by the radiographer, is used to force the radioactive material out into the guide tube where the gamma rays will pass through the specimen and expose the recording device.

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X-ray Radiography

Unlike gamma rays, x-rays are produced by an X-ray generator system. These systems typically include an X-ray tube head, a high voltage generator, and a control console.

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X-ray Radiography (cont.)

• X-rays are produced by establishing a very high voltage between two electrodes, called the anode and cathode.
• To prevent arcing, the anode and cathode are located inside a vacuum tube, which is protected by a metal housing.

63

X-ray Radiography (cont.)

• The cathode contains a small

filament much the same as in a light

bulb.

• Current is passed through the filament which heats it. The heat causes electrons to be stripped off.
• The high voltage causes these “free” electrons to be pulled toward a target material (usually made of tungsten) located in the anode.
• The electrons impact against the target. This impact causes an energy exchange which causes x-rays to be created.

Electrons

-

+

X-ray Generator

64

Imaging Modalities

65

Several different imaging methods are available to display the final image in industrial radiography:

• Film Radiography
• Real Time Radiography
• Computed Tomography (CT)
• Digital Radiography (DR)
• Computed Radiography (CR)

Film Radiography

• One of the most widely used and oldest imaging mediums in industrial radiography is radiographic film.
• Film contains microscopic material called silver bromide.
• Once exposed to radiation and developed in a darkroom, silver bromide turns to black metallic silver which forms the image.

66

Film Radiography (cont.)

• Film must be protected from visible light. Light, just like x-

rays and gamma rays, can expose film. Film is loaded in a “light proof” cassette in a darkroom.

• This cassette is then placed on the specimen opposite the

source of radiation. Film is often placed between screens to

intensify radiation.

67

Film Radiography (cont.)

• In order for the image to be viewed, the film must be

“developed” in a darkroom. The process is very similar to

phaaotographic film development.

• Film processing can either be performed manually in open

tanks or in an automatic processor.

68

Film Radiography (cont.)

Once developed, the film is typically referred to as a “radiograph.”

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Digital Radiography

70

• One of the newest forms of radiographic imaging is “Digital Radiography”.
• Requiring no film, digital radiographic images are captured using either special phosphor screens or flat panels containing micro-electronic sensors.
• No darkrooms are needed to process film, and captured images can be digitally enhanced for increased detail.
• Images are also easily archived (stored) when in digital form.

Digital Radiography (cont.)

71

There are a number of forms of digital radiographic imaging including:

• Computed Radiography (CR)
• Real-time Radiography (RTR)
• Direct Radiographic Imaging (DR)
• Computed Tomography

Computed Radiography

Computed Radiography (CR) is a digital imaging process that uses a special imaging plate which employs storage phosphors.

72

Computed Radiography (cont.)

X-rays penetrating the specimen stimulate the phosphors.

The stimulated phosphors remain in an excited state.

​

CR Phosphor Screen Structure

X-Rays

Phosphor Layer

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Protective Layer

Substrate

Phosphor Grains

Computed Radiography (cont.)

After exposure:

The imaging plate is read electronically and erased for re-use in a special scanner system.

74

Computed Radiography (cont.)

As a laser scans the imaging plate, light is emitted where X- rays stimulated the phosphor during exposure. The light is then converted to a digital value.

Optical

Motor

A/D

Converter

Imaging

Plate

Scanner

Photo-multiplier Tube

110010010010110

Laser Beam

75

Computed Radiography (cont.)

Digital images are typically sent to a computer workstation where specialized software allows manipulation and enhancement.

76

Computed Radiography (cont.)

Examples of computed radiographs:

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Real-Time Radiography

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• Real-Time Radiography (RTR) is a term used to describe a form of radiography that allows electronic images to be captured and viewed in real time.
• Because image acquisition is almost instantaneous, X-ray

images can be viewed as the part is moved and rotated.

• Manipulating the part can be advantageous for several reasons:
• It may be possible to image the entire component with one

exposure.

• Viewing the internal structure of the part from different

angular prospectives can provide additional data for analysis.

• Time of inspection can often be reduced.

Real-Time Radiography (cont.)

• The equipment needed for an RTR includes:

• X-ray tube
• Image intensifier or

other real-time detector

• Camera

• Computer with frame grabber

79

board and software

• Monitor
• Sample positioning system (optional)

Real-Time Radiography (cont.)

• The image intensifier is a device that converts the radiation that passes through the specimen into light.
• It uses materials that fluoresce when

struck by radiation.

• The more radiation that reaches the input screen, the more light that is given off.
• The image is very faint on the input screen so it is intensified onto a small screen inside the intensifier where the image is viewed with a camera.

80

Real-Time Radiography (cont.)

• A special camera which captures the light output of the screen is located near the image intensifying screen.
• The camera is very sensitive to a variety of different light intensities.

• A monitor is then connected to the camera to provide a viewable image.
• If a sample positioning system is employed, the part can be moved around and rotated to image different internal features of the part.

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Real-Time Radiography (cont.)

Comparing Film and Real-Time Radiography

Real-time images are lighter in areas where more X-ray photons reach and excite the fluorescent screen.

82

Film images are darker in areas where more X-ray photons reach and ionize the silver molecules in the film.

Radiographic Images

83

Radiographic Images

84

Radiographic Images

85

Advantages of Radiography

86

• Technique is not limited by material type or density.
• Can inspect assembled components.
• Minimum surface preparation required.
• Sensitive to changes in thickness, corrosion, voids, cracks, and material density changes.
• Detects both surface and subsurface defects.
• Provides a permanent record of the inspection.

Disadvantages of Radiography

87

• Many safety precautions for the use of high intensity radiation.
• Many hours of technician training prior to use.
• Access to both sides of sample required.
• Orientation of equipment and flaw can be critical.
• Determining flaw depth is impossible without additional angled exposures.
• Expensive initial equipment cost.

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ADVANCED NDE TECHNIQUES -I

What is PAUT?

89

• Phased Array Ultrasonic Testing

is an flaw

advanced ultrasonic technique used for detection, sizing, and imaging

• Uses multi-element (array) probes for increased beam steering and focusing compared to conventional UT

• Like having many small conventional

UT

probes in one pulsed at predetermined intervals

How Phased Array Works

• Each individual wave generated within the scan goes through the pulse/receive cycle as shown below

90

How Phased Array Works

• Each received analog A-Scan is digitized and

rendered into

2D display formats

• Signals from the entire equence are compiled into various image formats for evaluation

91

PAUT Views

92

• A data view is a 2D graphic rendering of the ultrasonic data
• Data views
• A-Scan
• B-Scan
• C-Scan
• S-Scan
• ToFD
• Ray Tracing
• Strip Charts

A-Scan View

93

• Source by which all

other views are created

• Amplitude vs time
• An A-Scan exists for

every sound beam

• One for each segment of a linear scan
• One for each angle of an S-Scan
• Rulers allow presentation of information in time, sound path, or true depth
• Amplitude is linked

to color pallet

B-Scan View

94

• A side view looking from the back side of the probe
• Represents data collected through the entire scan length for one A-Scan
• View changes dynamically as angle or VPA is scrolled

C-Scan View

95

• C-Scan is a plan view/top view
• View is generated based on gate positioning and mode (may be configured for gate A and B independently)
• Data can be presented in amplitude or position (thickness) formats

S-Scan View

96

• Presents all A-Scans within the group in an angular sector or sweep range
• A-Scans are converted to color coded lines representing amplitude
• May be corrected for delay and true depth relative to the ultrasonic axis as shown here

Time of Flight Diffraction (ToFD)

97

• The ToFD display offers a B-Scan representation of the diffracted signals detected in “pitch catch” conventional UT probes
• As with all other image views ToFD is built off individual A-Scans

Ray Tracing View

98

• Provides 2D representation of the weld, showing rebounds of the beam path and A gate position for the first, last and active focal law
• Should only be used as a reporting tool

Master Thoughts

99

• It’s still UT
• Anything UT can do PAUT can do better
• Technology enabled
• UT is not new, neither is PAUT
• Computer assistance of proven technology
• Advancements in transducer material/design
• Medical use and capability perfected
• Adapted for use in industrial sector
• Key elements
• Multi-dimension sizing offers more accurate assessment of integrity
• Weld images provided in multiple 2D orientations, giving 3D

of views

• Raises production while reducing costs

PAUT for Pipelines

API 1104 21^st

Edition

100

Acoustic Emission Testing and Application

101

Acoustic Emission Testing

Loaded the structure and cracks are formed

Oscillatory movement in atoms & molecules

Generate elastic stress wave in the sample and come out

1012

0

2

Sensor- Piezoelectric element

CONDITION FOR AE

• Load- No load (no cracks) no emission
• Controlled load – Part of the structure has to be loaded by

controlled load

• Load bearing structure – loaded anyway
• Inspect a big area/volume – Very Big area inspection

can be done in a single examination by placing a number of sensors at different location, where could be a potential source of damage

1013

0

3

Sources of AE

1014

0

4

• Stress field- stress wave propagate through the sample and received by the sensors

For example Metal

• Micro and macro crakes initiating and propagating
• Micro dynamical events such as twinning or slip
• It happen due to lattice defect which known as dislocation
• loaded the metallic system dislocation will move and due to that movement, it create slip
• Slip is the movement of atomic planes over one another as results dislocation move which lead the AE

Sources of AE

1015

0

5

• Fracture of brittle inclusions
• Chemical action like corrosion
• Phase Transformation due to change in volume

and strain effect due to temperature change

AE source from phase transformation

1016

0

6

Example Martensite in steel, at high temperature phase called Austenite

• When we heat it and quench it fast in liquid like water
• Two phase are very different kind of properties and structure
• Hence, the volume of parent phase and transformed phase are

different and due to that some stresses can be generated which may lead to acoustic emission

Composite Materials

1017

0

7

• Fiber fracture at medium strain level
• Delamination at high strain level
• Fiber pull out
• Matrix cracking and fibers debonding at low strain level
• Concrete
• Micro and macro cracks
• Separation of reinforcement members
• Mechanical rubbing of separated surfaces

Parameter of AE

σ σ

10¹8

0

8

Type1- Loading

Relationship between stress and cracks size

6 a² V ≥ 5 × 10⁴ 𝑥 ℎ (𝑤𝑎𝑡𝑡𝑠)

σ = stress

a = radius of detectable AE cracks

V = radial velocity of cracks propagation

h = distance between the source and receiver

x = smallest displacement that the sensor can sense

σ a V are called the source parameter for an acoustic emission events

Stress change,

where,

I = n × n square matrix c = stiffness tensor

c + ∆𝑐 = stiffness tensor of product phase

∆𝑐 = change in the stiffness

𝛽 ^∗= unconstrained shape

𝛽^𝑜 = pre – existing stress or residual stress

D = shape matrix

V = volume of transformed phase

If the stiffness ∆𝒄 << c and there is no residual strength that means 𝛽^𝑜 = 0

• Change in stress will depend on shape change (transformation one material another material) , c is the constant which is the material property
• This parameter will control the intensity or the level of

acoustic emission

∗

∆𝜎 𝑡 = 𝑐𝛽 𝑉 𝑡

− ∗

∆𝜎(𝑡) = 𝐼 + ∆𝑐𝐷 [(c + ∆𝑐)𝛽 −∆𝑐𝛽^𝑜] 𝑉(𝑡)

109

Characteristics of AE signal

• Cover the wide range of energy levels which depend on types of

sources and frequencies

• Two types of basic source
• Continuous emission-signals coming from rapidly occurring source
• Burst type- continuous emission source and suddenly burst
• Radiation pattern same as ultrasonic wave
• Radiate the energy in all direction, for a cracks of sufficient length it

can become directional

• Frequency is generally broadband
• Received frequencies cover a wide range from available to 400 KHz

or higher

110

Kaiser and Felicity effects

11

1

It describe a relationship between AE events and the previous load history

• Kaiser describe AE events occurring when the structure first loaded to threshold, unloaded and loaded again
• It states no AE is generated until the previous maximum load is exceeded and the structure that inspecting is still sound
• Emission that occurs in the later loading below the previous maximum load is due to structural damage

​

Kaiser effects -No emission before the previous maximum load

Felicity effects – Emission before the previous maximum load

Felicity

effects

Kaiser effects

AB- loading BC- unloading CB – reloading BD – loading DE – unloading ED – reloading

11

2

• F < D emission are occurring

before the previous

maximum load

• Steep rise in the emission
• GH - always happening with no increment in the load, this is known as load hold

• It is the particular value , load is not increasing but lot of emission coming out, which indicate some kind of damage

Felicity effects

Emission before the previous maximum load

Felicity ratio, Fr=

𝑃_𝑒 (𝑒𝑥𝑖𝑠𝑡𝑖𝑛𝑔 𝑙𝑜𝑎𝑑)

11

3

𝑃_𝑚(𝑝𝑟𝑒𝑣𝑖𝑜𝑢𝑠 𝑚𝑎𝑥𝑖𝑚𝑢𝑚 𝑙𝑜𝑎𝑑)

Fr > 1 – there is no damage has occurred since the last inspection

Fr < 1 – indicate of cumulative or permanent damage

Signal Parameter

Threshold

4

11

Time

• Counts N- Number of expression above the threshold is considered as AE
• Below the threshold, it is considered as noise
• It is the function of the threshold and frequency
• It depends of the magnitude of the AE sources
• It also depend upon the properties of the sample and the

sensor

Volts

Peak Amplitude

Time

​

• Highest measured voltage or amplitude
• Express in dB
• Directly related to the energy in the AE signal

Threshold

Volts

Peak Amplitude

11

5

Duration (D)

Duration

​

Volts

Threshold

​

Time

• It will depends on the magnitude and frequency of the AE source
• Duration can be used to identify different types of emission source
• The noise which are coming out from the some other source, which are not related to the defect hence useful for filtering the noise

11

6

Rise Time (R)

Threshold

Rise time

​

​

Volts

11

7

Time

​

• Time interval between first threshold crossing and the signal peak
• Related to the propagation of the wave between the source and the sensor

Measured Area Under The Rectifies Signal Envelope (MARSE)

• It used to qualify AE signal and filter out noise
• This the measure of the signal strength
• Sensitive to the duration and peak amplitude but does not take into account the user defined threshold and the operating frequency

​

Volts

​

Time

​

MARSE

​

Time

11

8

Volts

Data Display

Counts or energy

• Hit signal – A

Time

Signal

above the thresholds

above the

counts the

• If signal thresholds, signal

• Below thresholds counts as noise

Count rate or Energy rate

Count or Energy v/s time Count rate or Energy rate

v/s time

Time

• At particular instant, we know what is going inside the component
• At given time, how much

acoustic emission or

signals coming out at

certain location

119

Counts or energy vs Load

indicate that structure is good

Counts or energy

Load

• By the figure we have to know that lot of emission happen or lower amount of emission happen
• Very steep curve- small amount of increase in load lead a lot of emission
• With increasing the load very high emission generate which would indicate bad structure
• Gradual slop- Gradual increase in load with respect to load which

Bad

Good

120

Hit v/s Amplitude

Hits

Amplitude

Differential Plot

If we considered many hits are

how above

particular amplitude is known as cumulative plot

Amplitude

1000

Hits

100

10

At given hit with particular amplitude is known as Differential plot

12

1

Cumulative Plot

Source location

12

2

• This technique is primarily qualitative, it does not give quantitative information
• Location of source is very important to know about the zone from which these emission coming out
• Hence we provide some corrective measure around the particular area

Linear location scheme

If, t₁<t₂

velocity of sound wave inside the sample V X= V t

Difference t₁ and t₂ = ∆𝑡 X = V ∗ ∆𝑡

12

3

Zonal location technique

A , t₁

• B, t

2

• More than two sensor _{C, t}
• Placement of sensor is important- Potential source of AE
• Sensor should be placed in a pattern or geometric shaped
• Based on the possible source of location then, we target those area while inspecting and taking

^mc^eo^ar^sr^ue^rc^etive

t₁= t₂ =t₃

Source of emission at the center of the triangle Because sensor receiving signal at the same time

3 t₁≠ t₂ ≠ t₃

124

Elastic Wave Method – Principle

Source: Ref.5

125

Elastic Wave Method

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NDT Method

Principle Advantage Disadvantage

Corrosion Evaluation

Specific Equipment

Ultrasonic pulse velocity (UPV)

Mechanical energy propagates through the concrete as stress waves and converted into electrical energy by a second transducers

A large penetration depth and it is easy to use for estimating the size, shape, and nature of concrete damage

The evaluation of UPV data is a highly specialized task, which requires careful data collection and expert analysis

Pulse velocity

(V)

Transducers( transmitter and receiver), amplifier, and oscillator

Acoustic emission(AE)

Elastic wave are generated due to rapid release of energy from a localized source within an RC structure

A cost- effective and sensitive technique that can detect and locate the active cracks

Passive defects can not be effectively detected

AE parameter

Transducers, preamplifier, filter amplifier, and storage equipment , mechanical impa¹c²t⁶ors

Elastic Wave Method

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Hits-Detection of an AE signal

• In the below figure AE activity observed corresponds to the corrosion loss of steel reinforcement in a marine environment ( Melcher and Li)

​

Phase 1- Onset of corrosion is initiated and the phase is dominated by

the presence of oxygen and water

Phase 2- Corrosion loss decrease and stabilize

Phase 3 – corrosion penetrates inside

Phase 4 – expansion of corrosion products occurs due to anaerobic

corrosion

​

• Based on the above phase corrosion activity are characterized Onset of corrosion and the growth of corrosion products ( nucleation of cracks)
• On the basis of above phase AE technique could detect the

8

corrosion at an early stage

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Corrosion loss of steel reinforcement due to chloride immersion and cumulative AE hits and number of AE events during corrosiontest

Source- Ref.5

• AE hits to be one of the AE parameters used to study the onset of

corrosion and the nucleation of cracks in RC structure

• It observed that AE hits increased with an increase in the degreeof corrosion

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Signal strength (SS) and Cumulative Signal Strength (CSS)

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0

• Signal strength (SS) is one of the AE parameter, it is defined as the area under the voltage signal of AE over the duration of the waveforms.
• Since it provides a measure of the waveform energy released by the specimen, it is rational damage indicator (Velez et al.)
• The SS of prestressed specimen is attributed to the nucleation of cracks caused by the accumulation of corrosion products at the steel concrete interface
• The SS non prestressed specimen it could be indicator of early cracks

formation due to corrosion

• The CSS exhibits a clear rate change before the onset of corrosion according to electrochemical method, which suggest that the AE technique could detect the onset of corrosion
• The CSS rate increase slowly in Phase 1 indicating de-passivation of the layer surrounding the steel reinforcement and the onset of steel corrosion
• The presence of sudden rise at the end of phase 1 might indicate crack

initiation due to steel corrosion

• The rise of Phase 2 indicates corrosion activity
• The sudden rise at the end of phase 2 indicate cracks propagation

leading to macro- crack.

• If the sudden rise excluded from the CSS curve, dotted line will be

obtained and it is the agreement with conventional curve

131

The variation in the CSS parameter

Source- Ref.5

• AE detects the sudden release of micro-fractures in the RC structure. Increasing the steel diameter due to corrosion is found to increase

the absolute energy of AE (Ing et al.)

132

Rise amplitude (RA) and Average Frequency

• RA value = Rise Time/Amplitude
• Average Frequency = Counts/Duration

​

• Tensile type cracks – When AE signal with high average frequency and low RA value
• Shear type cracks – low average frequency and high RA value

Classification of cracks by AE indexes

Source- Ref.5

133

Earthquake

36

AE

Principle

Vibration of the earth produced by the rapid release energy

Measured the intensity (energy released) of the source

Parameter

Shocks, epicenter, origin, time, depth, magnitude and intensity

Threshold, peak amplitude, rise time , duration, measure area under the rectifies signal envelope

Instrument

Seismograph, Richer scale, Mercalli scale

Piezoelectric sensor

134

AE method, the same principle can be applied to determine the scaling

of the amplitude distribution of the AE waves during the fracture process

DIFFERENCE BETWEEN CONVENTIONALANDPROPOSED AE METHODOLOGY

Conventional AE technique for

seismic diagnosis of piles

AE technique proposed for seismic diagnosis of railway structures

Schematic illustration of procedure for AE experiment

in an inverted model pile

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Source- Xiu Luo at al. STUDY ON SECONDARY AE TECHNIQUE FOR SEISMIC DIAGNOSIS OF RAILWAY SUBSTRUCTURES. 13th World Conference

on Earthquake Engineering Vancouver, B.C., Canada

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A set-up for secondary AE monitoring under simulated train Lloading

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