Ultrasonic Testing

By Geetanjali Sharma

Devendra SinghAmit Kumar Jha

Defect & Discontinuity• Group A -Discontinuity, Imperfection & In

homogeneities• Group B –Defect ,Fault & Flaw

Discontinuity- An intentional or unintentional interruption in the physical structure or configuration of a part.

Defect- A condition or discontinuity having a size, orientation, nature or location that impairs the useful service of the part or that is rejectable according to specification or standard.

Need for NDT

• NDT or NDE are the terms used to represent the techniques that are based on application of physical principles employed for the purpose of determining the characteristics of the materials or components or systems and for detecting and assessing the in homogeneities and harmful defects without impairing the usefulness of such material or components or systems.

Types of NDT Methods

• 1.Ultrasonic Testing

• 2. Eddy Current

• 3. Magnetic Particle

• 4. Dye Penetration

• 5. Radiography

UT Testing

• This testing is used for finding surface, sub surface, internal defects in welding, castings, plate & forgings etc. It can also be used for tubes. It is more useful to find out planar defects.

• It can also detect exact depth of the defect.

Ultra sonic Testing Procedure

• Basic Procedure:

1.Prepare the surface to remove to obtain smooth surface.

2.Apply Couplant (Water ,Grease, oil etc.)

3.Pass Ultrasound, with help of Ultrasonic probes, ultrasound is reflected from defect boundary and revealed on CRT of UT machine.

Basic Principle

1. By Introducing short pulse of sound using a transducer into a job and receiving the echo from the defect or back wall then by finding out the time of journey the defect location can be determined.

• Distance = Velocity X Time

Sound reflection at a flaw

Probe

Flaw Sound travel path

Work piece

s

Principle of time of flight measurement : S=vt/2

2. Sound passes through homogenous material and is reflected from back wall. It will take definite time to do so, depending upon the material thickness and properties. If there is discontinuity in the object, the sound will be reflected from it and will take lesser time.

Plate testing

delaminationplate 0 2 4 6 8 10

IP

F

BE

IP = Initial pulse

F = Flaw

BE = Backwall echo T=D/VT1=D1/V

T>T1,Since D>D1

DD1

Ultrasonic Instrument

0 2 4 8 106

0 2 4 8 106

+-Uh

Ultrasonic Instrument

0 2 4 8 106

+ -Uh

Ultrasonic Instrument

0 2 4 8 106

+

+

-

-

U

U

h

v

Ultrasonic Instrument

Block diagram: Ultrasonic Instrument

amplifier

work piece

probe

horizontal

sweep

clock

pulser

IP

BE

screen

The ball starts to oscillate as soon as it is pushed

Pulse

Oscillation

Movement of the ball over time

Time

Time

One full oscillation

T

Frequency

From the duration of one oscillation T the frequency f (number of oscillations per second) is calculated: f=1/T

Types of Sound waves

1. Longitudinal waves/Compressive waves/ Zero Degree Beam/Straight Beam/ Normal beam

2. Transverse waves/Shear wave/Angle beam

3. Surface wave/Rayleigh wave

4. Plate wave/Lamb waves

Direction of oscillation

Direction of propagationLongitudinal wave

Sound propagation

Direction of propagationTransverse waveDirection of oscillation

Sound propagation

http://geophysics.eas.gatech.edu/classes/Geophysics/misc/pics/Rayleigh_wave.jpg

Plate Waves

Types of Probes

The transducer is capable of both transmitting and receiving sound energy.

1.Normal Probe

A piezoelectric element in the transducer converts electrical energy into mechanical vibrations (sound), and vice versa.

2.Angle Probe

Principles of Ultrasonic Inspection

• Ultrasonic waves are introduced into a material where they travel in a straight line and at a constant speed until they encounter a surface.

• At surface interfaces some of the wave energy is reflected and some is transmitted.

• The amount of reflected or transmitted energy can be detected and provides information about the size of the reflector.

• The travel time of the sound can be measured and this provides information on the distance that the sound has traveled.

Technicques of UT

• 1.Pulse Echo Method: In pulse-echo testing, a transducer sends out a pulse of energy and the same or a second transducer listens for reflected energy (an echo).

• Reflections occur due to the presence of discontinuities and the surfaces of the test article.

• The amount of reflected sound energy is displayed versus time, which provides the inspector information about the size and the location of features that reflect the sound.

Digital display showing signal generated from sound reflecting off back surface.

Digital display showing the presence of a reflector midway through material, with lower amplitude back surface reflector.

The pulse-echo technique allows testing when access to only one side of the material is possible, and it allows the location of reflectors to be precisely determined.

• The bigger the flaw is in the path of echo longer the indication & vice versa.

• If the defect will lye near the focus zone of the echo, the indication will become bigger.

N

Near field Far field

Focus Angle of divergenceCrystalAccoustical axis

D0

6

Calibration of UT Machines.• Range Calibration:-A block of known thickness

and material is used for calibrating x axis (time/depth).

• Sensitivity Calibration:-A block of similar material having standards shape & size reflectors are used for calibrating Y axis (amplitude/size).

IIW V1 Block

Test Techniques – Through-Transmission

0 2 4 6 8 10

2

11

• Two transducers located on opposing sides of the test specimen are used. One transducer acts as a transmitter, the other as a receiver.

• Discontinuities in the sound path will result in a partial or total loss of sound being transmitted and be indicated by a decrease in the received signal amplitude.

• Through transmission is useful in detecting discontinuities that are not good reflectors, and when signal strength is weak. It does not provide depth information.

T R

T R

11

2

Digital display showing received sound through material thickness.

Digital display showing loss of received signal due to presence of a discontinuity in the sound field.

Test Techniques – Through-Transmission

Test Techniques – Normal and Angle Beam

• In normal beam testing, the sound beam is introduced into the test article at 90 degree to the surface.

• In angle beam testing, the sound beam is introduced into the test article at some angle other than 90.

• The choice between normal and angle beam inspection usually depends on two considerations:

- The orientation of the feature of interest – the sound should be directed to produce the largest reflection from the feature.

- Obstructions on the surface of the part that must be worked around.

Angle beam probe calibration

IIW V2 Block

S1=25mmS2=50mm

Block Factor-25+50=75mm

1st echo-25mmIInd echo-25+BF(75)=100mm

Snell’s Law

Refraction : 1st critical angle

Calibration of angle probe

• Step1. Connect DAC Calibration Adapter to the UT instrument and manipulate its delay regulator to get the 7mm UT instrument digital distance indication. Adjust amplitude of the indicated pulse to the 100% of the screen height. Mark the top of the pulse as a point 1 of the DAC curve.

• Step2. Change the delay of DAC Calibration Adapter to get the 75mm UT instrument digital distance indication. Reduce the Gain of UT instrument in 11dB. Mark the top of the pulse as a point 2 of the DAC curve

• Step3. Change the delay of DAC Calibration Adapter to get the 38mm UT instrument digital distance indication. Increase the Gain of UT instrument in 5dB. Mark the top of the pulse as a point 3 and create DAC curve

Spectrum of soundFrequency range Hz Description Example

0 - 20 Infrasound Earth quake

20 – 20,000 Audible sound Speech, music

> 20,000 Ultrasound Bat, Quartz crystal

gas liquid solid

Atomic structures

• low density• weak bonding

forces

• medium density• medium bonding

forces

• high density • strong bonding

forces• crystallographic

structure

T

Distance travelled

From this we derive:

c=λ/T or c= λf Wave equation

During one oscillation T the wave front propagates by the distance :

Wave propagation

AirWaterSteel, longSteel, trans

330 m/s

1480 m/s

3250 m/s

5920 m/s

Longitudinal waves propagate in all kind of materials.Transverse waves only propagate in solid bodies. Due to the different type of oscillation, transverse wavestravel at lower speeds.Sound velocity mainly depends on the density and E-modulus of the material.

Behaviour at an interface

Medium 1 Medium 2

Interface

Incoming wave Transmitted wave

Reflected wave

0 2 4 6 8 10

s

s

Wall thickness measurement

Corrosion

Through transmission testing

0 2 4 6 8 10

Through transmission signal

1

2

1

2

T

T

R

R

Flaw

Ultrasonic Probes

socket

crystal

Damping

Delay / protecting faceElectrical matchingCable

Straight beam probe Angle beam probeTR-probe

Block diagram: Ultrasonic Instrument

amplifier

work piece

probe

horizontal

sweep

clock

pulser

IP

BE

screen

Weld inspection

0 20 40 60 80 100

s

aa'

d

x

a = s sinßa = s sinß

a' = a - xa' = a - x

d' = s cosßd' = s cosß

d = 2T - t'd = 2T - t'

s

Lack of fusion

Work piece with welding

Fß = probe angles = sound patha = surface distancea‘ = reduced surface distanced‘ = virtual depthd = actual depthT = material thickness

ß

Straight beam inspection techniques:Direct contact,

single element probe

Direct contact,

dual element probeFixed delay

Immersion testingThrough transmission

surface = sound entry

backwall flaw

1 2

water delay

0 2 4 6 8 10 0 2 4 6 8 10

IE IEIP IP

BE BEF

1 2

Immersion testing