51877225 ultrasonic testing

8/2/2019 51877225 Ultrasonic Testing

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Defect & Discontinuity

Group A -Discontinuity, Imperfection & Inhomogeneities

Group BDefect ,Fault & Flaw

Discontinuity- An intentional or unintentional

interruption in the physical structure orconfiguration of a part.

Defect- A condition or discontinuity having a size,

orientation, nature or location that impairs theuseful service of the part or that is rejectableaccording to specification or standard.


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Need for NDT

NDT or NDE are the terms used torepresent the techniques that are basedon application of physical principles

employed for the purpose of determiningthe characteristics of the materials orcomponents or systems and for detectingand assessing the in homogeneities and

harmful defects without impairing theusefulness of such material orcomponents or systems.


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Types of NDT Methods

1.Ultrasonic Testing

2. Eddy Current

3. Magnetic Particle 4. Dye Penetration

5. Radiography


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UT Testing

This testing is used for finding surface, subsurface, internal defects in welding,castings, plate & forgings etc. It can also

be used for tubes. It is more useful to findout planar defects.

It can also detect exact depth of the

defect.


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Ultra sonic Testing Procedure

Basic Procedure:

1.Prepare the surface to remove to obtainsmooth surface.

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

3.Pass Ultrasound, with help of Ultrasonic

probes, ultrasound is reflected from defectboundary and revealed on CRT of UTmachine.


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Basic Principle

1. By Introducing short pulse of sound usinga transducer into a job and receiving theecho from the defect or back wall then by

finding out the time of journey the defectlocation can be determined.

Distance = Velocity X Time


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Sound reflection at a flaw

Probe

FlawSound travel path

Work piece

s

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


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2. Sound passes through homogenousmaterial and is reflected from back wall. Itwill take definite time to do so, depending

upon the material thickness andproperties. If there is discontinuity in theobject, the sound will be reflected from it

and will take lesser time.


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Plate testing

delaminationplate0 2 4 6 8 10

IP

F

BE

IP = Initial pulse

F = FlawBE = Backwall echoT=D/V

T1=D1/VT>T1,Since D>D1

DD1


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Ultrasonic Instrument

0 2 4 8 106


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0 2 4 8 106

+-Uh



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0 2 4 8 106

+ -

Uh



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0 2 4 8 106

+

+

-

-

U

U

h

v



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Block diagram: Ultrasonic Instrument

amplifier

work piece

probe

horizontalsweep

clock

pulser

IPBE

screen


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The ball starts to oscillate as soon as it is pushed

Pulse


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Oscillation


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Movement of the ball over time

Time


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Time

One fulloscillation

T

Frequency

From the duration of oneoscillation T the frequency f(number of oscillations per

second) is calculated: f=1/T


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Types of Sound waves

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

2. Transverse waves/Shear wave/Anglebeam

3. Surface wave/Rayleigh wave

4. Plate wave/Lamb waves


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Direction ofoscillation

Direction of propagationLongitudinal wave

Sound propagation


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Direction of propagationTransverse wave

Direction of oscillation

Sound propagation


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


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Plate Waves


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Types of Probes

The transduceris capable ofboth transmittingand receivingsound energy.

1.Normal ProbeA piezoelectric elementin the transducerconverts electrical

energy into mechanicalvibrations (sound), andvice versa.


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2.Angle Probe


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Principles of Ultrasonic Inspection

Ultrasonic waves are introduced into a materialwhere they travel in a straight line and at aconstant speed until they encounter a surface.

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

The amount of reflected or transmitted energycan be detected and provides information aboutthe size of the reflector.

The travel time of the sound can be measuredand this provides information on the distancethat the sound has traveled.


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Technicques of UT

1.Pulse Echo Method: In pulse-echo testing, atransducer sends out a pulse of energy and thesame or a second transducer listens for reflectedenergy (an echo).

Reflections occur due to the presence ofdiscontinuities and the surfaces of the testarticle.

The amount of reflected sound energy isdisplayed versus time, which provides theinspector information about the size and thelocation of features that reflect the sound.


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Digital displayshowing signalgenerated fromsound reflectingoff back surface.

Digital displayshowing the presenceof a reflector midwaythrough material, withlower amplitude back

surface reflector.The pulse-echo technique allows testing when access to only oneside of the material is possible, and it allows the location ofreflectors to be precisely determined.


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The bigger the flaw is in the path of echo

longer the indication & vice versa.

If the defect will lye near the focus zone ofthe echo, the indication will become

bigger.

N

Near field Far field

Focus Angle of divergenceCrystalAccoustical axis

D0

6


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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 materialhaving standards shape & size reflectors are

used for calibrating Y axis (amplitude/size).


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IIW V1 Block


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Test Techniques Through-Transmission

0 2 4 6 8 10

2

11

Two transducers located onopposing sides of the test specimenare used. One transducer acts as atransmitter, the other as a receiver.

Discontinuities in the sound path willresult in a partial or total loss ofsound being transmitted and beindicated by a decrease in thereceived signal amplitude.

Through transmission is useful indetecting discontinuities that are notgood reflectors, and when signalstrength is weak. It does notprovide depth information.

T R

T R

11

2


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Digital displayshowing receivedsound throughmaterialthickness.

Digital displayshowing loss ofreceived signaldue to presenceof a discontinuityin the sound field.

Test Techniques Through-Transmission


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Test Techniques Normal and Angle Beam

In normal beam testing, the soundbeam is introduced into the testarticle at 90 degree to the surface.

In angle beam testing, the soundbeam is introduced into the testarticle at some angle other than90.

The choice between normal andangle beam inspection usuallydepends on two considerations:

- The orientation of the feature ofinterest the sound should bedirected to produce the largestreflection from the feature.

- Obstructions on the surface of the

part that must be worked around.


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Angle beam probe calibration

IIW V2 Block

S1=25mmS2=50mm

Block Factor-25+50=75mm

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


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Snells Law


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Refraction : 1st critical angle


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Calibration of angle probe

Step1. Connect DAC Calibration Adapter to the

UT instrument and manipulate its delayregulator to get the 7mm UT instrument digitaldistance indication. Adjust amplitude of theindicated pulse to the 100% of the screenheight. Mark the top of the pulse as a point 1 ofthe DAC curve.

Step2. Change the delay of DAC CalibrationAdapter to get the 75mm UT instrument digitaldistance indication. Reduce the Gain of UTinstrument in 11dB. Mark the top of the pulseas a point 2 of the DAC curve

Step3. Change the delay of DAC CalibrationAdapter to get the 38mm UT instrument digitaldistance indication. Increase the Gain of UTinstrument in 5dB. Mark the top of the pulse asa point 3 and create DAC curve


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SpectrumofsoundFrequency rangeHz Description Example

0 - 20 Infrasound Earth quake

20 20,000Audiblesound

Speech, music

> 20,000 UltrasoundBat, Quartzcrystal


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gas liquid solid

Atomic structures

low density

weak bondingforces

medium density

medium bondingforces

high density

strong bondingforces

crystallographicstructure


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T

Distance travelled

From this we derive:

c= /T or c=f Wave equation

During one oscillation T the wave frontpropagates by the distance :


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Wave propagation

Air

Water

Steel, long

Steel, 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, transversewavestravel at lower speeds.

Sound velocity mainly depends on the density and E-modulus of the material.


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Behaviour at an interface

Medium 1 Medium 2

Interface

Incoming wave Transmitted wave

Reflected wave


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0 2 4 6 8 10

s

s

Wall thickness measurement

Corrosion


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Through transmission testing

0 2 4 6 8 10

Through transmission signal

1

2

1

2

T

T

R

R

Flaw


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Ultrasonic Probes

socket

crystal

Damping

Delay / protecting faceElectrical matching

Cable

Straight beamprobe Angle beam probeTR-probe


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Block diagram: Ultrasonic Instrument

amplifier

work piece

probe

horizontalsweep

clock

pulser

IPBE

screen


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Weld inspection

0 20 40 60 80 100

s

aa'

d

x

a = s sin

a' = a - x

d' = s cos

d = 2T - t'

s

Lack of fusion

Work piece with welding

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


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Straight beam inspection techniques:

Direct contact,

single element probe

Direct contact,

dual element probe Fixed delay

Immersion testingThrough transmission


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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

51877225 ultrasonic testing

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