Ultrasonic Testing

Part 3

The Phenomenon of Sound

REFLECTIONREFRACTION

DIFFRACTION

Law of Reflection

• Angle of Incidence = Angle of Reflection

60o 60o

Inclined incidence(not at 90o )

Incident

Transmitted

The sound is refracted due to differences in

sound velocity in the 2 DIFFERENT materials

REFRACTION• Only occurs when:The incident angle is other than 0°

Water

Steel

Steel

Steel

Water

Steel

30°

Refracted

REFRACTION• Only occurs when:The incident angle is other than 0°

Steel

Steel

Water

Steel

30°

Refracted

The Two Materials has different

VELOCITIES

No Refraction

30°

30°

65°

Snell’s Law

I

R

Material 1

Material 2

2 Materialin

1 Material

Vel

inVel

RSine

ISine

Incident

Refracted

Normal

Snell’s Law

Perspex

Steel

S

CC

CC

S

When an incident beam of sound

approaches an interface of two

different materials:

REFRACTION occurs

There may be more than one waveform

transmitted into the second material,

example: Compression and Shear

When a waveform changes

into another waveform:

MODE CHANGE

Snell’s Law

Perspex

Steel

C

CS

If the angle of Incident is

increased the angle of

refraction also increases

Up to a point where the

Compression Wave is at

90° from the Normal

90° This happens at the

FIRST CRITICAL ANGLE

1st Critical Angle

C

27.4

S

33

C Compression wave refracted

at 90 degrees

1st Critical Angle Calculation

C

Perspex

SteelC

5960

2730

90

I

Sine

Sine

5960

2730SinI

458.0SinI

26.27I

S

190Sin

27.2

Snell’s Law• Calculate the 1st critical angle for a

perspex/copper interface

• V Comp perspex : 2730m/sec

• V Comp copper : 4700m/sec

5.355808.04700

2730SinI

2nd Critical Angle

C

S (Surface Wave)

90

C

Shear wave refracted at 90 degrees

57

Shear wave becomes a surface wave

2nd Critical Angle Calculation

C

Perspex

Steel

C

3240

2730

90

I

Sine

Sine

3240

2730SinI

8425.0SinI

4.57I

S190Sin

57.4

Snell’s Law

C

Perspex

Steel

C

20

48.3

2 Materialin

1 Material

Vel

inVel

RSine

ISine

5960

2730

48.3

20

Sine

Sine

4580.04580.0

Snell’s Law

C

Perspex

Steel

C

15

34.4

2 Materialin

1 Material

Vel

inVel

RSine

ISine

5960

2730

R

15

Sine

Sine

2730

596015SinSinR

565.0SinR

4.34R

Snell’s LawC

Perspex

Steel

C

20

S

48.3

24

1st.

2nd.

33°

90°

Before the 1st. Critical Angle: There

are both Compression and Shear

wave in the second material

S C

At the FIRST CRITICAL ANGLE

Compression wave refracted at 90°

Shear wave at 33 degrees in the

material

Between the 1st. And 2nd.

Critical Angle: Only SHEAR

wave in the material.

Compression is reflected out

of the material.

C

At the 2nd. Critical Angle: Shear

is refracted to 90° and become

SURFACE wave

Beyond the 2nd.

Critical Angle: All

waves are reflected

out of the material. NO

wave in the material.

Summary

• Standard angle probes between 1st and

2nd critical angles (45,60,70)

• Stated angle is refracted angle in steel

• No angle probe under 35, and more

than 80: to avoid being 2 waves in the

same material.

C

S

C S

One Defect Two Echoes

Sound Generation

• Hammers (Wheel tapers)

• Magnetostrictive

• Lasers

• Piezo-electric

magnetostrictive

Piezo-Electric Effect• When exposed to an alternating current a

crystal expands and contracts

• Converting electrical energy into mechanical

- + + - - +

Piezo-Electric Materials

QUARTZ

• Resistant to wear

• Insoluble in water

• Resists ageing

• Inefficient converter of

energy

• Needs a relatively high

voltageVery rarely used nowadays

LITHIUM SULPHATE

• Efficient receiver

• Low electrical

impedance

• Operates on low

voltage

• Water soluble

• Low mechanical

strength

• Useable only up to 30ºCUsed mainly in medical

Polarized Crystals

• Powders heated to

high temperatures

• Pressed into shape

• Cooled in very

strong electrical

fields

Examples

• Barium titanate (Ba Ti O3)

• Lead metaniobate

(Pb Nb O6)

• Lead zirconate titanate

(Pb Ti O3 or Pb Zr O3)

Most of the probes for conventional usage use

PZT : Lead Zirconate Titanate

Probes

Probes• The most important part of the

probe is the crystal

• The crystal are cut to a

particular way and thickness to

give the intended properties

• Most of the conventional crystal

are X – cut to produce

Compression wave

Z

X

X X

Y

Probes

• The frequency of the probe depends on

the THICKNESS of the crystal

• Formula for frequency:

Ff = V / 2t

Where Ff = the Fundamental frequency

V = the velocity in the crystal

t = the thickness of the crystal

Fundamental frequency is the frequency of the material ( crystal )

where at that frequency the material will vibrate.

Probes

• The Thinner the crystal the Higher the frequency

• Which of the followings has the Thinnest crystal ?

1 MHz Compression probe

5 MHz Compression probe

10 MHz Shear probe

25 MHz Shear probe

25 MHz Shear

Probe

Probe Design

• Compression Probe

– Normal probe

– 0°

Damping

Transducer

Electrical

connectors

Housing

Probe Design

• Shear Probe

– Angle probe

DampingTransducer

Perspex wedge

Backing

medium

Probe

Shoe

Probe Design

Twin Crystal

Advantages

• Can be focused

• Measure thin plate

• Near surface

resolution

Disadvantages

• Difficult to use on

curved surfaces

• Sizing small defects

• Signal amplitude /

focal spot length

Transmitter Receiver

Focusing

lensSeparator /

Insulator

Ultrasonic Displays

• A-Scan

• B-Scan End View

• C-Scan Plan View

• D-Scan Side View

• P-Scan or “projection scan” collects and

combines A, B, C & D Scan information

Ultrasonic Displays• A scan

The CRT (Cathode Ray Tube) display

The Horizontal axis :

Represents time base / beam path length / distance / depth

The Vertical axis :

Represent the amount of sound energy returned to the crystal

Ultrasonic Displays

• B scan

The End View Display

B

Ultrasonic Displays

• C scan

The Plan View Display

C

Ultrasonic Displays

• D scan

The Side View Display

D

Ultrasonic Test Methods

• Pulse Echo

• Through Transmission

• Transmission with Reflection (pulse echo techniques where the transmitter is

separate from the receiver - e.g. tandem testing, time

of flight)

Pulse Echo Technique

• Single probe sends

and receives sound

• Gives an indication of

defect depth and

dimensions

Defect OrientationONLY DEFECTS HAVING A SUITABLY ORIENTATED

REFLECTING SURFACE CAN BE DETECTED BY PULSE ECHO METHODS!!

Orientation favourable, sound reflected back to

point of origin

Orientation unfavourable, sound not reflected back

to point of origin

Through Transmission Testing• Transmitting and receiving probes on

opposite sides of the specimen

• Pulsed or Continuous sound

• Presence of defect indicated by

reduction in transmission signal

• No indication of defect location

• Easily automated

• Commonly integrated into plate rolling

mills - lamination testing

Through Transmission Technique

Transmitting and

receiving probes

on opposite sides

of the specimen

Tx Rx

Presence of defect

indicated by

reduction in

transmission signal

No indication of

defect location

Through Transmission Technique

Advantages

• Less attenuation

• No probe ringing

• No dead zone

• Orientation does not

matter

Disadvantages

• Defect not located

• Defect can’t be

identified

• Vertical defects

don’t show

• Must be automated

• Need access to both

surfaces

Transmission with ReflectionRT

Also known as:

Tandem Technique or

Pitch and Catch Technique

Transmission with

ReflectionT R

TANDEM TESTING

Transmission with Reflection

T R

TANDEM TESTING

Automated Inspections

• Pulse Echo

• Through Transmission

• Transmission with Reflection

• Contact scanning

• Gap scanning

• Immersion testing

Gap Scanning

• Probe held a fixed

distance above the

surface (1 or 2mm)

• Couplant is fed into

the gap

Immersion Testing

• Component is placed in a water filled tank

• Item is scanned with a probe at a fixed distance above the surface

Immersion Testing

Immersion Testing

Water

path

distance

Water path distance

Front surface Back surface

Defect