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This paper explains about the ultrasonic testing which uses high frequency sound energy to conduct examinations and make measurements. It also illustrate the general inspection principle and typical inspection configuration.

ASISH MANI ABRAHAM

SANJU TOM

7/10/2014

ULTRASONIC TESTING

Ultrasonic is a name given to the study and the application of sound waves having frequency higher

than those waves human ear can hear. Ultrasonic sound starts from 20 KHz and above and the testing

frequency ranges from 100 KHz to 25 MHz Ultrasonic examination can be conducted on a wide

variety of materials including casting, forging, weld and composites.

Infrasonic - Below 16 Hz

Sonic Sound - 16 Hz to 20000 Hz

Ultrasonic Sound - Greater than 20 KHz

SOUND WAVE FORMS

1) Compression Waves ( Longitudinal Waves)

Sound propogation is perpendicular to the specimen and particle movement is parallel

to the sound direction.

Compression Waves travel through Solid, Liquid and Gases.

Compression Waves can be generated by normal probe.

Sound’s propogation speed depends on the type, temperature and composition of

the ,medium through which it propagates.

Velocity of compression wave in steel is 5920 m/s.

2) Shear Waves ( Transverse Waves)

It is only used in solid substances.

The wave movement is right angle to the particle vibration.

Velocity of shear wave in steel is 3250 m/s.

The particle does not move along with the waves, they simply oscillate up and down

about their individual equilibrium position as the wave passes by.

Difference between Compression wave and shear wave

3) Surface Waves ( Rayleigh Waves)

It propagates in solid surface.

It travels through near surface and surface level penetration.

Propogation of surface wave is elliptical.

Only surface testing is possible using this wave.

4) Lamb Waves ( Plate Waves)

Lamb wave is produced by changing of surface waves.

It have no particular velocity.

The velocity changes with frequency and plate thickness.

There are two types of lamb waves: Symmetrical and Asymmetrical.

They are used to test very thin materials.

COUPLANT

A Couplant is used between transducer phase and the phase of test surface to ensure efficient sound

transmission from transducer to test surface. The presence of air between the transducer and the test

material causes a great difference in acoustic impedance at the interface. A couplant can be any of a

vast variety of liquids , semi-liquids , paste and even some solid that will satisfy the following

requirements.

A couplant must be easy to apply.

A couplant must be harmless to the test specimen and transducer.

A couplant must wet both the surface of the test specimen and face of the transducer

and excludes whole air between them.

SNELL’S LAW

MODE CONVERSION

1. First Critical Angle

The angle of incidence at which the longitudinal wave pass through the surface of the second

medium or perpendicular to the imaginary line is called First Critical Angle.

First Critical Angle = Sin-1 ( Vi/VR)

2. Second Critical Angle

The angle of incidence at which shear wave passes through the surface of the second

medium or perpendicular to the imaginary line is called Second Critical Angle. The refracted

shear wave is called surface wave and the refracted longitudinal wave is called plate wave or

lamb wave.Second Critical Angle = Sin-1(Vi/VR)

ULTRASONIC TESTING EQUIPMENT

Principle

When ultrasonic waves from a piezo-electric crystal are made to propagate in a material through

proper coupling it will be partially reflected or refracted when there is a change in medium or an

interface , say the presence of a discontinuity or the opposite surface of the sound entry. The energy of

the reflected ultrasonic waves depends upon the severity of the defect, the area or the orientation with

respect to the direction of sound entry. The reflected wave is picked by a receiver and amplified for

evaluation.

Major Components of Ultrasonic Testing Equipment.

I. Pulse Generator: It energise the timer and pulser transmitter.

II. Pulse Transmitter: It energise the piezoelectric crystal in short pulses of regular interval .

III. Probes: It generates the longitudinal sound waves and or shear sound waves.

IV. Receiver : It receives the electrical signal and amplify the signals and feed it in to sweep

and marker circuits.

V. Sweep Circuits: The horizontal line or base line of the ultrasonic testing screen is

controlled by sweep circuits. Normally the value of sweep reads from left to right at fixed

points.

VI. Marker Circuits: The amount of refraction in a particular area is displayed in CRT screen

with the help of a marker circuit.

VII. Timer: It is the heart of the equipment. It is the source of all timing signals in the UT

equipment.

PROBES

A probe sends a sound wave in to a test material. There are two indications , one from the initial

pulse of the probe , and the second due to the backwall echo. A defect creates a third indication and

simultaneously reduces the amplitude of the backwall indication.

TYPESOF PROBES

Normal Probe

Angular Probe

T-R Probe

Normal Probes

It is used to inspect thickness and measure flaws on bars, plates, casting, forgings and extrusions.

Structure Of Normal Probe

Calibration of Normal Probe

Calibration Block Used: IIW V1 Block.

Input Values:

Angle =0 degree

Range =100

Velocity =5920 m/s

Mode = T+R

Steps:

Horizontal Linearity Checking

Vertical Checking

H1/H2 = H3/H4

Sensitivity Check

S = Wavelength/2

Wavelength = Velocity/Frequency

=(6000*1000)/(4*1000000)

= 1.5mm

S= 1.5/2 =0.75mm

Resolution Check

Amplitude Check

dB = 20 log ( H2 /H1)

Penetration Check

Set Range=200 and Frequency= 25dB

Angle Probes

In this the echo of a discontinuity on the instrument display does not give us any direct information

about its position in the material. The only available information for determination of the reflector

position is the scale position and therefore the sound path, this means the distance of the discontinuity

from the index point of the probe. The mathematics of the right angled triangle help us to evaluate the

surface distance and the depth of a reflector which are both important for ultrasonic testing.

Calibration of Angle Probe

Calibration Block Used: IIW V2 Block

Input Values:

Angle = 70 degree

Range = 100mm

Velocity = 3250 m/s

Mode = T+R

Steps:

Horizontal Linearity Checking

Sensitivity Checking