2. 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.
3. 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
4. 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.
5. 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
6. 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)
7. 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.
8. 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.
9. 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
10. 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
11. 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.
