Ultrasonic

Question 1

How fast does sound travel through air, what is sound and what is frequency and how is it measured

Answer 1

A sound wave travels through the air.
 At sea level - at 0oC - at a speed of 332m/sec.
 We hear the sound when it hits a membrane in our ear & causes it to vibrate

 Sound travels through any medium that has molecules to move.
 It travels faster in more elastic materials as the vibrations are passed on more quickly.
 It travels faster in water or metal than it does in air, as liquids & solids are more elastic than air.
 Speed of sound increases with the stiffness, (elasticity) & decreases with density.

Sound is a series of vibrations, one way of measuring it is to:
 Count the number of vibrations per second –frequency.
 Frequency is measured in Hertz (Hz).
 One vibration in one second is 1 Hz.
 One thousand in one second is 1000 Hz or 1kHz
 One million vibrations in one second is 1MHz.

Question 2

What is the advantage of lower frequencies

Answer 2

There is an advantage for the lower frequencies.
 The lower the frequency, the more penetrating a sound wave is.
 That’s why foghorns give out very low notes & why the low throbbing notes from your neighbour’s stereo come through the wall.

Question 3

What range of frequencies is UT testing done

Answer 3

For most practical ultrasonic testing, the frequency range used is from 0.5MHz to 6MHz.
 Frequencies from 0.5MHz to 1.5MHz are used for testing materials with large grain structures, (concrete or cast iron castings).
 Frequencies from 2MHz to 6MHz are used for testing materials with fine grain structures (steels).

Question 4

How fast does a sound wave travel through material

Answer 4

 So far only the effect of the wave passing one point in the material has been considered.
 The wave itself is travelling along through the material.
 Ultrasonic waves travel through a solid, at the speed of sound, for a given type of wave, in a given material.
 So, the speed of travel of a sound wave, differs for different types of wave, & differs in different materials.

Question 5

Longitudinal/Compression Wave:

Answer 5

this type of wave propagation, the direction of oscillation of the atoms is the
same as the wave propagation

 This type of wave is given the symbol L.
 Velocity is given the symbol V.
 So, VL signifies the speed of propagation of a Longitudinal/Compression wave.

Used for => Thickness & lamination measurements

Question 6

Transverse/Shear waves

Answer 6

With this type of wave propagation, the direction of oscillation of the atoms is at 90o to the wave propagation
Transverse/Shear Wave:
 This type of wave is given the symbol T.
 VT signifies the speed of propagation of a Transverse/Shear wave.

Used for => Defect sizing & weld inspection

Question 7

What fundamental principles make UT a good choice

Answer 7

In order that ultrasonic waves can be used to measure depths & sizes within a material it is a fundamental principle that:
 The velocity of the sound wave remains constant for different samples of the same material.
 This is the case & furthermore, the ultrasonic wave obeys the Laws of Light.
 We can, therefore, predict its behaviour.

Question 8

Why is wavelength important when calculating the return signal

Answer 8

 The Wavelength is important because:
 The shorter the wavelength, the smaller the flaws that can be discovered but
 The shorter the wavelength, the less the ultrasound will penetrate the test material.

Question 9

What happens when a UT signal passes through a material

Answer 9

As the UT signal passes through a material.
 A pressure or stress front will be initiated.
 This presents resistance to the passage of the sound.
 The amount of resistance depends on the properties of the material.
 Known as Acoustic Impedance (Z).
 Is the reason that ultrasound travels at different speeds in different materials.

Attenuation means the loss of energy.
Acoustic Attenuation.
 Is the reduction in energy of the wave as it passes through a material.
 The strength of the UT signal will be reduced, & eventually, so low that it can’t be detected.

Question 10

UT waves are said to obey the laws of light, why

Answer 10

The Ultrasonic Wave:
 Travels at a known speed.
 In a straight line.
 Obeys the Laws of Light.
To predict the direction that a wave travels, it’s necessary to determine what happens when it meets an interface.
Boundary between two different materials.

An Interface will include:
 The outside edges of a component - the back wall.
 The surfaces of a crack or porosity bubble.
At these interfaces, the direction of travel of the wave will be determined by:
 The Law of Reflection & the Law of Refraction.

Question 11

Law of Reflection

Answer 11

This states that the angle the reflected wave makes with the normal angle to the interface from which the wave is being reflected is the same as the angle that the incident wave makes with the same normal angle.

When the angle of incidence is 0°, the reflected angle is also 0°, so the wave is reflected back along the incident direction.

Question 12

Law of Refraction (Snell’s Law)

Answer 12

At an interface, part of the ultrasonic wave is reflected and the rest will pass into the second material. The path in the second material will still be a straight line, but the direction of this wave will not be continuous with the direction of the incident wave as it will have been turned through an angle that can be determined by Snell’s Law.

Question 13

What is a Piezo-electric Transducer

Answer 13

Transducer.
 A device that changes energy from one form into another.
Sound.
 Is a stress wave of mechanical energy.
Piezo-electric Transducer.
 Changes electric energy into mechanical energy & vice versa.

The Piezo-electric Effect.
 Curie Brothers observed it in quartz crystals.
 Polarised Ceramic used instead of quartz crystal.
 Varying the thickness, alters the frequency.

When an electric potential is applied the crystal will vibrate & produce mechanical energy

When the crystal is squeezed by the returning mechanical energy an electrical potential is produced

Question 14

Explain the transducer

Answer 14

The Transducer.
 Mounted in a probe assembly for protection & to enable electrical connections to be made.
The crystal is fitted with silver foil electrodes to apply the voltage across the crystal.
The crystal is attached to the base by the mounting, which acts as a fixing, & as a backing to the crystal.

A pulse of ultrasound from a Piezo-electric crystal has a length of several wavelengths.
A Piezo-electric crystal continues to vibrate after it is hit by an electrical charge.
 This affects sensitivity, as the longer the pulse length, the worse is the resolution.
 In most probes, a slug of Tungsten-loaded Araldite is put behind the crystal to dampen the sound.

Probes are designed to transmit a signal into a specimen with maximum efficiency.
There are four types of probe:
 Single Crystal Probes.
 Twin Crystal Probes.
 Compression/00 Probes.
 Angle Probes.

Question 15

Explain single crystal probes

Answer 15

Single Crystal Probe.
Uses a single Piezo-electric crystal that both transmits & recieves.
The crystal must:
 Transmit the signal.
 Stop ringing.
 Ring down to rest.
 Pick up any reflected signal.
 Ring up to produce electrical energy.

Question 16

Explain twin crystal probes

Answer 16

Twin Crystal Probe.
 Has separate crystals for Transmitting & Receiving.
 The two are mounted in the same housing but are isolated electrically & acoustically, usually by cork.
 One is constantly Tx while the other is constantly Rx.
 The electrical isolation is achieved by providing two co-axial connectors.

Question 17

What the advantages and disadvantages of single crystal probes

Answer 17

Single Crystal Probes
Advantages
 Good power output.
 Greater penetration.
Disadvantages
 Poor near zone resolution.
 Cannot measure thin plate.

Question 18

What the advantages and disadvantages of twin crystal probes

Answer 18

Twin Crystal Probes
Advantages
 Good near zone resolution.
 Can measure thin plate.
 Can be focused to any depth.
Disadvantages
 Less power output.
 Less penetration.

Question 19

Explain the spread of the sound wave

Answer 19

The spread of waves from a Piezo-electric crystal has been likened to a torch beam.
 Just as light from a torch diminishes with distance, so sound pulses get weaker the further they travel.
 Sound produced from a UT crystal derives from many points along the surface of the crystal.
 This results in a sound field that has many waves interfering with each other.

Dead Zone - Nothing useful can be obtained from here.

Near Zone - An area of fluctuating intensities, used for Wall Thickness & lamination measurements.

Far Zone - Beam diverges at a predictable rate &can be used to accurately size small defects.

Question 20

Pulse Echo Technique

Answer 20

Pulse Echo Technique in which a single probe is used to both transmit and receive ultrasound. In addition to the fact that access is required from one surface only, further advantages of this technique are that it gives an indication of the type of defect, its size and its exact location within the item being tested.

The major disadvantage is that pulse echo inspection is reliant upon the defects having the correct orientation relative to the beam in order to generate a returning signal to the probe and is not, therefore, considered fail safe

Question 21

Use of DTM’s

Answer 21

Thickness Measurement
The main use of UT is the measurement of the time the signal takes to travel between specific interfaces.
The instruments are either:
 Digital Thickness Meter.
 A-scan Flaw Detector.

DTMs measure thicknesses using
 Longitudinal waves.
 Propagated by a compression probe.
 transmitted into the material at 00 angle.

A DTM
Designed to give a single readout for each application of the probe.
Only gives a readout from the major reflector. Its main limitation
A-scan is designed to display multiple reflections.

DTM Advantages:
 Quick & easy to use.
 Divers & ROV’s use them.
 Only small amount of training.
 Only isolated cleaning required.
 Reads through firmly adhered paint (SA2).

Question 22

Summary of a DTM

Answer 22

 The Cygnus DTM uses a 00 single crystal probe.
 Uses Pulse Echo Technique for thickness measurement.
 Check calibration at the worksite (pre-cal) logged by the data recorder.
 Clean enough for probe access (SA2).
 On completion check calibration (post-cal) also logged.

Care of a DTM
 Clean all terminations, plugs, leads & controls.
 Wash off housings with fresh water.
 Charge all batteries as recommended.
 Don’t overcharge batteries, some may evolve Hydrogen gas & cause an explosive hazard.
 Store in a dry place.
 Never operate damaged equipment.