Ultrasonic Testing - Training Notes

Ultrasonic Testing

TRAINING NOTES

ROB MAXWELL

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HEORY

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Ultrasonic is a term used in acoustics (the science of sound) when dealing with vibratory waves whose frequencies are beyond the limits of the hearing range of the average person. Ultrasonic waves are of the same nature as audible sound waves, i.e. they are stress waves and can only exist within the media. The energy of an ultrasonic wave is transferred from one point to another by vibrating the particles in the material through which they are being

propagated.

In this respect they differ from light and other forms of electromagnetic radiation which can travel freely through a vacuum. In other respects these two forms of energy obey similar laws of propagation1_.

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SOUND SPECTRUM

When a block of metal is struck with a hammer, the sound of the blow is heard and the accompanying vibration of the block can be felt. The sound is conveyed by airborne waves. These audible oscillations2 _{are called sonic waves. The frequency of these waves are unlikely}

to exceed 5000 cycles/second.

The term Hertz (Hz) is used to measure the frequency of vibration. In ultrasonics this frequency occurs many thousands of times per second. A more suitable way of stating frequency is to use the term kilo (k) and mega (M) to describe units of a thousand and a million respectively.

1 The movement of a wave through a medium.

Frequency Uses

Under 1 KHz Early underwater navigation. 16 – 20 KHz Upper limit to human hearing. 19 – 20 KHz Alarm systems.

40 KHz

Underwater signalling and cleaning. Testing of large grain material.

500 KHz (½ MHz) Upper limit to underwater signalling. 1 MHz – 5 MHz Common range for testing material. 5 MHz – 10 MHz Used on fine grain material.

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LONGITUDINAL OR COMPRESSION WAVES (NORMAL PROBE)

A longitudinal wave is a wave formed by individual particles oscillating in the direction of propagation.

An ultrasonic wave is supported and propagated through a material by particles that oscillate around their point of equilibrium. To simplify this statement let us consider Fig 2.

Shown in Fig 2, we have a model consisting of a series of identical pendula and for simplicity, this may be looked upon as representing a solid body prior to the application of ultrasonic energy. Position A and B illustrate the point where a pulse of ultrasonic energy will be applied. Once a pulse has been initiated pendulum A is forced towards pendulum B, transmitting its internal energy.

Fig 3

As each pendulum has to protrude somewhat from its neutral positions in order to strike the next one, the next pendulum will start its oscillation a fraction later than the preceding one.

Compression

Rarefaction

Consequently, the pendula do not oscillate in the same rhythm, but all of them with the same frequency. Longitudinal or compressed waves may be propagated through solids, liquids and gasses.

TRANSVERSE OR SHEAR WAVES (ANGLE PROBES)

Contrary to longitudinal waves the individual particles of the transverse wave oscillate vertically in the direction of propagation (Fig 4 & Fig 5).

Fig 4 Fig 5 Fig 6

The use of the pendula, example serves well to illustrate the effect of transverse wave. Fig 6.

Fig 7

For this example it has been shown that a pre-requisite for the formation of transverse waves are molecules being ties firmly together. As this is only the case with solid bodies, then it will follow that transverse or shear waves are not supported in liquids or gas.

RAYLEIGH WAVES (SURFACE)

Rayleigh waves travel over the surface of a solid and bear a rough resemblance to waves on the surface of water.

Reflections of Rayleigh waves from cracks in the surface or from discontinuities lying just beneath the surface may be seen on an oscilloscope screen.

Rayleigh waves are also called surface waves since their depth of penetration is usually no more than one wavelength.

LAMB WAVES

Fig 9

Lamb waves occur in plate material, in two basic forms which in practice are usually mixed. Lamb waves consist of a mixture of zigzag reflected longitudinal and transverse waves with a mutual phase relationship in which some particles oscillate in a direction 90˚ to the plate surfaces and others of varying angles.

The ability of Lamb waves to flow in thin plates make them applicable to a wide variety of problems requiring the detection of subsurface discontinuities. As lamb waves have a surface component, surface scale or dirt will affect the trace causing damping or reflection, and it is therefore necessary to ensure surface cleanliness and also to limit the couplant to the probe edge only.

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VELOCITY

The velocity of sound in a medium depends on the density and the elastic constants of the material. If follows that material with different densities will have different velocities, for example: Material Comp velocity (m/s) Shear velocity (m/s) Material Comp Velocity (m/s) Shear Velocity (m/s) Air 322 Oil 1440 Aluminium 6400 3130 Perspex 2740 1320

Brass (70-30) 4372 2100 Steel (Mild) 5960 3240 Cast Iron 3500 2200 Steel

(Stainless) 5740 3130 Copper 4760 2325 Water 1480 Gold 3240 1200 Tungsten 5174 2880 Iron 5957 3224 Zinc 4170 2480 Lead 2400 790 Zirconium 4650 2300

Note: Comp velocity used for Longitudinal Probes, Shear velocity used for angle probes. The velocity of a surface wave for any given material is approximately 90% of the shear velocity.

Velocity difference can cause problems if testing a material other than the material on which the calibration has been conducted.

As an example, if having calibrated on a steel block, a probe is then placed onto a copper plate of 20mm thickness, the screen would indicate a thickness of 25mm.

This can be proven by the following calculation: Actual thickness * (V Steel / V Copper)

20 mm * (5960 / 4760) = 25.04mm

The velocity is different by a ratio of 1.25:1

If the thickness indicated is divided by the factor we arrive at 20mm – the true thickness (25.04mm / 1.25 = 20mm).

FREQUENCY

The frequency of a wave form is the repetition rate, or the number of cycles or oscillations in a given time, and is usually expressed as cycles per second (Hertz).

From Fig 11 it will follow that the higher the frequency (greater number of oscillations or cycles per second) then the crest of the waveform will come closer together, thereby producing a shorter wavelength.

Multiple units of frequency are expressed as kilohertz (KHz) which is equal to one thousand oscillations per second, it may also be written as MHz which is equal to one million oscillations per second.

WAVELENGTH

Wavelength expressed as (λ) lambda is given as the distance between two successive crests in the waveform, this distance varies with frequency and velocity.

Fig 10

The wavelength formula is as follows:

Three examples follow to show the effect of frequency and velocity: 1. Find the wavelength of a 5MHz longitudinal probe in mild steel.

Longitudinal velocity in mild steel = 5960 m/s Probe frequency = 5 * 106 _Hz

λ = v/f is 5960/(5 * 106₎

=0.001192m

= 1.192mm (0.001192 * 1000 < mm in a meter)

2. Find the wavelength for a 5MHz longitudinal probe in copper. Longitudinal velocity in copper = 4760 m/s Probe frequency = 5 * 106 _Hz

λ = v/f is 4760/(5 * 106₎

=4760 / 5000000 =0.000952m

= 0.952mm

3. Find the wavelength for a 5MHz angle probe in copper. Transverse velocity in copper = 2325 m/s Probe frequency = 5 * 106 _Hz

λ = v/f is 2325/(5 * 106₎

=2325 / 5000000 =0.000465m

= 0.465mm

Consider the three examples:

Wavelength for 5MHz longitudinal probe in steel = 1.192mm Wavelength for 5MHz longitudinal probe in copper = 0.952mm Wavelength for 5MHz transverse probe in copper = 0.465mm

We have proven that a 5MHz longitudinal probe has a shorter wavelength in copper than it has in steel and that a traverse or angle probe has a wavelength approximately half that of the longitudinal probe for the same frequency in the same material.

An important point to remember is, that in theory, a probe can detect a defect whose diameter is approximately one tenth of its wavelength. However, in practice on third to one half seems to be the more acceptable figure.

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A wave train refers to a short group of waves, before or after which there are no waves. This formation is generally referred to as a pulse.

Pulse takes several forms:

It may start and drop rapidly (Fig 12) It may build and decay gradually (Fig 13)

It may build up rapidly and decay exponentially (Fig 14)

The type of wave illustrated in Fig 14 is referred to as a decayed train and this is probably the most commonly used type of pulse in ultrasonics. Almost all probes incorporate assisted damping to the crystal in the form of backing. This backing medium must have a higher acoustic impedance3 _{than the crystals. The reason for the damping in a single probe is the}

very fact that the crystal has to produce short, and above all sharp bursts of energy. Ideally the crystal motion must end abruptly from its previous pulse so that the reflected energy excites a relatively inactive crystal and not one that is already in a stage of oscillation. Fig 15 and Fig 16 show the effect of probe ringing time with assisted clamping.

Fig 15 Fig 16

What type of pulse width can we expect to be displayed on the cathode ray tube time base? Pulse width will depend on the frequency of the probe used and is also a function of pulse energy. In other words one must apply an electrical pulse to the crystal that is wide enough to cause the transducer to reach maximum oscillations, at the same time remembering that an increase in pulse width will markedly reduce resolution.

With a lower frequency probe the applied electrical pulse will be wider and the resolution will be inferior to that of a higher frequency probe that required a shorter pulse.

The effect of pulse width with regard to resolution will be discussed later.

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Ultrasonic energy is produced by a crystal or transducer that is subject to the phenomenon known as the PIEZO ELECTRIC effect.

DEFINITION

The piezo electric effect is the property of certain crystals which when subjected to electrical energy convert this energy to mechanical energy (sound) and vice versa.

THE PIEZO ELECTRIC EFFECT

Electrical Current (IN) Sound Waves Electrical Current (OUT) An electrical charge to a crystal produces mechanical energy (Ultrasound).

(Conversely) Mechnical pressure on the crystal produces an electrical charge at electrodes.

CRYSTALS

Crystals used in the production of ultrasound and subject to the piezo electric effect are discussed later. A crystal may be one which has occurred naturally, as in the Quartz or it may be a crystal produced in polycrystalline form by a process of calcination and sintering at high temperatures. To produce various frequencies of ultrasonic energy the thickness of the crystal and the applied alternating current to the crystal are prime factors. For instance if a high frequency alternating charge is applied to a crystal that has been made to oscillate at the same frequency, then it will follow the applied field, causing the faces to vibrate in respect of each other at that frequency. It is possible however to use a 5MHz probe with the frequency switch positioned at the 2.5MHz position. The 5MHz probe will still oscillate but will be greatly reduced in output.

The reason for this is the alternating current to the probe does not match the crystal thickness and therefore does not achieve maximum crystal oscillation. In other words the crystal is not operating at its resonant4 _frequency.

The fundamental resonant frequency (Ff) of the crystal is inversely5 _{proportional to the crystal}

thickness.

Fundamental resonant Frequency (Ff) = velocity(v) / 2 *

thickness(t)

Example. What frequency will a 1mm crystal of Barium Titanate operate at? v = 4,400,00mm/sec

Ff = v/2t

Ff = 4,400,000 / 2 * 1 = 4,400,00 / 2

Ff = 2.2MHz (220,000Hz / 100,000)

The frequency will produce maximum crystal oscillation i.e. maximum output.

4 Resounding / echoing 5 of course, related

The cut of a crystal and the way it is mounted with the probe both have direct bearing on the type of crystal motion produced.

Fig 17

Fig 17 (a) shows the direction of crystal motion for an x-cut crystal and Fig 17 (b) the direction of crystal motion for a Y-Cut crystal.

The types of vibration that can be produced (torsional, longitudinal etc.) are achieved by the particular crystal chosen and the cut of the plate.

Crystals for consideration are:

QUARTZ

A naturally occurring compound which is hard and very stable both chemically and physically. It occurs as a six-sided prism with a pyramid attached to each end. If the point of the opposite corners are joined they provide the X-axis. The X-axis are the electrical ones and produce the ultrasonic vibration which is required.

BARIUM TITANATE

This is a pre-polarised crystal, or ceramic with the chemical composition of a crystal i.e. Barium carbonate is backed together with titanium dioxide at a temperature of around 1250˚C.

The domains of the crystal or ceramic, when subjected to an intense electric field of about 24V/mm at 140˚C6 _{and allowed to cool, become orientated to the direction of the field and}

polarisation is achieved.

After polarisation the activity factor of the probe will drop about 50% in 24 hours and thereafter will remain fairly constant although slow deterioration will take place.

6 The Curie point - The temperature at which a phase change in the magnetic or ferroelectric properties of a substance occurs, especially the change from ferromagnetism to paramagnetism that occurs with increasing temperature.

The resultant crystal is fairly good but it should be appreciated that heating above Curie point will destroy the piezo effect.

LEAD ZIRCONATE TITANATE

The best and recently the most widely used of all the pre-polarised crystals. Sensitivity is excellent with very little ‘grass’. Now becoming the accepted crystal for Non-destructive Testing.

LEAD METANIOBATE

Less sensitive than Barium Titanate but has a high internal damping coefficient and is thereby capable of transmitting very narrow pulses.

Having briefly discussed the merits of various crystals our next logical step will be to consider the various probes and their associated technicalities.

PROBES

We can distinguish two main groups of probes, probes producing longitudinal or compression waves (L-Wave) vertically through the surface and probes producing transverse waves (S-Wave) which are transmitted into the specimen at an angle with respect to the surface.

SINGLE PROBEOR TRANSCEIVER

A probe with only one crystal in its construction is known as a single probe, the mode of operation being pulse echo. In simple terms, the single crystal produces and also receives the ultrasonic energy. This is possible because the electrical charge is fed to the crystal in the form of rapid pulses with a number of micro seconds delay between each pulse. Each electrical charge produces a pulse of ultrasonic energy, which in turn are passed into a specimen via the coupling medium.

Fig 20a Fig 20b

The energy travels through the specimen and continues to do so until it reaches an interface in Fig 20a, this is the backwall. Reflection takes place and ultrasonic energy returns towards the crystal Fig 20b. The returning mechanical energy excites the crystal during the non-productive intervals and thereby produces an electrical charge which is fed back into the circuit to provide the signals for the cathode tube display.

COMBINED DOUBLE PROBES

The combined double probe is constructed as its name suggests, by the use of two crystals in one probe. One crystal continually transmitting whilst the other crystal is continually receiving the reflected energy. Fig 21 shows such a probe.

Fig 21

It will be noted that the acoustic barrier is corrugated on the inside to reduce the effect of cross talk chatter, also, a layer of cork is interposed between the perspex faces. This cork is known as the acoustic separator. The use of a penetrating oil as a couplant with this type of probe should be discouraged, as it is merely a matter of time before the oil, through a process

of penetration and saturation, breaks down the acoustic separation, giving rise to ‘standing echoes’ on the cathode ray tube.

The probe, by the use of two crystals, eliminate the dead zone and thereby allow for the detection of defects close to the surface.

Single or twin probes may be longitudinal or transverse in ultrasonic output.

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THE ULTRASONIC BEAM

DEAD ZONE

The dead zone is a zone where it is not possible to detect defects. The dead zone is shown as the transmission signal at the start of the time base. Its depth can be seen on a calibrated time base as the amount of time base occupied by the transmission signal. The dead zone is the ‘ringing time’ of the crystal and is minimised by the damping medium behind the crystal. The dead zone increases when the frequency is decreased, therefore a 5MHz single probe will have a smaller dead zone than a 2.5MHz.

NEAROR FRESNEL ZONE

The ultrasonic beam remains parallel and has the same diameter as the crystal over a distance known as the near zone.

Fig 24

Within the near zone there exists varying intensities of waves at the edge of the crystal (Fig 24) giving rise to unreliable signal amplitudes.

This means that signal height from the same size defect may increase when positioned further from the crystal.

The formula used to calculate the near zone is: D2_{f / 4v}

Example

Calculate the near zone for a 20mm crystal size, longitudinal velocity, 2.5MHz probe in

Steel (velocity = 5960m/s) D2_{f 4v} = 202 _{* 2.5 * 10}6 = 400 * 2.5 * 1,000,000 = 1,000,000,000Hz =1,000Mhz = 4 * 5960 = 23,840 D2_{f / 4v = 0.0419m = 41.9mm} Water (velocity = 1480m/s) D2_{f 4v} = 202 _{* 2.5 * 10}6 = 400 * 2.5 * 1,000,000 = 1,000,000,000Hz =1,000Mhz = 4 * 1480 = 5,920 D2_{f / 4v = 0.1689m = 168.9mm}

FAROF FRAUNHOFER ZONE

Beyond the near zone is the far zone. In the far zone the beam diverges resulting in a decay in sound intensity as the distance from the crystal is increased, just as a beam of light from a torch gets weaker the further it travels.

In the far zone, large and small reflectors follow different laws.

Large reflectors (larger than the beam) follow the Inverse Law. The amplitude is inversely proportional to the distance i.e. If the distance is doubled the amplitude is reduced by half (6 dB).

Small reflectors (smaller than the beam) follow the Inverse Square Law. The amplitude is inverse proportional to the square of the distance i.e. If the distance is doubled the amplitude is reduced by quarter (12 dB).

BEAM SPREAD, CRYSTAL SIZE, FREQUENCY

When an ultrasonic beam is produced it is propagated in a rectilinear fashion (straight line) from which it diverges only slightly. This divergence is a function of frequency and crystal diameter.

If the frequency is increased for a certain diameter of crystal the solid angle decreases and if the frequency is decreased for the same crystal diameter then the solid angle increases. This means that when using lower frequencies we must choose probes with larger crystal diameters if we wish to reduce the effect of an increased beam spread.

The half beam spread formula is as follows: ASin (kλ / D) - ASin is the inverse Sin Where:

D =

The constant k is used to calculate the beam spread intensity. k = 1.22 for extreme edges of the beam.

k = 1.08 for 10% (20 dB) edge k = 0.56 for 50% (6 dB) edge. Example:

Calculate the beam spread of ultrasonic waves travelling through mild steel, the waves are generated from a 10mm, 5MHz crystal.

Wavelength in steel is v / f =5960 / (5 * 106_{) =0.001192m =1.192mm}

Extreme edge of beam: k = 1.22

Half Beam Spread =ASin (k * λ/ D) =ASin (1.22 * 1.192 / 10) =ASin (0.1454) = 8.36˚

After considering the above example we will now calculate the half beams spread for a 2.5MHz longitudinal probe using the same size crystal.

Wavelength in steel is v / f =5960 / (2.5 * 106_{) =0.00238m =2.38mm}

Extreme edge of beam: k = 1.22

Half Beam Spread =ASin (k * λ/ D) =ASin (1.22 * 2.38 / 10) =ASin (0.2903) = 16.88˚

Note the considerable increase in beam spread.

REFLECTION AND REFRACTION

Ultrasonic high frequency vibrations react in many ways as that of light. They can be focused into a beam, refracted and reflected. It is this ability which makes it possible to utilise sound energy as a means of flaw detection.

REFLECTION

When a beam of sound energy strikes a boundary, it is normally reflected at the same angle as the incident beam.

Therefore it can be said that the angle of incidence equals the angle of reflection. REFRACTION

When sound waves pass from one medium to another a change in wavelength takes place due to the differing acoustic velocities of each medium. As a result, the angle at which the sound enters the second medium does not equal that of the first medium.

This is known as refraction. This refraction can be compared with the action of a beam of light when passing from air into water.

SNELL’S LAW

Snell’s Law determines the angular relationship between the incident and refraction beam on transmission between the media of different acoustics velocities.

Snell’s Law is used when calculating the angle to which the perspex wedge must be machined in order to produce a given refracted angle in the test material.

The formula can also be used to find the angle of refraction that a given probe will produce when examining materials other than steels e.g. copper and aluminium.

Example:

Calculate the incident angle used in the machining of the Perspex wedge to produce a 70˚ refracted angle in steel.

Velocity in Perspex (longitudinal) = 2740m/s Velocity in Steel (transverse) = 3240 m/s Incident angle used:

= Sin-1_{(V1 x Sin(˚)) / V2)} = Sin-1_{(2740 *Sin(70) / 3240)} = Sin-1_{(2740 *0.9397 / 3240)} = Sin-1 _(0.7947) =53.63˚ CRITICAL ANGLES

As the incident angle is increased from the normal the refracted wave is predominantly longitudinal, although the shear mode exists at an insignificant strength.

When the incident angle reaches the first critical angle (27.4˚) the longitudinal component will be totally internally reflected through 90˚. At this point only transverse waves exist in the second medium

V Perspex (long) = 2740 m/s

V Steel (long) = 5960 m/s Sin B = Sin(90˚) = 1 Sin A = Sin-1_{(V1 * Sin B / V2)}

= Sin-1_{(2740 * 1 / 5960)}

= Sin-1_(0.4597)

= 27.37˚

When the incident angle reaches the second critical angle (57.7 ˚) the transverse wave is totally internally reflected through 90˚.

Calculate the Second Critical Angle – Perspex to Steel V Perspex (long) = 2740 m/s

V Steel (trans) = 3240 m/s Sin B = Sin(90˚) = 1 Sin A = Sin-1_{(V1 * Sin B / V2)}

= Sin-1_{(2740 * 1 / 3240)}

= Sin-1_(0.8457)

= 57.74˚

Angle A as previously stated is the angle of the perspex wedge and it will be appreciated from Fig 28 that a wedge angle between 28.7˚ and 56˚ will produce transverse waves of refracted angles from 35˚ to 80˚.

RESOLUTION

The effect of pulse width with regard to resolution will now be discussed.

First of all it is necessary to enlarge on the fact that a pulse energy is made up of several waves produced from oscillations of the crystal during a given number of micro seconds.

We will now consider the pulse width in steel for a crystal that has produced a pulse of energy for a period of two microseconds (2 µ sec). The width of the pulse will be approximately

12mm.

This pulse width of 12mm would not be able to resolve these two defects because their echoes would overlap. Therefore this pulse width would not satisfactorily resolve defects closer than approximately 6mm. Defects around this 6mm separation band would give an indication on the back of the main signal on the cathode ray tube. Defects below this value would be lost in the main signal envelope.

Fig 31 and Fig 32 show the pulse from the 3mm separation and the 6mm separation. Fig 31 (3mm Separation) Fig 32 (6mm separation) To determine the pulse width:

Pulse width = (velocity x number of waves) / frequency

On summarising, good resolution demands a very short pulse so that the reflections of one defect lying close to another is not lost in the received signal of the first. It should be further noted that the higher the frequency, the shorter the pulse width. Also the higher the

frequency the shorter the wavelength, thereby giving greater sensitivity to small defects. The two combinations give good defect detectability and good resolution.

Fig 33 Fig 33 shows two examples of resolution.

ABSORPTIONOR ATTENUATION

Attenuation or weakening of the echoes, is a combination of absorption and scattering and

leads to energy loss, both in the material under test and in the probe itself.

Absorption of the wave can refer to loss through heat due to internal friction. The absorption factor is greater with high frequencies die to more rapid particle movement.

Other factors which attenuate the acoustic intensity of the beam are find inclusions, micro

porosity, the crystal structure and composition. Scattering of ultrasound is similar to the effect

of fog or smoke on light. It occurs when the frequency of the probe chosen has a wavelength which approaches the grain size of the material under test. Due to grain size irregularity, scattering is multi-directional.

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PULSE GENERATOR (MASTER TIMER)

To regulate the output of the transmission pulse and to supply the time base circuit.

TIME BASE (DEPTH RANGES)

Which by virtue of the relationship between the distance travelled by ultrasonic waves in unit time (velocity) can be used as a distance of depth scale when locating defects or measuring thickness.

TRANSMITTER

To provide short pulses of electrical energy to excite the transmitter probe.

RECEIVER / AMPLIFIER

To pick up and magnify the signal coming from the receiver probe. The energy then being applied (as a voltage) to the ‘Y’ plates.

ATTENUATOR

Controls the relationship of volts in / volts out across the ‘Y’ plates thereby controlling signal heights. Does not affect amplifier linearity.

DISPLAY UNIT

For presenting visually the transmitted and received signals in their proper time sequence with indication of relative amplitude. Two types of display may be utilised a Cathode Ray Tube (CRT) which are now superseded by the digital display.

The CRT is a highly evacuated tube and housed within this tube are an electron gun used to produce a pencil beam of electrons, a deflection system and a luminescent screen coated with special phosphor to render the movement of the electron beam as visible light.

In the electron gun the beam is initiated by heating a coated cathode by a small filament, electrons are emitted and these in turn are attracted by a positively charged anode. Between the cathode and anode we have the grid, its function being to limit electron flow.

The anode assembly can be produced in the form of apertures, their function being to accelerate the electron flow. The electrons are now in the form of a fine beam which will impinge on the luminescent screen and surrender its energy in the form of visible light. Located at the front of the electron gun there is the deflection system. Application of an electrical potential to the ‘X’ deflector plates will cause the beam to be deflected in the horizontal plane. When an electrical potential is applied to the ‘Y’ plates, deflection will occur in the vertical plane.

We now have a pointer that has no mass, in fact, weightless. This pointer can be deflected in two dimensions having the ability to faithfuly reproduce high frequency signals as visible light. The Pulse Generator provides the initiating pulse, in other words triggers off the time base circuit and causes the spot to commence its movement across the face of the tube and to trace out the green base line; it also triggers off the transmitter, causing a large pulse of energy to be sent to the probe.

The frequency with which the pulse generator performs these functions is known as the Pulse Repetition Frequency (PRF). The PRF could vary between 50 and 2000 pulses a second. The greater the rate of the PRF then the brighter becomes the green line of the time base. Due to this simultaneous triggering, it is assumed that the transmission pulse is located on the left hand side of the screen. Sometimes it is necessary to delay this pulse for various forms of testing.

The Pulse Transmitter circuit is the part of the equipment which delivers the current to the crystal. High tension supplies to this circuit are required of 1 to 2 kV and it is into this circuit that we introduce frequency selection. The time base circuit is required to obtain a linear transverse of the spot from left to right in the horizontal sweep. During this sweep there will be deflections in the vertical plane from reflections within the specimen and these must be displayed on the screen directly proportional to time.

Sweep times must be variable and depend on the depth range, therefore sweep times from 20 micro seconds to 1 millisecond may be required.

If we compare this period of time to the PRF rete we can see that the spot has some time to spare and, in fact, has a small amount of time to wait at the left hand side before

commencing further excursions.

The receiver accepts and amplifies the returning electrical signals and these signals are so minute that amplification rates from 10,000 to 100,000 times as required. Another

requirement of the amplifier is that it has to accommodate various frequencies and has to have adequate bandwidth.

A further requirement of the amplifier is that it should have the ability to reproduce all signals on the CRT whether large or small, at an amplitude or height which is in direct proportion to the energy at the receiver crystal. That is to say, that the input/output relationship should be linear.

These are but a few of the functions of an ultrasonic unit. They have been included to enable the student to have some appreciation of the electronics of ultrasonics.

The standard flaw detector utilises the pulse-echo (A-Scan) presentation.

A-SCOPE PRESENTATION

A form of cathode ray tube in rectangular co-ordinates, in which pulse amplitude is

represented as a displacement along one axis, and time is represented as a displacement along the other axis.

B-SCOPE PRESENTATION

A form of cathode ray tube display, in rectangular co-ordinates, in which the travel time of an ultrasonic pulse is represented as a displacement along one axis, and probe movement (generally rectilinear) is represented as a displacement along the other axis. In the display, reflected pulses are shown as bright marks on a dark background, or vice versa.

C-SCOPE PRESENTATION

The line-by-line presentation of flaw data obtained by scanning the major surface of the material line-by-line (non-intersecting lines) so that discontinuities are shown in terms of probe position at the moment of detection. The presentation may be on a cathode ray tube screen or recorded on paper or film i.e. a two-dimensional presentation.

D-SCOPE PRESENTATION

A two –dimensional graphical projection on to a plane normal to the test surface and normal to the projection of the beam direction on the test surface, showing the apparent size and position of reflections in the volume inspected by scanning an area of test surface.

DECIBEL NOTATION (dB)

The decibel is a unit of comparison. It is the measurement of changes in sound intensities and has a logarithmic base. It is possible to calculate the dB difference between signals from two reflectors whose size ratio is known.

e.g.

If two reflectors, equidistant from the probe, have a size ratio of 10:1, what would be the dB difference in their echo heights?

Formula: dB = 20 * log(h1/h2) where h1 & h2 are the echo heights. = 20 * log (10/1)

=20 * 1

Ratio of 2:1 = 20 * log (2/1) =20*0.30

Difference = 6 dB

By transmitting the formula, it is possible to determine the ratio of sizes of two reflectors whose dB difference is known.

e.g.

Tow reflectors, equidistant from the probe, have a dB difference of 14 dB between their echo heights. What is their ratio of reflective areas?

dB = 20 *log (h1/h2)

14 = 20 *log (h1/h2)

(14/20) = log (h1/h2)

Antilog 0.7 = (h1/h2) Note: Log(5) = 0.7

5 = (h1/h2)

Ratio = 5:1

CALCULATIONSOF dB RATIO’S – PRACTICE

1. The signals from two defects equidistant from the probe have heights of 10mm and 40mm. What is the decibel difference between the two signals?

2. Express a ratio of 5:1 in decibels.

3. What is the ration of signal amplitudes if the dB difference between the echoes is 40 dB?

4. Two reflectors equidistant from the probe had a difference of 8 dB between their echo heights, what is the ratio of their surface area?

Check answers HERE

The table below gives approximate dB and amplitude ratio equivalents. Amplitude Ratio dB Amp Ratio dB Amp Ratio dB Amp Ratio dB Amp Ratio dB Amp Ratio 1 1.12 6 2 11 3.55 16 6.31 21 11.22 2 1.26 7 2.24 12 3.98 17 7.8 22 12.59 3 1.41 8 2.51 13 4.47 18 7.94 23 14.13 4 1.59 9 2.82 14 19 8.91 24 15.85 5 1.78 10 3.16 15 5.62 20 10 25 17.78

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The surface condition of the specimen is of prime importance. There are many ‘Schools of thought’ on this subject and over-exuberant claims are constantly made by people who state that more than a fair degree of roughness is quite acceptable. More often than not these claims are made by people with little or no practical experience and, unfortunately, their comments are taken as read by management. When the ultrasonic operator attempts to perform his task of testing a specimen, and eventually requests some surface preparation, he is then considered as over critical and a burden to production costs.

Surface at all times should be of reasonable nature, and a guide to the degree of irregularity that can be tolerated can be stated as approximately one tenth of a wavelength. Beyond this figure, coupling conditions deteriorate.

The theoretical critical roughness is given by the equation: Rc = (λ1v2) / ( 2(v2-v1)) = (λ2v1) / (2(v2 –v1))

Whereλ1 is the wavelength in couplant

V1 is the velocity of sound in the couplant λ1 is the wavelength in the test piece V2 is the velocity of sound in the test piece

An ultrasonic test applied to a specimen with a rough surface beyond the limits defined above would bear little relationship to a further test applied to the same specimen after preparation. The reason for this is as follows. Probe control wold be much easier, making the point of reflection of defects easier to locate and measure. A smaller dead zone, would be created because less gain would be required, due to improved coupling conditions. The use of a higher frequency probe being made possible by the improved surface. All of the aforementioned would make a far superior test.

In some industries an unprepared surface is quite acceptable. The applied test is only required to find defects which are of certain dimensions. The size of defect, the surface condition and the probe frequency are all compatible. The coupling be water which is ejected onto the surface via an irrigated probe.

General comments on good surfaces in the following conditions will now be made.

LOOSE SCALE

Loose scale on a good surface should be removed. One layer of tightly adhering scale does not normally produce any significant hazards, but heat treatments do e.g. double stress relief or repeated tempering. These operations can create a double, tightly adhering scale, which presents an almost impenetrable acoustic barrier causing little or zero transmission.

PEENING

Peening on the surface of a casting should be regarded with the greatest suspicion as invariably this has been done to remove sand or to tighten or blend surface holes. These surface holes could very well be gas tails leading to cavities, or shrinkage.

SLIGHT RUST

Slight rust on the surface should be wire brushed, if this is not done it will mix with the couplant and become quite a solid mass under the movement of the probe.

In certain instances the use of a thin polythene sheet enhances the coupling conditions. To use this method it is first necessary to apply the coupling medium, one which has a high viscosity (grease, polycell, etc.) to the surface of the specimen. The polythene sheet is then placed on the couplant and smoothed over until all traces of air bubbles have been removed from below the sheet. When this has been accomplished, a thin smear of couplant on the exposed surface of the polythene is all that is necessary to provide a very good surface condition.

An added advantage of this method is the fact that it reduces probe wear, and presents a cleaner trace on the cathode ray tube.

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R

At any interface between two media of differing acoustic impedance acoustic mismatch causes part of the energy to be reflected back and the remainder to be transmitted through the interface.

The specific Acoustic Impedance is derived from the acoustic impedance of velocity and density.

z = pv

p = density of material

v = velocity of sound through the material

For ultrasonic wave’s incident at normal angle to the interface, the percentage of energy transmitted is:

(4z2 * z1 / (z2 + z1)2) * 100%

Where z1 and z2 are acoustic impedance’s in materials 1 and 2 the percentage of energy

reflected is:

((z2 - z1)2 / (z2 + z1)2) * 100%

If the energy in a material arrives at a boundary in air, the value of z2 is small compared to z1,

so that reflection is practically complete, therefore when it is required to transmit ultrasonic energy through an interface it is necessary to use a COUPLANT.

The nearer the acoustic impedance of the couplant to that of the two solids the greater will be the transmission.

COUPLANT

The couplant used in ultrasonic inspection appears to be very much a matter of personal preference; some operators use grease to eliminate the air between the transducer and the specimen, while others prefer polycell, water or oil. Glycerine is a thick syrupy liquid, soluble in water and is very good, but expensive.

Many are the mediums used, but it is worth knowing that in terms of efficiency some are much better than others. We shall consider some examples in the respect using the following formula.

Efficiency of couplant is a function of acoustic impedance of couplant and test piece. Acoustic Impedance z = Density * Velocity of Sound

Transmission = 4*z1*z2*100 / (z2 + z1)2 % z1 = A1 Test piece Steel = 45 * 105 _g/cm2_/sec z2 = A1 Couplant Oil = 1 * 105 _g/cm2_/sec Water = 1.5 * 105 Glycerine = 2.46 * 105 Polycell = 1.8 * 105 _(Approx.) Example Oil Couplant = 4* z1*z2*100 / (z2 + z1)2 = 4*45*1*100 / (45 + 1)2 =18,000 / (46)2 =18,000 / 2116 =8.5% Water = 4* z1*z2*100 / (z2 + z1)2 = 4*45*1.5*100 / (45 + 1.5)2 =27,000 / (46.5)2 =27,000 / 2162.25 =12.5% Glycerine = 4* z1*z2*100 / (z2 + z1)2 = 4*45*2.46*100 / (45 + 2.46)2 =44,280 / (47.46)2 =44,280 / 2252.45

=19.7% Assuming compression waves and a very smooth surface Reflection R = (z1 – z2)2*100 / (z1 + z2)2

% reflection + % transmission = 100%

Example of reflection at a water to steel interface:

R = (z1 – z2)2*100 / (z1 + z2)2

= (45 – 1.5)2 _{* 100 / (45 + 1.5)}2

= 1892.25*100 / 2162.25 = 189225 / 2162.25

= 87.5%

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I

When a wave is incident upon a face of other than 90˚, mode conversion may take place i.e. the transformation of a shear wave to a compression wave. Mode conversion may occur in the most simple of geometric shapes, a classic example of such an occurrence can be given from the IIW calibration block.

Fig 34.

Fig 34 (i) shows a compression probe placed at a convenient position on the IIW calibration block so as to obtain multiple reflections of 100mm. Fig 34 (ii) shows the end elevation, or through section, and the subsequent transformation of waves from compression (C) to shear (S) and back again to compression. Fig 34 (iii) shows the corresponding cathode ray

presentation.

The effect of mode change can be observed on a weld when examination of the root run is undertaken using a 60˚ probe. Fig 35 shows a root concavity of smooth contour, and the

effect of mode change is shown. It may well be that the conversion from shear to compression may occur only once, that is from the concavity to the dressed surface of the weld, then back again for conversion back to shear, in other cases it may return to the probe as a compression wave after many reflections of only a few degrees separation. The net product of such an occurrence is to indicate the effect on the screen at a beam path equal to the total beam paths, in this case, well into the parent material. Fortunately we can damp down these signals by placing an oily finger at position x thereby confirming mode conversion. In any event, such a suspect area as indicated by the mode conversion in the parent material would have been investigated from the other side of the weld.

The golden rule in ultrasonic inspection is to predetermine any areas where mode conversion may occur by careful preparation of the geometry of the specimen.

This enables the technician to pre-calculate beam path distance etc. from a drawing, thereby appreciating most of the variables that can happen before he starts the ultrasonic

Spurious indications are, as the name suggests, signals that occur and present irrelevant indications such as signals ghosting across the screen through electrical disturbances. The occurrence of a signal on the time base may be due to a build-up of couplant in front of an angle probe on flat or curved surfaces although it is fair to state that these signals are more predominant from a curved surface with the probe positioned for a traverse scan Fig 36.

Fig 36

Other such indications happen when a combined double compression probe is used on a rough surface with the attenuator set to give quite a high sensitivity level. Spurious

indications will be displayed as very strong signals in group formation occupying a position on the time base of between 15m to 20mm. This position and size of group depends on the degree of surface roughness and the size of the probe.

The reason for such spurious indications are due to reflection grating, which, in simple language, means that sound waves are reflected from one crystal to another across the

surface producing indications on the cathode ray tube at positions equal to the distances travelled.

Transmission of the beam still takes place under these conditions and good back wall

indications are obtained. To the less experienced ultrasonic technician these signals may be misinterpreted as inclusions or lamina defects.

Fig 37 illustrates the conditions described and the associated cathode ray tube display. Fig 37

One further example that is worthy of mention is the reflections obtained from a probe that has a refracted angle around 45˚.

Indications can be obtained from such a probe when the smallest amount of surface irregularity is present on the incident surface.

Fig 38

Fig 38 shows a situation where the signal heights from the surface irregularity could quite conceivably be greater than that of the defect for the indicated probe position.

For the technician with little experience in this respect it is suggested that practical tests are carried out to appreciate just how strong such indications can be.

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SINGLE PROBES

Before the time base is calibrated, the graticule scale does not represent any distance. The scale is divided into ten vertical lines, running the full height of the screen and fifty small lines at the bottom of the display.

Once the time base is calibrated to a known distance, each line has then a definite meaning. For example, when the time base has been calibrated to measure 0-50mm; each of the small lines represents 1mm.

Before calibration is attempted, two factors must be known; the time base distance required and the thickness of the available calibration block.

Calibration is achieved by placing a probe on the calibration block and aligning the repeated echoes to the corresponding graticules. In order to do that, one must first identify the

meaning of each line. As stated above, when calibration for 0-50mm; the small lines represent 1mm, the large lines must then each represent 5mm (10 lines, screen 50mm, 50 / 10 =

5mm). If the calibration block is 20mm thick, the first echo would be 20mm and the second echo would be 40mm.

The same method is used for any calibration required, by dividing the time base by the thickness of the calibration block and placing the echoes in their respective positions. For example, a time base calibration to 100mm using a 25mm thick calibration block would have four echoes, placed at 25, 50, 75 and 100mm.

The same time base (100mm) using a 20mm thick calibration block, would have 5 echoes placed at 20, 40, 60, 80 and 100mm; and so on.

The key, then, to any calibration is to divide the required time base by the thickness of the calibration block and place the echoes in the correct position.

An important point to remember is that the distance from echo to echo, is the thickness of the calibration block being used. For that reason it is not possible to calibrate the time base, using only one echo!

CALIBRATION OF SHEAR WAVE PROBES

Unlike compression waves, shear waves do not make use of a known thickness, but use a radius to achieve calibration. The main reason for this is to enable calibration of different angles from only one radius.

The method of calibration is similar to that used for compression, but the probe must be

positioned to give the maximum response from the radius. At that point the beam is radial and is travelling the radial distance. For example, the calibration block known as the V1 block has a radius of 100mm and when the echo is maximised the distance travelled by the beam is 100mm. Therefore repeat echoes each indicate an additional 100mm.

The echoes are repeated from slots, as indicated in Fig 41 above. I there were no slots in the calibration block, the beam would not show repeat echoes, which is the case when using a calibration block known as the V2.

It will be noted that the V2 block has two radii of 25mm and 50mm. You will recall that, as stated in the section on calibration of compression waves, it is not possible to calibrate using only one echo. With the V2 calibration block, echoes from 25mm and 50mm can be used to calibrate the time base for a required distance.

Some practice is needed to be confident when calibrating shear waves and it must be re-stated tha the calibration is achieved, not only when the leading edge of the echo is on the correct graticule, but also when the echo is maximised.

As an example an explanation of a calibration will now be given

It is required that a time base, reading from 0 – 100mm is to be used. The calibration block, which is available, is the V2. This has two radii of 25mm and 50mm.

1. Using the delay control, place the initial pulse at 0 on the time base.

2. Apply couplant to the calibration block and place the shear wave probe on the block, with the beam projected to the 25mm radius.

3. Using the delay and calibrate controls obtain two echoes. The distance between the first and second echoes is 75mm. Not, as one would expect, 25mm. The reason for this is that this block does not have and slots, like the V1. The sound beam does not repeat at 25mm, but reflects form the flat surface and is then projected to the 50mm radius. The second echo is then at the distance travelled from the 25mm radius to the 50mm radius. Add these together and you get 75mm.

4. With the delay control place the first echo on the graticule which represents 25mm, and the second echo on the graticule which represents 100mm. (First echo 25m, second echo is 25mm + 75mm = 100mm).

5. Turn the probe round so that the beam is projected at the 50mm radius, and make any adjustment using the calibrate control, to place the leading edge of the echo on the graticule which represents 50mm. it is important that the second echo is placed on 50mm as an additional check on the accuracy of the calibration.

The time base is now calibrated to measure 0 – 100mm.

PROBE INDEX POINT

The V1 block has many dimensions, which can be used for a great number of time base calibrations. It has one other important function, making use of the slot at the centre of the

radius. When the sound beam is reflected, as it passes from the probe shoe into the block, the exact point on the probe where this occurs is called the index point. With the echo from the 100mm radius maximised, the part of the probe which is aligned with the slot is the point at which the index point is to be marked on the probe.

G

Before any attempt is made to examine a particular weld or component by ultrasonics, it is necessary to appreciate the geometric considerations involved in that inspection.

In many instances the geometric configuration may be quite simple while in others it will be complex and require careful thought as to the correct technique to apply so that the

possibility of incorrect interpretation through mode change etc. will be avoided

With the qualification schemes for the approval of non-destructive testing personnel now operating it becomes increasingly apparent that a critical and precise approach in the methods of ultrasonic testing and recording of such information are of the highest priority.

If the ultrasonic technician has a constant system of recording and also a graphic

presentation, all the ultrasonic examinations he makes will be consistent and of the highest integrity.

Such a system will now be discussed and will serve to introduce a new word into the field of NDT; the word being Ultragraph.

THE ULTRAGRAPH

The Ultragraph should show three important aspects of a section of defective weld.

1. A cross section or side elevation of the defect(s) at various points indicating their size. 2. A plan view or radiograph type of presentation indicating the length of the defect. 3. A plot of the root contours in the case of a full penetration butt weld showing excess

penetration and lack of penetration.

The Ultragraph is accompanied by a recording sheet which contains all the relevant plotting information obtained

At the time of the test.

One of the main advantages of this system is the fact that all plotting is done on a drawing board in warm and pleasant surroundings after the scanning techniques have been completed and the plotting data compiled.

This is far removed and much more desirable than attempting to attain accuracy with greasy plastic slides and a chinagraph pencil that has a point, after a little use, of between 1 and 2 mm. it is quite ludicrous to lay any claim to accuracy under these conditions, furthermore the accumulation of such errors would very quickly exceed any acceptable plus or minus

tolerance on defect size as set by the qualification examining body.

Let us now proceed to a simple weld configuration in a pipe to pipe weld joint.

It will be noted that the root gap is 3mm and that the land is also 3mm. The preparation is a U prep and a decrease in angle is introduced at 37mm to reduce the amount of weld volume required.

Form this drawing the beam path distance, half and full skip distance can be measured. The choice of probe angle will respect to the junction zone can be selected remembering that the ultimate angle would be one of 90˚.

Beam path, half and full skip distances can be calculated mathematically. These are as follows, complete with examples. (These are not related to the above dimensions).

FULLAND HALF SKIP DISTANCE

The skip distance factor for any probe is found by 2t TAN(a).

This is twice the material thickness multiplied by the tangent of the angle of refraction (obtained from trigonometric tables, calculator).

Example:

Probe angle 45˚, plate thickness 20mm

2t *TAN(a)

2t = 20mm x 2 = 40mm TAN(45) = 1

40 x 1 = skip distance = 40mm (½ skip / 2) To calculate half skip distance

t*a

20mm x 1 = 20mm ½ skip BEAM PATH

To calculate beam path distance (BP)

BP = t / COS(a)

The beam path (BP) is the distance travelled from the index point of the probe to the first reflection point of the material.

To obtain this distance it is necessary to divide the cosine of the angle of refraction into the thickness of the material.

Example : BP = t / COS(a) Probe = 45˚ t = 20mm BP = 20 / COS(a) = 20 / 0.707

= 28.2mm

The foregoing formula can be deduced from the following: CB / AC = TAN(a)

Where: CB is the half skip distance AC is the thickness t of the material Example:

CB / AC = 20mm / 20mm = 1 (TAN of 45˚)

Calculation of the Beam Path and Surface distances mathematically provides a very reliable means to accuracy, however, the student who does not wish to use this method can use the geometric approach i.e. produce a drawing and determine the various distances by physically measuring the drawing.

Many complex shapes are examined by ultrasonics and parts of the component can be most difficult to examine either because of accessibility or geometric configuration. Such a problem is illustrated in Fig 50.

It can be seen that with the concave weld profile it is impossible from the stub side to examine the toe of the weld (an area where we may observe toe cracking) the changing contour of the header section makes any inspection from that face most difficult and almost impossible to plot under actual working conditions.

Another important point which should be borne in mind when the examination of a cylindrical object is contemplated using angle probes, is the IRRADIATION FACTOR, from Fig 51 you will observe the effect of a curved surface and will appreciate that irradiation is a function of diameter and angle.

IRRADIATION FACTOR

Formula

Irradiation Factor = (1-SIN(a))/2

Example:

Where a = 45˚ = (1-SIN(a))/2

= (1-SIN(45))/2 = (1 - 0.707)/2 = 0.293 / 2

Irradiation Factor = 0.146

Irradiation Factor * Diameter = Irradiation Depth The factors for various angles are as follows:

20˚ 25˚ 30˚ 35˚ 40˚ 45˚ 50˚ 55˚ 60˚ 65˚ 70˚ 75˚ 80˚ 0.32 9 0.28 9 0.25 0 0.21 3 0.17 9 0.14 6 0.11 7 0.09 0 0.06 7 0.04 7 0030 0.01 7 0.00 8 Having taken an initial look at weld profile let us now discuss the method of testing such a weld, bearing in mind our geometric considerations.

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ESTING

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The next important step will be to inquire as to the nature of the parent material and the weld metal (mild steel 2¼% chrome, 1% moly, etc.) and to any heat treatment operation carried out on the metal.

Attention should be paid to the overburden or cap of the weld. This should be ground off, (where possible) and the surface polished. It is also necessary to polish the surface on each side of the weld to ensure good coupling. Measuring from the centre point of the weld, the area requiring polishing is approximately twice the thickness of material under inspection. The parent material should be scanned on both sides using longitudinal waves to establish that it is free from laminations or other defects. Precise readings of the thickness during this operation should be noted from both sides and any areas of reduction marked for special attention later.

Next, scan the weld area noting any areas offering response.

Mark the centre line of weld circumferentially and then draw in the half skip positions

circumferentially as pre-determined from the drawing. This is the point where the probe index is beamed directly

into the centre line of the root. Finally, draw in the lines which represent the outer limits of scanning.

The use of a back or front stop is highly recommend for the initial root scan as this will provide a guide for the probe and allow the technician to direct most of his attention to the CRT. The method of root interpretation to be in accordance with the instructions received in the school.

TEST SENSITIVITY

First signs of ‘grass’ from weld area up to the maximum beam path distance, or as

recommended by the specification. The body movement of the probe during examination of the weld should be smooth and well controlled with no greater lateral movement than half the crystal width. The probe should be transversed from the limit of scan line through the centre line – moved laterally a half crystal width and transversed back to the limit of scan, this sequence should be followed until the circumference has been internally scanned. This operation should be carried out from both sides.

TRANSVERSE FLAWS

Materials prone to transverse cracking in weld and heat affected zones should be scanned in the appropriate direction again using the techniques leant in the school.

NB: Angle probe irradiation factors are used here.

It will be appreciated that the difficult part of an examination of a weld is the interpretation of the time base presentation.

To state that a rigid code or set pattern of Trace Formation can be followed, would be

extremely misleading. The number of variables are quite considerable, for example the shape of the Trace Pulse from a quartz transducer will vary from that of Barium Titanate. Equipment characteristics will be evident when a known flaw is scanned with two ultrasonic detectors. The variations may be small, but in some cases the difference will be very noticeable.

The choice of probe angle and its frequency, all have an effect on the resolution of the pulse presentation. Good resolution is of great importance, it enables the technician to observe reflecting facets of the flaw displayed on the time base in a well-defined form, without this ability the facets of group porosity for instance would take up a broad trace with insufficient break up of peaks to display the various time intervals, from index point to reflecting facets of the pores.

A signal of high amplitude (signal pulse height) does not necessarily mean that the flaw is a large one and conversely a small display of amplitude may be from a large defect not

presenting face to the angle of ultrasonic propagation, further investigation using other angles would give conclusive results.

From a series of carefully controlled probe movements much information can be gained and used to plot the flaw position within the weld. The welding technique must be known, and the type of flaw that may occur in that technique fully understood. It should also be noted that similar time base presentations can present themselves from different flaws.

Accurate interpretation of the time base presentation is very much a matter of experience, but there are basic probe movements and trace formations that result from certain flaws.

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1. Lack of fusion side wall 2. Porosity & piping

3. Slag spots & linear inclusions 4. Lack of penetration

5. Cracks – longitudinal & transverse

LACK OF FUSION SIDE WALL

The time base presentation for lack of side wall fusion tends to be quite sharp, and providing the probe angle selected is at right angles, only a little movement of the signal along the time base will be observed when the probe scans the flaw region. Some variations of this will be experienced and will depend upon the extent of the lack of side wall fusion.

Detection of this type of defect on thin wall tubing is best carried out from a back stop position using the minimum number of skips. To provide adequate coverage of fusion area, the test should be set up for this type of flaw only, and the probe carefully transversed around the tube using the back stop to maintain a constant weld to index point relationship. Any lack of side wall fusion would present itself at a position on the time base as previously calculated. This signal would tend to be sharp and reasonably high in amplitude. A scan from each side is necessary.

If a natural specimen containing this flaw is available for “Setting up” this would prove to be of immense value.

POROSITY & PIPING

The time base presentation of a single pore is sharp with clean rising edges, the following movements will provide reasonable confirmation.

a. Orbit the probe around the flaw, and observe the time base presentation. If the signal height remains constant, i.e. no reduction in amplitude, this then indicates a spherical reflector.

b. Obtain maximum amplitude and rotate the probe on its axis, reduction of signal height will be rapid.

c. A lateral scan will produce an equal rise and fall of the signal, with little signal width on the time base presentation. As the probe is traversed toward the pore, the leading edge of the beam makes contact, the signal presents itself fairly quickly although the gain to maximum is steady and fall off rapid. The probe movement will depend on the angle and depth of the flaw. The movement of the signal along the time base for this operation will be fairly long, again depending on the probe angle depth.

Much the same conditions exist for a rounded isolated pipe. In the case of group porosity, there are present many reflecting faces at different beam path lengths. The time base presentation is a broad one, and its width depends on the shortest and the longest reflected travel path taken by the Ultrasonic beam. The left and right hand edges are clean but any signals superimpose themselves across the area and the time base presentation has a somewhat jagged appearance across the top.

Orbiting the probe produces much the same information. Rotation of the probe causes a large number of maximum signals to be observed, which rise and fall rapidly.

SLAG SPOTS & LINEAR INCLUSIONS

Slag inclusions present themselves in various ways, for a small inclusion time base presentation similar to that for a small pore could be observed. Slightly larger inclusions produce a signal that is forked, indicating irregularity of the reflecting face.

Orbiting the probe for this type of flaw produces a time base presentation where a number of maximum amplitudes can be observed.

Rotation of the probe produces a slower fall of than for a gas pore.

Linear inclusions maintain a good amplitude when the probe is in lateral movement. If the amplitudes are a number of maximum and minimum, the whole length of the scan (deducing of course beam spread in the horizontal plane at that depth) must be taken as the flaw length. This is necessary owing to the fact that it was not possible to separate the flaws due to beam overlap.

LACK OF PENETRATION

Lack of root penetration being a root condition where unfused edge or edges provide good ultrasonic reflectors. In the case of a butt weld where for various reasons the specified root gap has closed and an unsuccessful attempt has been made to penetrate through the time base presentation in this case is a signal of quite some amplitude, singular and very sharp, with clean edges indicating a constant root condition; the type of signal to be expected from a machined face. Some linearity may be observed when the probe is in lateral movement.

Orbiting the probe in an arc, using the flaw as a central point, produces a time base

presentation where a fairly quick fall off of maximum amplitude occurs both on the right and left hand arc from the right angle probe position. Variations of the above occur and depend on wall thickness and probe angle.

Pipe curvature must also be considered.

Rotation of the probe produces a fairly quick rise and fall and does not indicate a great deal of movement, along the time base.

Traversing the defect produces a very characteristic time base presentation when the leading edge of the ultrasonic beam contacts the defect. A signal is observed to the right hand side of the previously calculated root position. Further traversing of the probe produces the maximum amplitude and this should present itself as near limits to root position on the time base,

depending on the height of the lock of penetration. Further traversing will produce a steady fall off of signal. The significant points to make are that the probe movement will be quite substantial and the signal travel along the time base will have been extensive, just how extensive would be a feature of the height of the flaw, and probe angle.

The observed signal from such an area with complete lack of penetration when traversed is one that is extremely smooth with gradual build up to maximum and gradual fall off and occupies a considerable amount of time base.

CRACKS

Longitudinal and transverse cracks are good ultrasonic reflectors, if we were to observe the face of the crack magnified only several times we wold see multiple facets, in fact a quite jagged saw tooth formation. These are the faces of reflection that present themselves to the ultrasonic beam. Cracks have random orientation, varying height and length, thus, their detection by ultrasonics is relatively simple.

The time base presentation will not always be a simple matter of interpretation, small fine cracks may produce a presentation surprisingly similar to a fine slag inclusion, much again depends on probe angle and orientation of flaw.

In general a crack produces signals of quite high amplitude and because the facets are so close to one another, these reflections superimpose themselves on this signal. Many high intensity blips (half cycles) can be observed both on the left hand rising edge and on the right hand flank. Slight rotational probe movement will cause these high intensity blips to move up and down the signal rapidly.

A crack that has height and various orientations can produce signals at different time intervals, their peaks being ragged.

Orbiting of the flaw may cause loss of signal amplitude, to what extent depends on flaw orientation.

Traversing of the flaw produces the following: when the leading edge makes contact the signal amplitude rises quickly, the amplitude fall off is fairly rapid, time base travel is dependent on flaw orientation, height and probe angle.

Lateral scanning of this type of flaw will probably reveal other facets of reflection at longer or shorter beam path lengths. On the time base presentation these show that the flaw

orientation has changed to some degree, and a number of maximum amplitudes may be observed.

Other signals not placed in the above categories but worthy of mention are as follows:

TWIN PEAKING FROM ARGON ARC ROOT

During the welding process a slight notch effect is formed, in some instances this is quite pronounced, the effect on the time base presentation is to produce twin peaking, with the root signal in the middle of the peaks. Amplitude of signal depends on the degree of notching or shrinkage and the probe angle. A single notch effect may be observed, this will probably be due to weld position and other variables.