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1.6 Fundamental Properties

1.6.1 Basic principle of the sound

This chapter introduces fundamental concepts necessary for mathematically and physically describing the phenomena of wave propagation, which are key to understanding how ultrasonic waves propagate for the ultrasonic testing of various materials.

Materials are made up of many small particles known as atoms. When a force is exerted on an atom, it moves from its rest or equilibrium position (also called displacement) and exerts a force on the adjacent particles or atoms. These adjacent particles are moved from their position, and this continues through a medium. The particles will return to their original position when the force is removed. This can be visualized using Newton’s cradle in the figure below.

Fig1.1- Newton’s Cradle

In metals, the displacement occurs due to their elastic properties. When a force or energy is applied to a metal surface, it moves inward, back to its rest or original position and/or to a maximum distance in the opposite direction. If we plot the movement and vibration against time, it will produce a sine wave or cycle as shown below.

 

Fig1.2- Sine Wave generated by vibration

 

Note: Points “a” represent nodes and are the points of zero sound pressure, while points “b” indicate antinodes, which are the points of maximum amplitude or pressure.

 Frequency (measured in Hertz [Hz]) is the number of cycles or sine waves in a given time, usually one second. A greater number of cycles in a given time represents a higher frequency and vice versa.

Fig1.3- Frequency (2CPS)

 

The word “mechanical wave” (as opposed to electromagnetic waves like light or x-rays) is used to describe the distribution of energy through a medium by transferring energy or force from one particle to the next.

The transmission of mechanical energy (vibrations) through solids, liquids, and gases is known as sound (pressure wave). Sound can be propagated through a range of frequencies (Hz). Humans can hear sounds between 20 Hz and 20,000 Hz.

Ultrasound refers to sound with a frequency above human hearing. It can propagate through most elastic or near-elastic media, such as solid, liquid, or gaseous, but cannot travel through a vacuum. (Sounds below about 16 Hz are called infrasound.)

In ultrasonic testing, mechanical energy in the form of ultrasound is generated from a transducer and causes the displacement of atoms within the specimen.

 A transducer is a device that converts one type of energy to another form. In the case of ultrasonic testing, it converts mechanical energy to electrical or electrical energy to mechanical energy.

The most common type of ultrasonic transducers or probes utilise piezoelectric material. Piezoelectric material is a polarised material that converts electrical energy to mechanical energy (sound) and vice versa.

In the diagram below, it illustrates the piezoelectric effect. The active element has electrodes/wires connected to two of its opposite faces. When electrical energy is applied through these wires to the crystal (piezoelectric material), the particles of the polarised element align themselves with the electric field applied, resulting in induced dipoles within the crystal structure. This alignment of the particles will cause the material to change dimensions (also known as electrostriction). This causes the crystal to expand and contract, forming mechanical vibrations (sound). In addition, the crystal will produce an electric field when the material changes dimensions as a result of an imposed mechanical force or sound (also known as the ferroelectric effect). Therefore, a transducer can both send and receive energy.

The details about the types of crystals will be explained later in this textbook.

Fig1.4- Piezoelectric Crystal

 

1.6.2 Sound Velocity

The speed (rate) at which a sound wave travels through a medium is called velocity or acoustic velocity. Sound can travel through different media (solids, liquids, and gases) at different rates or speeds. The two properties affecting the sound velocity in a material are:

 

Elastic properties are different for different materials. Elastic properties relate to the tendency of a material to maintain its shape and not deform when a force is applied to it. A material such as steel will experience smaller deformation than rubber when a force is applied to it. Steel is a rigid material while rubber deforms easily and is a more flexible material. At the particle level, a rigid material is characterised by atoms and/or molecules with strong forces of attraction for each other. These forces can be thought of as springs that control how quickly the particles return to their original positions. Particles that return to their resting position quickly are ready to move again more quickly, and thus they can vibrate at higher speeds. Therefore, sound can travel faster through media with higher elastic properties (like steel) than it can through solids like rubber, which have lower elastic properties.

The phase of matter has a large impact on the elastic properties of a medium. In general, the bond strength between particles is strongest in solid materials and weakest in the gaseous state. As a result, sound waves travel faster in solids than in liquids, and faster in liquids than in gases. While the density of a medium also affects the speed of sound, the elastic properties have a greater influence on the wave speed.

The density of a medium is the second factor that affects the speed of sound. Density describes the mass of a substance per volume. A substance that is more dense per volume has more mass per volume. Usually, larger particles have more mass. If a material is more dense because its particles are larger, it will transmit sound slower. Sound waves are made up of kinetic energy. It takes more energy to make large particles vibrate than it does to make smaller particles vibrate. Thus, sound will travel at a slower rate in the more dense object if they have the same elastic properties. If sound waves were passed through two materials with approximately the same elastic properties, such as aluminium (10 psi) and gold (10.8 psi), sound will travel about twice as fast in aluminium (6.3 km/s) than in gold (3.2 km/s). This is because aluminium has a density of 2.7 grams per cubic cm, which is less than the density of gold, which is about 19 grams per cubic cm. The elastic properties usually have a larger effect than the density, so it is important to consider both material properties.

The wave speed remains constant in a particular material and will not change because the particle density and elasticity are fixed for a given material.

Another point to consider is the density of different media with regard to how tightly and closely the particles are packed. In general, more dense (closely packed particles) materials have a relatively higher sound wave velocity than less dense materials. The sound velocity will change only as it moves from one medium to another.

It will take longer for sound to travel through gas (air) than through liquid (water) and longer through liquid (water) than through solid (steel). Air, in general, is a poor transmitter of ultrasound because its particle density is so low that it is difficult for energy to transfer from particle to particle. That is why we apply the couplant, such as oil, between the probe and the specimen.

Temperature is also a condition that affects the speed of sound to a degree. For example, steel shows slight changes in velocity when the temperature rises above 60°C.

 

1.6.3 Mode of Sound Vibration

Sound waves propagate due to the vibrations or oscillatory motions of particles within a material. In air, sound travels by the compression and rarefaction of air molecules in the direction of travel. However, in solids, molecules can support vibrations in other directions. Hence, a number of different types of sound waves are possible. The propagation of waves is often described in terms of what are called “wave modes.”

 The most common types of sound wave are:

 

Longitudinal Wave

Longitudinal waves also called compression waves and pressure waves. In longitudinal waves the particle motion (displacement of the medium) is in the same direction as the wave travels as each particle moves, it pushes or pulls the adjacent particle through elastic interconnection (back and forth motion). Gases, liquids, and solids have elasticity, so compressional waves can travel in all of them.

The speed of sound (velocity) is fastes for longitudinal wave as compared with other types of wave used in ultrasonic testing.

 

Fig1.5- Longitudinal Wave (Particle Motion)

 

The velocity of longitudinal wave is calculated using the following equation:

Where,

VL= Longitudinal wave velocity

E= Young’s module of elasticity

p= Density of material

µ= Poisson’s ratio

 

Shear Wave

In the shear mode, molecules vibrate up and down, perpendicular to the direction of propagation. For this reason, the Shear mode is also called the transverse wave, as particle motion is transverse to the direction of sound energy. In the shear or transverse mode, particles of a solid move like beach balls floating on the surface of the sea – they move up and down as a wave passes.

This type of sound wave can only propagate when the molecules are joined together in a solid. Solids have both rigidity and elasticity, whereas air and water, like other gases and liquids, lack rigidity. Shear or transverse waves cannot travel through gases or liquids due to this reason.

The velocity of sound (speed) in the shear wave is lower than it is in longitudinal mode by approximately half the velocity of the longitudinal wave.

 

Fig1.6- Shear Wave (Particle Motion)

 

The velocity of shear wave is calculated using the following equation:

Where,

VS= Shear wave velocity

E= Young’s module of elasticity

p= Density of material

µ= Poisson’s ratio

 

Surface Wave

Waves travel across the surface of relatively thick solid materials, penetrating to a depth of one wavelength. Surface waves combine both longitudinal and transverse motion, resulting in an elliptical motion. These waves are also known as Rayleigh waves, and they can only propagate on the surface of solids.

 The velocity of sound in surface waves is about 90% (or nine-tenths) of the velocity of shear waves.

The velocity of surface wave is calculated using the following equation:

Where,

VR= Surface wave velocity

E= Young’s module of elasticity

p= Density of material

µ= Poisson’s ratio

 

Fig1.7- Surface Wave (Particle Motion)

 

Surface waves have the ability to follow the contour of surfaces, including curved surfaces, as long as there are no sharp points or changes on its way. It should be noted that surface waves can be easily damped and absorbed by excess couplant or by touching the surface of the part ahead of the probe, hence the term “finger damping technique.” This method is useful for confirming the location of a discontinuity found on the test surface. Care must be taken to interpret the signal correctly because surface waves will respond to any dirt, grease, or contamination on the test surface.

 

Plate Wave

Lamb or plate waves are similar to surface waves, except they can only propagate in thin plate materials when the plate thickness is about the same as half the wavelength. The propagation of Lamb waves depends on the density and elastic material properties of a component and is also greatly influenced by the test frequency, incident angle, and material thickness.

Lamb or plate waves travel at velocities that vary with the plate thickness and the wavelength. Particle motion is elliptical, as with surface waves.

 

 

Plate waves are generated by using longitudinal or shear waves, which develop either symmetrical or asymmetrical waves.

 

Fig1.8- Plate Wave (Asymmetrical Wave)

 

Fig1.9- Plate Wave (Symmetrical Wave)

 

Plate waves occupy the entire thickness of the part, and without saturating the part, the wave cannot exist. Lamb waves are useful in detecting discontinuities in thin sheet materials and tubular products.

 

Resonance or Standing Wave

Resonance wave techniques were previously used for thickness measurement and bond or lamination inspection but are rarely used nowadays. A standing wave can be defined as the characteristic of a vibrating body to resonate or vibrate sympathetically with a vibration source. A resonant condition will exist anytime a continuous longitudinal wave is introduced into a specimen and reflected in phase with the incoming wave. Resonance will occur only when the thickness of a specimen is equal to a half wavelength or an exact multiple of a half wavelength. This is also called harmonics.

The fundamental resonant frequency is the lowest frequency at which a specimen will resonate, and harmonics are exact multiples of the fundamental (minimum) resonant frequency. The minimum resonant frequency can be found using the following equation:

Where,

F= Fundamental resonant frequency

V= Velocity of longitudinal wave

T= Thickness of material

Fig1.10-Resonance or Standing Wave Propagation

 

1.6.4 Frequency (Hz)

Earlier in this section, we discussed vibrations and displacement, and when we plot the movement and vibration against time, it produces a sine wave or cycle, as shown below.

Fig1.11-2Hz Frequency

 

Frequency, measured in Hertz (Hz), is the number of cycles or sine waves in a given time, usually 1 second. As sound is a series of vibrations, one method to measure frequency is to count the number of vibrations/cycles per second (CPS). The frequency is measured in hertz, where one cycle per second is equal to 1 Hz (1 CPS = 1 Hz). In non-destructive testing, high frequencies are often used, so they are usually measured in Megahertz (MHz).

1,000Hz = 1kHz

1,000,000 = 1 MHz

The dominant frequency of an ultrasonic transducer is fixed for a given probe. However, the frequency can be adjusted by changing the thickness of the crystal. Thicker crystal elements generate lower frequency ultrasound, while thinner crystal elements generate higher frequency. The dominant frequency of an ultrasonic transducer also depends on the velocity of the crystal, in addition to its thickness.

There are advantages and limitations to both high and low frequency probes, and a Level 2 operator must be aware of these in order to select the correct frequency for the inspection application. The summary effects of low and high frequency will be discussed later in this textbook.

 

1.6.5 Wavelength (Lambda [λ])

A wavelength is considered to be the distance between two successive displacements (cycles). It can also be defined as the distance a wave travels during one complete cycle, which is the distance from one point of a sine wave to the corresponding point of the next sine wave as shown in the figure below.

Fig1.12- Wavelength (mm)

 

The symbol λ is used to represent a wavelength and is called Lambda. The wavelength is typically measured in millimetres (mm).

The wavelength of a sound wave varies with two factors:

If we know the velocity and frequency of a sound wave, we can calculate its wavelength using equation 1.

Equation 1

Where;

λ = Wavelength (mm)

V= Velocity (km/s)

f=Frequency (MHz)

An easy way to remember how equation 1 works is to break it down into its components using a triangle. This triangle consists of three parts that represent the velocity, frequency, and wavelength of a sound wave, and shows how they are related.

If we want to calculate the wavelength using equation 1, we can rearrange the equation by covering up the λ symbol. This leaves us with the formula V/f, which represents the velocity of the sound wave divided by its frequency. Similarly, if we need to find the velocity or the frequency, we can cover up the corresponding variable in the equation. For example, covering up the V symbol gives us the formula f x λ, while covering up the f symbol gives us V/λ.

Effect of changing frequency in wavelength:

 Example 1: Let’s work through an example problem to calculate the wavelength of a sound wave in a material with a velocity of 5.9 km/s, using a 5 MHz probe. To do this, we can use equation 1:

Plugging in the given values, we get:

Therefore, the wavelength of the sound wave in this material is approximately 1.18 mm.

 

Example 2: Let’s consider another example problem to calculate the wavelength of a sound wave in a material with a velocity of 5.9 km/s, using a 10 MHz probe.

Plugging in the given values, we get:

Therefore, the wavelength of the sound wave in this material is approximately 0.59 mm.

By comparing the results of the two example problems we just solved, we can see that the wavelength of a sound wave in a given material (with a fixed velocity) is inversely proportional to its frequency. That is, as the frequency of the sound wave increases, its wavelength decreases, and vice versa. This relationship is often referred to as the ‘opposite effect’ because changes in one variable have an opposite effect on the other variable.

This relationship between wavelength and frequency is important because it affects many aspects of sound wave behavior, such as how the wave interacts with different materials or how it propagates through different media.

We can visualize the relationship between frequency and wavelength using a sine wave diagram. As the frequency of the wave increases, the wavelength decreases and the wave appears more compressed. Conversely, as the frequency decreases, the wavelength increases and the wave appears more stretched out. This can be seen in the following figures, which show the same sine wave at different frequencies and wavelengths:

As you can see, the wavelength and frequency of the wave are inversely proportional and affect the overall shape and behavior of the wave. This visual representation can help us better understand how sound waves propagate through different media and how they interact with different objects.

Effect of using same probe frequency on different materials with different acoustic velocity:

 

Example 3: Let’s say we want to calculate the wavelength of a 5 MHz probe in a material with an acoustic velocity of 5.9 km/s. Using the formula λ=V/f, we can determine the wavelength as follows:

Plugging in the given values, we get:

Therefore, we can see that the wavelength of a 5 MHz probe in this material is approximately 1.18 mm. This information can be useful in selecting the appropriate probe frequency for a specific application, as it determines factors such as the sensitivity, resolution , attenuation and penetration depth of the sound wave.

 

Example 4: Suppose we want to calculate the wavelength of a 5 MHz probe in a material with an acoustic velocity of 4.5 km/s. Using the formula λ=V/f, we can determine the wavelength as follows:

Plugging in the given values, we get:

 

Therefore, we can see that the wavelength of a 5 MHz probe in this material is approximately 0.9 mm.

From examples 3 and 4, it is evident that the wavelength decreases as the velocity decreases and increases if the velocity increases (directly proportional). It is important to remember that the velocity is fixed in a given material, and in the above examples, a change in velocity means a change in material, such as from steel to copper.

It is also important to note that the velocity does not change when the frequency changes.

The wavelength of ultrasonic waves is crucial because a change in wavelength can affect other important factors such as sensitivity, attenuation, penetration, and resolution. Some of these factors will be discussed in this section, while the remaining will be covered in the next sections of this textbook.

 

1.6.6 Pulse Length

Pulse length (or width) is sometimes referred to as wave train length. It is a measurement of pulse duration expressed in time, usually in microseconds (µs). The leading and trailing edges of a pulse are measured at a defined level below the peak amplitude, typically 10%.

For example, when you strike a bell, the metal continues to vibrate for several seconds, causing the sound to decay gradually. As the vibrations weaken, the sound eventually fades away. However, if you place your hand on the bell, the vibrations stop abruptly, causing the sound to die away quickly. In other words, you dampen the sound.

Pulse length can be controlled electronically by adjusting the damping value in a UT set. This value controls the duration of the pulse applied to a crystal by pulser circuits. Pulse length can also be controlled mechanically by mounting damping material behind the crystal and frequency. Of the two methods, mechanical damping is considered more efficient.

Backing or damping material is often used in probes to reduce ringing time and shorten the pulse length. In most probes, a slug of tungsten-loaded Araldite is placed behind the crystal for this purpose. It is important to note that pulse length, duration, and width are interchangeable terms, but should not be confused with wavelength.

Fig1.14- Low Damping (Top), Heavy Damping (Bottom)

 

Ultrasound waves with longer pulse length contain more energy, making them more penetrating. However, longer pulse length can result in reduced sensitivity and resolution, which creates a need for compromise between these factors.

 

Probe Bandwidth 

The range and band of frequencies that make up an ultrasonic pulse is called the bandwidth. A probe does not generate only one frequency; it generates a band of frequencies centered around its dominant frequency. The frequency indicated on a transducer is the central or center frequency, which primarily depends on the backing material.

Highly damped ultrasonic transducers respond to frequencies both above and below the central frequency. This broad frequency range provides the transducer with high resolving power. However, less damped transducers have a narrower frequency range and lower resolving power but greater penetration.

The central frequency also determines the capabilities of a transducer. Lower frequencies (0.5 MHz – 2.25 MHz) provide greater energy and penetration in materials, while high-frequency crystals (15.0 MHz – 25.0 MHz) provide reduced penetration but greater sensitivity to small discontinuities. High-frequency transducers, when used with the proper instrumentation, can dramatically improve flaw resolution and thickness measurement capabilities. Broadband transducers with frequencies up to 150 MHz are commercially available.

Mechanical damping using a damping material behind the crystals or elements is the most efficient way to control the bandwidth of a probe. Probes can be classified based on their relative bandwidth, including:

 

Narrow Bandwidth, low damping (Long Pulse)

To generate a long pulse over a very narrow band of frequencies, lower damping (typically between 15 – 30% or high Q Factor) is used. The characteristics of using a long pulse probe are:

Fig1.15- Narrow Bandwidth (Long Pulse)

 

Medium Bandwidth

Typically, damping of 31-75% is used to achieve a bandwidth for good general-purpose probes. These probes have:

 

Broad Bandwidth (Short Pulse)

Higher damping around 76-100% (Low Q Factor) results in broad bandwidth or a shorter pulse over a wide range of frequencies. This is a common type of bandwidth used in ultrasonic testing probes due to good resolution and flaw detection. The shorter pulse provides higher resolution (both axial and near the surface) and shorter dead zone.

Fig1.16- Broad Bandwidth (Short Pulse)

 

1.6.7 Pulse Energy (Voltage)

The pulser circuits apply voltage from 100V to 400V to the transducer to increase the amount of sound energy applied to the test specimen. Higher voltage maximises penetration, while lower voltages improve near surface resolution and conserve battery life.

 

1.6.8 Signal Amplitude

The amount of returning sound energy received by the receiver circuit is called signal amplitude. In wave motion, amplitude refers to the maximum displacement of material particles, while in electronics it refers to the magnitude of a signal expressed as a positive or negative voltage.

Ultrasonic flaw detectors are equipped with an amplifier that is fitted between the receiver circuit and the display. The amplifier increases (amplifies) the strength of the signal/voltage received on the screen, making it easier to detect, interpret, or measure the indications received.

Gain (dB) determines the amount of amplification of the signals from the material. The gain of an amplifier can be adjusted by changing the dB value in the ultrasonic system.

Fig1.18- block diagram of ultrasonic testing pulse echo

 

1.6.9 Sensitivity

In ultrasonic testing, sensitivity refers to the ability to detect the smallest detectable discontinuity, which is typically half the wavelength (1/2 λ).

 

Example: What is the minimum detectable discontinuity that you can find with a 5MHz probe in a material with a velocity of 6.3km/s?

Answer:

The smallest detectable discontinuity in the above example is 0.63mm.

Sensitivity is directly related to the wavelength, which means that the same factors affecting the wavelength will also affect the sensitivity. For example, a higher probe frequency decreases the wavelength, enabling the detection of smaller discontinuities (i.e., higher sensitivity). If we move the same probe to another material with lower velocity, the wavelength becomes shorter, thus increasing the sensitivity in that particular material.

 

1.6.10 Attenuation

Attenuation refers to the loss, damping or decreasing of ultrasound energy as it travels through a material. When sound travels through a material, its intensity or pressure diminishes with distance or when there is an air gap between the transducer and the test specimen. Sound can also lose its energy due to mode conversion. For example, when a sound wave hits an interface between two materials with different velocities at an angle other than 90°, longitudinal wave may be converted to shear wave.

However, there are other components that may weaken a sound wave. The two main components are absorption and scattering.

 

Absorption is the component of the attenuation resulting from the conversion of ultrasonic energy into other forms of energy, such as thermal or heat. It occurs as the sound pulse hits the molecules of the test material and makes them vibrate. The energy lost in vibrating the molecules turns into heat. The rate of absorption varies from one material to another and even from one type of steel to another. It is very high in perspex, nylon and lead and is low in aluminium.

 

Scatter is the random reflection of the wave in directions other than its original direction of propagation and is usually caused by various factors, including:

Microstructure of test specimen (Grain Structure)

The grain structure of a test specimen has a significant impact on its acoustic properties and attenuation. Attenuation can occur when sound energy is reflected from grains in the test material. The larger the grains, the larger the boundary, and the more scatter occurs. The random signals or noise at the bottom of the CRT (cathode Ray Tube) or display screen are caused by reflections from grain boundaries in the test material (attenuation), and this is referred to as “grass” or “hash”. More grass is seen from cast iron or brass than from small-grained materials like refined steel.

A steel forging usually has a fine grain structure and a low damping effect (low attenuation) on the sound beam. However, a casting product generally has a coarser grain structure, which makes it more difficult to pass sound through and more attenuating. For materials like cast iron, the carbon graphite at the grain boundaries causes high damping of ultrasound and high attenuation.

The longer the wavelength of a wave (lower frequency), the less energy is scattered. When the wavelength is smaller than the grain size, a sound wave is scattered very quickly. Therefore, a low-frequency probe with its longer wavelength is used to test such components, usually at the cost of decreased sensitivity to discontinuities and resolution.

Large (Coarse) grain structure – High attenuation

Fine grain structure – minimum to no attenuation

 

Size and shape of the discontinuity

When ultrasonic waves pass through common polycrystalline elastic engineering materials (that are generally homogeneous but contain evenly distributed scatterers, e.g., gas pores and segregated inclusions), the waves are partially reflected at each discontinuity, and the energy is said to be scattered in many different directions.

Fig1.19- Poor Sound Reflection (Top), High Reflection (Bottom)

The shape and surface condition of a discontinuity also influence the amount of scattering or returning sound energy displayed on the screen. Volumetric discontinuities are generally more of a concern than linear or planar types. A discontinuity with a rough surface will tend to scatter the reflection more than a smooth flaw. For example, non-metallic inclusions are typically rough and would scatter the sound more than a crack-like discontinuity.

 

Orientation of a discontinuity

Discontinuities are best detected when their major plane is at a right angle to the sound wave generated. When a discontinuity is not normal (at 90 degrees) to the incident wave, the reflected wave will be at an angle. The result is a reduction in the returning sound energy (amplitude) of the discontinuity displayed on the screen.

Angular flaws, such as weld discontinuities, are best detected using an angle probe for contact testing or by tilting the probe to achieve the necessary directionality in immersion testing.

Fig1.20- Reduction in the returning sound energy

 

Geometry of the component

A good back surface reflection indicates a good response from the material being tested. If the back surface is not parallel to the entry surface or is curved, the reflected energy will be directed away from the transducer, similar to light falling on a mirror at an angle, and may cause partial or total loss of sound energy.

The physical shape or contour of a part must be considered when attempting to discern whether a discontinuity indication is true or non-relevant.

Fig1.21- Reduction in the returning sound energy due to the geometry

 

When testing a long and/or narrow component, non-relevant indications may be produced due to the reflection of the spreading beam when it hits the sides of the component. The mode-converted shear wave signal may be generated at a steep angle to the opposite side and will appear on the right side of the first backwall echo.

Fig1.22- Reduction in the returning sound energy due to the geometry

 

These are the reasons why probes with smaller beam spreads, twin crystals, or angle probes are used to inspect such components.

 

Rough front (entry) or backwall surfaces

The entry surface or back of the test specimen can greatly affect ultrasonic wave propagation. Rough surfaces can cause undesirable effects such as a reduction in discontinuity and back surface echo due to the distortion of wave directivity. If the entry surface is rough, sound can scatter and be directed away from the transducer. This effect can be reduced by applying more couplant on the surface and using a lower frequency probe.

The rough backwall of the test specimen can also have the same effect as the rough entry surface. Internal uneven general corrosion acts in the same way as a rough surface, resulting in the sound being reflected in an unfavorable direction and causing partial or total loss of ultrasound. The use of a twin crystal probe in this case may reduce the scattering effect.

Fig1.23- Effect of rough front and back surface on ultrasound

 

1.6.11 Penetration

The ability of ultrasound waves to travel deep into a material is called penetration. It is related to the factors that affect attenuation. The higher the amount of attenuation, the lower the penetration will be in the material. A low-frequency probe, with its longer wavelength and pulse length, has greater penetration in a given material than a high-frequency probe.

 

Fig1.24- Low frequency, long wavelength, long pulse length probe provides better (higher) penetration

 

Fig1.25- High frequency, short wavelength, short pulse length probe provides poor (lower) penetration

 

1.6.12 Resolution

Resolution or resolving power is the ability of an equipment/probe combination to separate (distinguish between) the two or more echoes from reflectors that are closed together in depth or time. Near surface resolution refers to the ability to distinguish a discontinuity close to a surface or boundary (initial pulse or dead zone). For example ability to resolve a discontinuity close to the backwall.

To have good resolution, a probe must present two signals on a screen from two separate reflectors. If it has poor resolution, the echoes from the two reflectors appear as one signal on the screen. In the early days of ultrasonic testing, 100mm, 91mm, and 85mm steps were used at the radius end of the V1 block to test resolving power. However, today this is regarded as too crude a test, and AS 2083 recommends that we should be able to recognise two discrete echoes less than two wavelengths apart. By discrete echoes, they mean signals split by more than 6dB or to more than half the total height of the signals.

A high-frequency probe, with its shorter wavelength and pulse length, has greater resolution. Resolving power is poor in single crystal probes due to the presence of a dead zone. Near-surface resolution can be improved by using delay line or twin crystal probes.

 

Fig1.26- High frequency, short wavelength, short pulse length separating two discontinuities closed to each other at the depth of 10mm & 11mm – High Resolution or Resolving Power

 

Fig1.27- Low frequency, long wavelength, long pulse length not able to distinguish the signal from two discontinuities closed to each other at the depth of 10mm & 11mm – Poor Resolution or Resolving Power