Ultrasonic Testing Level 1 Training

In this lecture, we go back in time into the history of Non Destructive Testing so you'll be able to know how the ultrasound appeared for the first time and the pioneer of this field.

There are many different Non Destructive Testing methods used in the industry, but each one of these techniques has it's pros  and counts. In this lecture I present a detailed comparison between those so you'll be able to know when to use each of them and when not to.

Ultrasonic Testing is based on particle's vibration which occurs when applying an electrical energy to the piezoelectric element that converts it into a mechanical energy that causes the particles to move. In this video we will see that in action to better understand the ultrasonic vibration.

The Frequency, Wavelength and Velocity are linked together in such a manner that any change of those three parameters may affect the others. Because all waves move through a vacuum at the same speed, the number of wave crests passing by a given point in one second depends on the wavelength.  The equation that relates wavelength and frequency for waves is: λf=v where λ is the wavelength, f is the frequency and v is the velocity.

In  this Lecture we're going to explain the formula that links those parameters together, so you'll know how changing one of them will affect the others.

Wave diffraction is a general phenomenon in ultrasonics. The sharp tip of a well defined internal defect like a crack will diffract an incident ultrasonic beam, creating a spherical wave front whose arrival at the probe can be used to locate the tip and measure the depth of the crack. Common angle beam transducers are used for this test. Higher transducer frequencies produce the strongest diffraction signals.

In this lecture we take a deeper look on the diffraction theory and how it's manipulated in ultrasonic testing for some of the techniques that are relying completely on it like the Time of Flight Diffraction (ToFD)

All material substances are comprised of atoms, which may be forced into vibrational motion about their equilibrium positions. Many different patterns of vibrational motion exist at the atomic level; however, most are irrelevant to acoustics and ultrasonic testing. Acoustics is focused on particles that contain many atoms that move in harmony to produce a mechanical wave. When a material is not stressed in tension or compression beyond its elastic limit, its individual particles perform elastic oscillations. When the particles of a medium are displaced from their equilibrium positions, internal restoration forces arise. These elastic restoring forces between particles, combined with inertia of the particles, lead to the oscillatory motions of the medium.

In solids, sound waves can propagate in four principal modes that are based on the way the particles oscillate. Sound can propagate as longitudinal waves, shear waves, surface waves, and in thin materials as plate waves. Longitudinal and shear waves are the two modes of propagation most widely used in ultrasonic testing.

Certain single crystal materials exhibit the following phenomenon: when the crystal is mechanically strained, or when the crystal is deformed by the application of an external stress, electric charges appear on certain of the crystal surfaces; and when the direction of the strain reverses, the polarity of the electric charge is reversed. This is called the direct piezoelectric effect, and the crystals that exhibit it are classed as piezoelectric crystals.

Conversely, when a piezoelectric crystal is placed in an electric field, or when charges are applied by external means to its faces, the crystal exhibits strain, i.e. the dimensions of the crystal change. When the direction of the applied electric field is reversed, the direction of the resulting strain is reversed. This is called the converse piezoelectric effect.

An ultrasonic search unit consists of an appropriate housing with electrical connector, backing material for dampening, active piezoelectric material, lens for acoustic focusing, or time delay material if required. In addition, a thin coupling layer, protective layer, or wear plate may be used ahead of the active element.

The effectiveness of the search unit for a particular application depends on Q, bandwidth, frequency, sensitivity, acoustic impedance, and resolving power.

The I.I.W. VI block is used to determine the resolution of a flaw detector using a normal beam probe. This block has three target reflectors at ranges of 85 mm, 91 mm and 100 mm. The probe is placed on the block as shown in Figure 13a and echoes from the three reflectors are obtained. The separation of the echoes from each other indicates the degree of resolution of the flaw detector for that particular probe.

Another block (described in B.S. 3923: Part 3: 1972) used for the determination of resolution of flaw detectors using either normal beam or angle beam probes is shown in Figure 5.26 of the Training Handbook. With this block the resolution is determined by the minimum distance apart that flaws can be indicated clearly and separately. In use the probe is placed on the center line of the block over the change in radius from one step to the next. Its position is adjusted so that echoes from the two radii are of the same height and approximately 1/2 full screen height. The steps are said to be resolved when their echoes are clearly separated at half maximum echo height or less.

The speed of sound is the distance traveled per unit of time by a sound wave propagating through an elastic medium. In dry air at 20 °C (68 °F), the speed of sound is 343.2 meters per second (1,126 ft/s). This is 1,236 kilometers per hour (667 kn; 768 mph), or about a kilometer in three seconds or a mile in about five seconds.

In fluid dynamics, the speed of sound in a fluid medium (gas or liquid) is used as a relative measure of speed itself. The speed of an object divided by the speed of sound in the fluid is called the Mach number. Objects moving at speeds greater than Mach1 are traveling at supersonic speeds.

The speed of sound in an ideal gas is independent of frequency, but does vary slightly with frequency in a real gas. It is proportional to the square root of the absolute temperature, but is independent of pressure or density for a given ideal gas. Sound speed in air varies slightly with pressure only because air is not quite an ideal gas. Although (in the case of gases only) the speed of sound is expressed in terms of a ratio of both density and pressure, these quantities cancel in ideal gases at any given temperature, composition, and heat capacity. This leads to a velocity formula for ideal gases which includes only the latter independent variables.

When sound travels through a medium, its intensity diminishes with distance. In idealized materials, sound pressure (signal amplitude) is only reduced by the spreading of the wave. Natural materials, however, all produce an effect which further weakens the sound. This further weakening results from scattering and absorption. Scattering is the reflection of the sound in directions other than its original direction of propagation. Absorption is the conversion of the sound energy to other forms of energy. The combined effect of scattering and absorption is called attenuation. Ultrasonic attenuation is the decay rate of the wave as it propagates through material.

Attenuation of sound within a material itself is often not of intrinsic interest. However, natural properties and loading conditions can be related to attenuation. Attenuation often serves as a measurement tool that leads to the formation of theories to explain physical or chemical phenomenon that decreases the ultrasonic intensity.

The acoustic impedance Z is the ratio of sound pressure (measured in Pa) to volume flow (measured in cubic meters per second). Let's consider a duct or pipe with cross sectional area A. If the wavelength of sound is large compared to the lateral dimensions of the duct, a plane wave will propagate along the duct. Let's suppose for the moment that there are no reflections coming back from the other end of the duct. In that rather special case, we have a one dimensional (plane) wave travelling to the right, with pressure p and particle velocity u in phase, as we saw in the Sound wave equation.

The Dead Zone is a zone where it is not possible to detect defects. Due to imperfect damping of the crystals some waves will interfere with the returning waves. This problem can be overcome by using twin crystals, one transmitting, and one receiving. The higher the probe frequency the shorter the Dead Zone.

The Near Field is an area of 'turbulence' and varying sound intensity. Due to the effect of interference in the near field the signal height from the same size of defect may increase when it is positioned further away from the crystal. Similarly, small defects may be completely overlooked.In the Far Field the beam diverges and the signal height from the same size of defect decreases in relation to the distance in accordance with the inverse square law.

It can be seen from the formula, (Figure 21 of the Training Handbook) that by increasing the probe diameter or increasing the frequency (shorter wavelength), the solid angle of the beam will decrease.

It is convenient to define the beam 'edge' as the point, across the beam, where the intensity of sound has fallen to one half, or sometimes one tenth of the intensity at the center of the beam. Whenever possible we use the Far Field in ultrasonic testing, the near field usually being accommodated within the Perspex shoe of the probe.

Most transducers use a piezoelectric element. When piezoelectric ceramics were introduced, they soon became the dominant material for transducers due to their properties and their ease of manufacture into a variety of shapes and sizes. The first piezo-ceramic in general use was barium titanate and that was followed during the 1960s by lead zirconate titanate (Pb(Zr,Ti)O3, PZT) compositions, which is now the most commonly employed ceramic for making transducers.

For ferroelectric materials the piezoelectric effect takes place only below the Curie temperature, for barium titanate it is 110 °C, for PZT 320 °C.

To compare different piezoelectric materials, it is popular to compare everything to quartz. A basic conflict is given through the fact that for the maximization of the transmission other crystal properties are relevant than for the maximization of the receiving properties. Sometimes it can be of use to choose one material for the transmitter and another material for the receiver.

The choice of the transducer material has also to include a consideration of the load (immersion medium, wedge or delay line material.

If an acoustic wave meets an interface of two materials with different impedances (impedance mismatch) a part of the energy is reflected while the other part is transmitted. For perpendicular incidence the reflection coefficient R and transmission coefficient T in terms of pressure are defined as ?=(Zt−Zi)/(Zt+Zi) ; ?=(2∗Zt)/(Zt+Zi)

Zi and Zt are the acoustic impedance for the incident and the transmitting material, respectively. Clearly, the transmission coefficient is always positive, the reflection coefficient, however, can be positive or negative. A change of sign corresponds to a phase change of the reflected wave.

Refraction is a change of beam direction at a boundary two media in which ultrasound travels at different velocities. It is caused by a change of wavelength as the ultrasound crosses from the first medium to the second while the frequency remains unchanged. We recall that:

Velocity= frequency x wavelength

Therefore. When velocity changes but frequency remains the same, the wavelength must undergo change.

When an ultrasonic wave obliquely impinges on an interface between two media as shown in Fig.3, several things happen depending on the incident angle of the wave as well as the material properties of the two media. One of important things is refraction in which a transmitted wave has a different angle from the incident. The refraction is basically caused by the velocity difference on either side of the interface. The refraction angle can be calculated from Snell’s law if the velocities of the two media and the incidence angle are known.

Snell's law (also known as the Snell–Descartes law and the law of refraction) is a formula used to describe the relationship between the angles of incidence and refraction, when referring to light or other waves passing through a boundary between two different isotropic media, such as water, glass and air.

In optics, the law is used in ray tracing to compute the angles of incidence or refraction, and in experimental optics to find the refractive index of a material. The law is also satisfied in meta materials, which allow light to be bent "backward" at a negative angle of refraction with a negative refractive index.

When the ultrasonic wave passes from one medium (material) to another it changes speed. This is because the speed of a wave is determined by the medium through which it is passing. When the wave speeds up as it passes from one material to another, the angle of refraction is bigger than the angle of incidence.

For example, this happens when the wave passes from water to aluminum or from glass to water.

First Critical Angle

Before the angle of incidence reaches the first critical angle, both longitudinal and shear waves exist in the part being inspected. The first critical angle is said to have been reached when the longitudinal wave no longer exists within the part, that is, when the longitudinal wave is refracted to greater or equal than 90°, leaving only a shear wave remaining in the part.

Second Critical Angle

The second critical angle occurs when the angle of incidence is at such an angle that the remain shear wave within the part is refracted out of the part. At this angle, when the refracted shear wave is at 90° a surface wave is created on the part surface.

Beam angles should always be plotted using the appropriate industry standard, however, knowing the effect of velocity and angle on refraction will always benefit an NDT technician when working with angle inspection or the immersion technique.

Through-transmission ultrasonic testing (UT) is used for detection, verification, sizing, and growth rate monitoring of cracks in piping, vessels, cylindrical shapes, and sometimes non-cylindrical shapes. Through-transmission UT is a two transducer technique in a pitch-catch arrangement. While there are many types of UT techniques, because of the wide variety of component shapes, sizes, and orientations it is sometimes valuable to have an alternative technique for verification, such as through-transmission.

Pulse Echo Method: Sound pressure on axis (schematic) for the incident wave (top) and the wave reflected from a reflector in form a circular disc (bottom).

Rough ultrasonic velocity measurements are as simple as measuring the time it takes for a pulse of ultrasound to travel from one transducer to another (pitch-catch) or return to the same transducer (pulse-echo). Another method is to compare the phase of the detected sound wave with a reference signal: slight changes in the transducer separation are seen as slight phase changes, from which the sound velocity can be calculated. These methods are suitable for estimating acoustic velocity to about 1 part in 100. Standard practice for measuring velocity in materials is detailed in ASTM E494.

The angle beam technique is used to transmit ultrasonic waves in to a test specimen at a predetermined angle to the test surface. According to the angle selected, the wave modes produced in the test specimen may be mixed longitudinal and transverse, transverse only, or surface wave modes. Usually, transverse wave probes are used in angle beam testing. Transverse waves at various angles of refraction between 35° and 80° are used to locate defects whose orientation is not suitable for detection by normal beam techniques.

The basic electronic instrument used in pulsed ultrasonic testing contains a source of voltage spikes (to activate the sound source-that is, the pulser) and a display mechanism that permits interpretation of received ultrasonic acoustic impulses. Figure 89 shows a block diagram of the basic unit. The display can be as simple as a digital meter for a thickness gage or a multidimensional representation of signals over an extended area of interest.

The basic electronic instrument used in pulsed ultrasonic testing contains a source of voltage spikes (to activate the sound source-that is, the pulser) and a display mechanism that permits interpretation of received ultrasonic acoustic impulses. Figure 89 shows a block diagram of the basic unit. The display can be as simple as a digital meter for a thickness gage or a multidimensional representation of signals over an extended area of interest.

Scientists and engineers have developed a new family of ultrasonic testing systems based on miniaturized electronic modules. In combination with a modular designed software architecture and suitable PC hardware, these new products permit custom configurations for a wide range of client-specific applications, from simple PC-aided manual ultrasonic inspections through fully automated inspections using compact portable systems with up to four channels or very sophisticated multi-channel systems for the ultrasonic inspection of heavy components. Highly integrated electronic circuits and the powerful processing capabilities of today’s PC systems allow the integration of electronic components even for multi-channel systems into portable computers, thus providing compact and simple to operate instruments to the ultrasonic inspector in the field.

Calibration refers to the act of evaluating and adjusting the precision and accuracy of measurement equipment. In ultrasonic testing, several forms of calibration must occur. First, the electronics of the equipment must be calibrated to ensure that they are performing as designed. This operation is usually performed by the equipment manufacturer and will not be discussed further in this material. It is also usually necessary for the operator to perform a "user calibration" of the equipment. This user calibration is necessary because most ultrasonic equipment can be reconfigured for use in a large variety of applications. The user must "calibrate" the system, which includes the equipment settings, the transducer, and the test setup, to validate that the desired level of precision and accuracy are achieved. The term calibration standard is usually only used when an absolute value is measured and in many cases, the standards are traceable back to standards at the National Institute for Standards and Technology.

Calibration refers to the act of evaluating and adjusting the precision and accuracy of measurement equipment. In ultrasonic testing, several forms of calibration must occur. First, the electronics of the equipment must be calibrated to ensure that they are performing as designed. This operation is usually performed by the equipment manufacturer and will not be discussed further in this material. It is also usually necessary for the operator to perform a "user calibration" of the equipment. This user calibration is necessary because most ultrasonic equipment can be reconfigured for use in a large variety of applications. The user must "calibrate" the system, which includes the equipment settings, the transducer, and the test setup, to validate that the desired level of precision and accuracy are achieved. The term calibration standard is usually only used when an absolute value is measured and in many cases, the standards are traceable back to standards at the National Institute for Standards and Technology.

It is clear that materials have natural properties such as density, conductivity and elastic modulus. Surfaces, representing material boundaries have perhaps rather more insubstantial properties but we still think of some of these properties are natural, like color. There are other properties, however, which are easy to define but whose value seems to depend on the technique or scale of measurement: hardness, for instance. Roughness seems to be such a property, with the added difficulty that is not always so easy to define as a concept.

In ultrasonic pulse/echo immersion inspection, the transducer beam transmits through the water-solid interface and is reflected back by flaws in the part, producing a flaw signal that can be used to find, locate, and size flaws. In many applications, for example, the monitoring of the cleanliness of steel billet and the detection of flaws in pipes, interfaces between water and parts are not flat, but rather curved. Curved surfaces can modify flaw signals, making the performance of an inspection process more difficult to predict and the inspection results more difficult to interpret. A solution to this problem is to use physics-based simulation tools to design and assess the performance of inspections. In this work, we present a particular example, the use of models to aid in the characterization of the cleanliness of steel billet.

Small crystallites (also called grains) in polycrystalline metals can scatter a propagating ultrasonic wave in various directions, leading to an attenuation of the beam and producing back-scattered noise that can mask the signals reflected from small flaws in pulse/echo inspections. Attenuation and back scattering are thus important in studying the flaw detectability of a given material. Attenuation and back scattering are also fundamental properties of ultrasonic wave propagation in poly-crystalline materials, and can be directly related to micro-structure of the material. Hence, the study of the relationships between micro-structure and ultrasonic attenuation and back-scattering properties are foundations for understanding both flaw detection and material characterization.

Discontinuities are interruptions in the typical structure of a material. These interruptions may occur in the base metal, weld material or "heat affected" zones. Discontinuities, which do not meet the requirements of the codes or specification used to invoke and control an inspection, are referred to as defects.

A discontinuity is best evaluated when its size (extension) is known. The operator's wish to accurately know the "real reflector size" is understandable therefore it is expected that an nondestructive testing method, such as ultrasonic testing, give this information. However, due to the fact that on the display only the echo can be interpreted, this means the reflected sound coming from the discontinuity, it is very often difficult, and in some cases even impossible, to reliably assert the size of the reflector. In fact, the echo height plays the decisive part when evaluating discontinuities during manual Ultrasonic Testing.