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Ultrasonic Testing
Chapter 6
Ultrasonic Testing AN INTRODUCTION TO THE WORLD OF SOUND Physicists and acoustic engineers tend to discuss sound pressure levels (SPLs) in terms of frequencies, partly because this is how our ears interpret sound. What we experience as “higher pitched” or “lower pitched” sounds are pressure vibrations having a higher or lower number of cycles per second. In a common technique of acoustic measurement, acoustic signals are sampled in time and then presented in more meaningful forms such as octave bands or time frequency plots. Both of these popular methods are used to analyze sound and better understand acoustic phenomena. The fundamental principles being the same, there are more advanced developments in the field, and basic principles are dissected and used to address more specific needs of the ever-growing industry. Some of these specialized “subsections” or new developments in ultrasonic testing are listed below. In this chapter, some of these will be addressed, but for details, readers are directed to the specialized industry expert companies because some of these are proprietary techniques. l l l l l l l
UT: shear wave ultrasonic testing AUT: automated ultrasonic testing PAUT: phased array ultrasonic testing TOFD: time of flight diffraction ultrasonic testing GWUT: guided wave ultrasonic testing LGWUT: long-range guided wave testing SWUT: surface wave ultrasonic testing technology
THEORY OF SOUND WAVE AND PROPAGATION In fluids such as air and water, sound waves propagate as disturbances in the ambient pressure level. Although this disturbance is usually small, it is still audible to the human ear. The smallest sound that a person can hear, known as the threshold of hearing, is nine orders of magnitude smaller than the ambient pressure. The loudness of these disturbances is called the SPL, and
Applied Welding Engineering. http://dx.doi.org/10.1016/B978-0-12-804176-5.00026-8 Copyright © 2016 Elsevier Inc. All rights reserved.
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it is measured on a logarithmic scale in decibels. Mathematically, SPL is defined as: SPL ¼ 20 log10 ðP=Pref Þ where Pref is the threshold of hearing and P is the change in pressure from the ambient pressure. Table 3-6-1 gives a few examples of sounds and their strengths in decibels and Pascals. The entire sound spectrum can be divided into three sections: audio, ultrasonic, and infrasonic. The audio range falls between 20 and 20,000 Hz. This range is important because its frequencies can be detected by the human ear. This range has a number of applications, including speech communication and music. The ultrasonic range refers to the very high frequencies: 20,000 Hz and higher. This range has shorter wavelengths that allow better resolution in imaging technologies. Industrial and medical applications such as ultrasonography and elastography rely on the ultrasonic frequency range. On the other end of the spectrum, the lowest frequencies are known as the infrasonic range. These frequencies can be used to study geological phenomenon such as earthquakes. As stated in the introduction, the term ultrasonic is the name given to the study and application of sound waves having frequencies higher than those that can be heard by human ears. Ultrasonic nondestructive testing is the use of ultrasonic sound spectrum to examine or test materials or measure thickness without destroying the material. The testing frequencies range from 100,000 cycles per second (100 kHz) to 25,000,000 cycles per second (25 MHz). Ultrasonic testing does not give direct information about the exact nature of the discontinuity. This is deduced from a variety of information, such as materials property and its construction.
TABLE 3-6-1 Pressure Amplitude and Decibel Level Example of Common Sound
Pressure Amplitude 6
Pa
Decibel Level
Threshold of hearing
20*10
0 dB
Normal talking at 1 m
0.002 to 0.02 Pa
40 to 60 dB
Power lawnmower at 1 m
2 Pa
100 dB
Threshold of pain
200 Pa
134 dB
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THEORY OF SOUND Sound is the mechanical vibrations of particles in a medium. When a sound wave is introduced in a material, the particles in the material vibrate about a fixed point at the same frequency as the sound wave. The particles do not travel with the wave but react to the energy of the wave. It is the energy of the wave that moves through the material. The length of a particular sound wave is measured from through to trough, or from crest to crest. The distance is always the same. This distance is known as the wavelength (l). The time taken for the wave to travel a distance of one complete wavelength (l) is the same amount of time it takes for the source to execute one complete vibration. The velocity of sound (V) is given by the following equation: V¼lF where, l is the wavelength of the wave and F is the frequency of the wave. A number of sound waves travel through the solid matter; some of them are listed below. In a longitudinal wave, also called a compression wave, the particles vibrate back and forth in same direction as the motion of the sound. The ultrasonic vibrations in liquids and gases propagate in longitudinal waves only. This is because liquids and gases have no shear rigidity. In a shear wave, also called transverse wave, the particles vibrate back and forth in a direction that is at right angle to the motion of sound. It is also possible in some specific limits, to produce shear waves that travel along the free boundary or surface of a solid. These surface or Rayleigh waves penetrate the material to a depth of only a few particles. In solids, all the three modes of sound waves can propagate. The shortest ultrasonic wavelengths are of the order of the magnitude of the wavelength of visible light. Because of this, ultrasonic wave vibrations possess properties very similar to the light waves that are they can be reflected, focused, and refracted. High-frequency particle vibrations of sound waves are propagated in homogeneous solids in the same manner as directed light beams. Sound beams are reflected either partially or totally at any surface acting as a boundary between the object and the gas, liquid, or other type of solid. The ultrasonic pulses reflect from discontinuities, thereby enabling detection of their presence and location. Some of the terminology specially associated with ultrasonic testing methods is described next.
PIEZOELECTRICITY Piezoelectricity refers to the electricity produced by a vibrating crystal and its reversion back to vibrations of the crystal.
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When electric current is applied into the crystal, the crystal transforms the electrical energy to mechanical vibrations and transmits the vibrations through a coupling medium into the test material. These pulsed vibrations propagate through the object with a velocity that depends on the density and elasticity of the material.
SOUND BEAM REFLECTION High-frequency sound wave act in a similar way as light waves. If the wave is interrupted by an object, most of the sound beam is reflected. Crystals or transducers then pick up these sound beams and present them as vertical deflections of a horizontal trace or a line base on a cathode ray tube (CRT) or an oscilloscope. This type of presentation is called A-scan, other presentations being the B-scan, which presents a cross-sectional image of the discontinuity and the material being inspected. The C-scan presentation displays the discontinuity in plan-view.
SOUND BEAM FREQUENCIES Most ultrasonic testing is available within 400 kHz to 25 MHz. These vibrations are beyond the audible range and propagate in the test material as waves of particle vibrations. Sound beams of all frequencies can penetrate fine-grained material without difficulty. When using high frequencies in coarse-grained material, interpretation becomes difficult because interference in the form of scattering is noted. Depth of penetration is better achieved by lower frequencies. The selection of specific frequency for testing is mainly dependent on the material property and goal of the testing. Frequencies up to 1 MHz are generally a good choice, and they have better penetration and have less attenuation, and they scatter less by coarse grains and rough surfaces. The disadvantage of low frequencies is that they have large angle of divergence, so they cannot resolve small flaws. On the other side, the high-frequency transducers emit more concentrated beams with better resolving power, but they are more scattered by coarse grains and rough surfaces. Frequencies above 10 MHz are normally not used in contact testing because the higher frequency transducers are thinner and fragile. As the frequency of sound vibrations increases, the wavelength correspondingly decreases and approaches the dimension of the molecular or atomic structure. In immersion testing, however, all frequencies can be used because there is no physical contact between the transducer and the material being tested.
SOUND BEAM VELOCITIES Ultrasonic waves travel through solids and liquids at relatively high speed, but they are relatively rapidly attenuated or die down. The velocity of a
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TABLE 3-6-2 Sound Velocities in Various Mediums Longitudinal Velocity 3
cm/mSec
Material
Density g/cm
Air
0.001
0.033
738
Water
1.00
0.149
3,333
Plastic (Acrylic)
1.18
0.267
5,972
Aluminum
2.8
0.625
13,981
Steel
0.56
Cast Iron
0.35 to 0.56
Mercury
13.00
0.142
3,176
Beryllium
1.82
1.28
28,633
specific mode of sound is a constant through a given homogeneous material. The ultrasonic wave velocities through the various materials are given in Table 3-6-2. The difference in sound velocity is due to the difference in the density and elasticity of each material. Yet it may be noted that density alone is not able to account for the variations because it may be noted that beryllium has high sound velocity, although it is less dense than aluminum, and the acoustic velocity of water and mercury is nearly the same even though the density of mercury is 13 times greater than that of water. While discussing properties of sound, we introduced that the sound beams are refracted and subjected to mode conversion, resulting in a combination of shear and longitudinal waves. What principles govern the of mode transformation? When a longitudinal ultrasonic wave is directed from one medium into another of different acoustic properties at an angle other than normal to the interface between the two media, a wave mode transformation occurs. The resultant transformation depends on the incident angle in the first medium and on the velocity of sound in the first and second media. In each transformation, there is an equal angle of reflection back into the first medium. Snell’s law is used to calculate angle transformations based on the sound path angles and the sound velocities of the two media (Figure 3-6-1).
SNELL’S LAW OF REFLECTION AND REFRACTION Figure 3-6-1 shows an illustration of Snell’s law of reflection and refraction. We note that the sine of the incident angle a is to the sine of d (longitudinal) or e (shear) refracted angle as sound velocity of the
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(a) b
a
c
Second medium Air Snell’s Law of Reflection
(b)
b
a
c
First medium Second medium
d
e
Legend: Longitudinal wave Shear wave
Snell’s Law of Refraction FIGURE 3-6-1 (a) and (b), Snell’s law of reflection and refraction.
incident medium 1 is to the sound velocity of the refracted medium 2. The same relationship exists when mode conversion occurs within the same medium, using the sound velocities of different waves in the equation. This can be used to calculate refracted and reflected angles. For part A of the sketch, these equations can be written as: sin a=sin d ¼ Longitudinal velocity in medium 1=Longitudinal velocity in medium 2 sin a=sin e ¼ Longitudinal velocity in medium 1=Shear velocity in medium 2
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or sin a ðlongÞ=sin c ðshearÞ ¼ Longitudinal velocity in medium 1=Shear velocity in medium 1 In B part of Figure 3-6-1, the equation will be: sin a ðshearÞ=sin c ðlongÞ ¼ Shear velocity in medium 2=Longitudinal velocity in medium 2 We note that as the incident angle is increased from normal, this results in only the longitudinal wave. This longitudinal angle d also increases until it reaches 90 degrees. At this point, no more longitudinal wave is entering the second medium. This angle of the medium is called the first critical angle. As the incident angle is further increased, the shear angle e also increases until it becomes 90 degrees. At this point, the entire shear wave in the second medium is transformed into the surface wave. This is called second critical angle. These calculations use simple centerline of the beam as input. The actual application, however, is more complex because the sound beam has width and divergence. The amplitude of the sound is also higher at its center line, and it gradually dies down on the outer edges.
UNDERSTANDING THE VARIABLES ASSOCIATED WITH ULTRASONIC TESTING The sound velocity through a given material is the distance that sound energy will propagate in that material in a given time and is a function of material density, the material’s acoustic impedance, and the material’s temperature. Because sound velocities are relatively high, the most expression is in meters or feet per second. The sound velocity of a shear wave in a given material is usually one half of a longitudinal and about 1.1 times that of a surface wave. Specific velocities are tabulated in handbooks and are used for calculations to determine the angle transformations. The effect of temperature on sound velocity is normally not very significant in most metals but must be considered when calculating angles in plastics if they are used as wedges for shear wave search units. The frequency of ultrasound used for testing is usually between 1 and 6 MHz. The most common frequency for weld inspection is 2.25 MHz. The transducer element, when excited, resonates at its natural frequency. The resulting frequency is not a single frequency but a relatively narrow band of frequencies. Of these, one or more respond with highest amplitude. The frequency is related to the thickness of transducer element. Frequency is decreased as the thickness of the element is increased.
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The piezoelectric property of the transducer is affected by the natural frequency of the element and must be considered to obtain maximum sound amplitude. It may be noted that broadband pulse generation is usually effective and used with portable equipment. As explained earlier in discussion of the theory of sound, the wavelength (l) is the function of velocity (V) and frequency (F): V¼lF The expected minimum size of reflector (flaw) detectable with ultrasonic sound is about one half wavelength (l/2) as measured in a direction perpendicular to the direction of sound propagation.
SELECTION OF TEST EQUIPMENT A variety of ultrasonic test applications have prompted industry to develop specialized test equipment. The enormous development of electronics has allowed manufacturers to add more sophisticated functional features to their equipment, increasing portability, leading to better results, and taking away several calculations from the hands of the operators and adding them to the machine as features. The equipment with miniaturization in size and higher speed is very common features of this new equipment. Other developments include: l l l l l l
l l l l l l
Typically, real-time 320 240 pixels (QVGA) A-trace (40-Hz update) display Distance amplitude flaw gating (DAG) Weld trig Freeze and insta-freeze for spot weld applications Precision thickness measurement capability, combined flaw & thickness mode, 2 MB of memory for substantial storage capability and USB connectivity High-speed scrolling and encoded B-scan IP and IF gating Adjustable damping Multi-color liquid crystal display (LCD) screens Other features such as SplitView, SplitScan, AutoTrack, and Quarter VGA resolution
But in its very basic form, the ultrasonic principles are the same, and the equipment available is based on those principles. Longitudinal wave ultrasound is generally limited in use to detecting inclusions and lamellar types of discontinuities in the base material. Shear waves are most valuable in detection of weld discontinuities because of their ability to furnish three-dimensional coordinates for discontinuities. As stated earlier in this section, the sensitivity of shear wave is about twice that of longitudinal wave, the frequency and search unit size being constant.
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0.080 Hole 0.080
0.6
0.6 0.4
15
3.6
55 100
1.5
2.2 4
Type 1 R=1
0.125 6.6
Type 2
0.92 1
30
50
165
35
4
Type 2
300 60° 70°
R=4 6
Type 1 R = 25
3
23 25 2
60° 70° 2
9
91
12
0.080
15
40° 50° 60°
40° 50° 60°
1.2 1.42
2
8
80°
8 4 US CUSTOMARY DIMENSIONS
5
10
R = 100 15
20
80°
200 100 SI DIMENSIONS (MM)
Notes:
1. The dimensional tolerance between all surfaces involved in retaining or calibrating shall be within +0.005 inch (0.13 millimeter) of detailed dimension. 2. The surface finish of all surfaces to which sound is applied or reflected from shall have a maximum of 125 µin. r.m.s. 3. All materials shall be ASTM A36 or acoustically equivalent. 4. All holes shall have a smooth internal finish and shall be drilled 90 degrees to the material surface. 5. Degree lines and identification marking shall be indented into the material surface so that permanent orientation can be maintained. 6. Other approved reference blocks with slightly different dimensions or distance calibration slots are permissable.
FIGURE 3-6-2 Calibration block.
Shear wave angles are measured in the test material from a line perpendicular to the test surface. The search unit’s angle selection is based on the expected flaw orientation. Usually, it is a good practice to use more than one search angle to ensure proper detection of flaws. Three of the most common angles used for shear wave testing are 70-, 60-, and 45-degree probes (Figure 3-6-2).
A-Scan Equipment In A-scan systems, the data are presented as returned signal from the material under test. The data are presented on an oscilloscope. The horizontal base line on the oscilloscope screen indicates from left to right the elapsed time, and the vertical deflection shows signal amplitude. For a given velocity in the specimen, sweep can be calibrated directly across the screen in terms of distance or depth of penetration into the sample. Conversely, when the dimension (thickness) of the specimen is known, the sweep time may be used to
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determine ultrasonic velocities. The height of the indications or “pips” represents the intensities of the reflected sound beams. These are used to determine the size of the discontinuity, the depth, or distance to discontinuity from any given surface the sound beam is either entering or reflecting back. The main advantage of this type of presentation is that it gives the amplitude that can be used to determine the size and position of the discontinuity.
B-Scan Equipment B-scan is especially useful when the distribution and shape of large discontinuities within a sample cross section is of interest. In addition to the basic components of the A-scan equipment, B-scan provides the following functions. 1. Retains the image on the oscilloscope screen by use of a long persistence phosphor coating 2. Deflection of the image-tracing spot on the oscilloscope screen is synchronized with the motion of the transducer along the sample. 3. Image-tracing spot intensity modulation or brightness is in proportion to the amplitude of the signals received.
C-Scan Equipment C-scan equipment is intended to provide a permanent record of the test when high-speed automatic scanning is used in ultrasonic testing. C-scan equipment displays the discontinuities in a plan view. It does not give the depth or orientation of the discontinuity.
TESTING PROCEDURE Most of the testing is carried out as per the written procedures for specific work. These are based on the applicable code of construction. Hence, the discussion here is of general application, and specifics must be developed meeting the work requirements. Before testing with shear wave angle units, it is good practice and some codes mandate that the material is scanned with a longitudinal unit to ensure that the base material is free from such discontinuities that would interfere with shear wave evaluation of flaws. Some of the basic rules of testing are: 1. The sound path distance is basically limited to a specified limit generally up to 10 inches. 2. Three basic search unit angles used are 70, 60, and 45 degrees as measured from a line normal to the test surface of the material.
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3. It is assumed that any flaw will be normal to the test material surface and parallel to the weld axis. The flaw orientation would be the most serious direction for flaws in most welds. 4. The 70-degree search unit would provide the highest amplitude response from the type of flaw described earlier followed by the 60- and 45-degree search units. Hence, the order of preference in use shall be the same. 5. The relative amplitude response from the flaw is in direct proportion to its effect on the integrity of the material and weld. The generally accepted diminishing order of flaw severity in welds and material is: a. Cracks b. Incomplete fusion c. Incomplete penetration d. Inclusions (e.g., slag) e. Porosity 6. Ultrasonic indications are evaluated on a decibel amplitude basis. Each indication to be evaluated is adjusted with the calibrated decibel gain or attenuation control to produce a reference level height on the CRT, and the decibel setting number is recorded as indication level a. 7. The reference level b is attained from a reflector in an approved calibration block. The reflector indication is maximized with search unit movement and then adjusted with the gain or attenuation control to produce a reference-level indication. This decibel reading is the reference level. 8. The decibel attenuation factor c, used for weldments testing, is at the rate of two decibels per inch of sound path after the first inch. Example: A 6-inch sound path would produce an attenuation factor of (6 1) 2 ¼ 10. 9. Decibel rating d for flaw evaluation is in accordance with the construction code requirement, with gain control this is attained by applying the equations a b c ¼ d. However, for equipment with attenuation control, b a c ¼ d. Case studies associated with actual calibration testing of welds are included.
Role of Coupling in Testing The couplant material is used to maintain full contact of the transducer with the material surface. This allows the transfer of sound wave. It also helps full coverage of the test surface during testing. Coupling material should be hydraulic in nature and have good wetting properties to cover the material surface. The couplant materials used include water, oil, grease, glycerin, and cellulose gum powder mixed with water. The cellulose gum powder is most common material used for testing. It has significant advantage over others because it is low cost; its viscosity can be
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changed with the addition of more water; it is not a slipping hazard; it does not form a contaminant film on the material surface; and its residue is easily removable, so it is not an obstacle for any further work such as repair involving welding. Before testing, the equipment is checked for linearity and calibrated to cover the scope of work. The calibration blocks used for calibrating ultrasonic equipment before testing can be standard or very specific to the task at hand. A typical IIW block is shown in Figure 3-6-3. Both U.S. customary unit and metric unit calibration blocks are shown. Note that the block has the beam exit angle for correcting the angle of sound beam exit from the transducer. This is an important step in the accuracy of the test. Most standards and code mandate that before beginning calibration and testing, the beam exit angle is reestablished, and all future calculations are based on the correct angle of the beam path. This is one of the basic calibration steps; further detailed calibration steps are also involved in testing.
ATOMIZATION OF ULTRASONIC TESTING SYSTEMS At the beginning of chapter, we introduced some terms such as AUT, TOFD, pulse echo, and so on. We will discuss a few of these as an introduction because more and more ultrasonic inspections are now being done by automated systems. Most of the AUT systems are applied to and associated with girth welds, but that is not to assume that the technique is not used for longitudinal butt weds. Techniques used are to be based on zonal discrimination of the weld cross section, whereby the weld is divided into approximately equal vertical inspection sections (zones), each being assessed by a pair of ultrasonic transducers. These inspection zones are typically 2 to 3 mm (0.08e0.12 in) in height. For most applications, this requires the use of contact-focused transducers. This is essential to avoid excessive overlap with adjacent signals and interference with signals originating from off-axis geometric reflectors. The system is able to provide an adequate number of inspection channels to ensure the complete volumetric examination of the weld through thickness in one circumferential scan. The instrument is capable of providing a linear “A” scan presentation for each selected channel. The AUT inspection channels allow the volume of the weld scanned to be assessed in accordance with the inspection zones. It is important to have calibrated Instrument linearity within 5% of the ideal acceptable linearity for both linear and logarithmic amplifiers. Each inspection channel is suitable for selecting pulse-echo or through-transmission mode, gate position and length for a minimum of two gates, and gain. Recording thresholds is set so that the system can select display signals between 0% and 100% of full screen height for simple amplitude and transit time recording and from 0% to 100% for B-scan or “mapping” type recording
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of data. B-scan mapping is described later. Two recordable signal outputs per gate are available, in either analog or digital form, and they are representative of signal height and time of flight. These systems are made suitable for recording on a multichannel recorder or computer data acquisition software display.
THE RECORDING SYSTEM A marker circuit suitable for connection to the recorder or acquisition system is used. Its accuracy is very important for full and proper coverage of the weld, especially the circumferential welds. This acquisition is important as a means of electronically determining circumferential weld distance to an accuracy of at least 10 mm (0.4 in); often an optical encoder is used as a distance marker. The programmed scan lengths ensure that all probes cover the maximum circumferential distance required for a given circumference of the weld. The suitable correction factor is used for equipment that has the encoders traveling on a track or welding guide band to ensure that the circumferential distance recorded on the chart corresponds to the transducer position on the circumferential weld outer surface. Each transducer for weld discontinuities records for confirmation of the acoustic coupling arranged on the chart or display.
MAPPING (TIME OF FLIGHT DIFFRACTION) B-scan or “mapping” displays are used for volumetric flaw detection and characterizations, and TOFD techniques are often added to improve characterization and sizing. TOFD techniques are often used to augment pulse-echo techniques; however, a TOFD technique is not intended to replace the pulseecho techniques. When TOFD technique is used for mapping, the recording system should be capable of a 256-level greyscale display and be capable of recording full R-F waveforms for the TOFD transducer pairs. The above is only a brief a typical AUT system. The details of AUT inspection of welds should be assessed. Often an engineering critical assessment is carried out to assess the level of critical flaws that should be evaluated and accepted or rejected. The selection of specific equipment and its limits and advantages should be evaluated in terms of the specific quality level required for the specific weld.
Ultrasonic Testing AN INTRODUCTION TO THE WORLD OF SOUND Physicists and acoustic engineers tend to discuss sound pressure levels (SPLs) in terms of frequencies, partly because this is how our ears interpret sound. What we experience as “higher pitched” or “lower pitched” sounds are pressure vibrations having a higher or lower number of cycles per second. In a common technique of acoustic measurement, acoustic signals are sampled in time and then presented in more meaningful forms such as octave bands or time frequency plots. Both of these popular methods are used to analyze sound and better understand acoustic phenomena. The fundamental principles being the same, there are more advanced developments in the field, and basic principles are dissected and used to address more specific needs of the ever-growing industry. Some of these specialized “subsections” or new developments in ultrasonic testing are listed below. In this chapter, some of these will be addressed, but for details, readers are directed to the specialized industry expert companies because some of these are proprietary techniques. l l l l l l l
UT: shear wave ultrasonic testing AUT: automated ultrasonic testing PAUT: phased array ultrasonic testing TOFD: time of flight diffraction ultrasonic testing GWUT: guided wave ultrasonic testing LGWUT: long-range guided wave testing SWUT: surface wave ultrasonic testing technology
THEORY OF SOUND WAVE AND PROPAGATION In fluids such as air and water, sound waves propagate as disturbances in the ambient pressure level. Although this disturbance is usually small, it is still audible to the human ear. The smallest sound that a person can hear, known as the threshold of hearing, is nine orders of magnitude smaller than the ambient pressure. The loudness of these disturbances is called the SPL, and
Applied Welding Engineering. http://dx.doi.org/10.1016/B978-0-12-804176-5.00026-8 Copyright © 2016 Elsevier Inc. All rights reserved.
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it is measured on a logarithmic scale in decibels. Mathematically, SPL is defined as: SPL ¼ 20 log10 ðP=Pref Þ where Pref is the threshold of hearing and P is the change in pressure from the ambient pressure. Table 3-6-1 gives a few examples of sounds and their strengths in decibels and Pascals. The entire sound spectrum can be divided into three sections: audio, ultrasonic, and infrasonic. The audio range falls between 20 and 20,000 Hz. This range is important because its frequencies can be detected by the human ear. This range has a number of applications, including speech communication and music. The ultrasonic range refers to the very high frequencies: 20,000 Hz and higher. This range has shorter wavelengths that allow better resolution in imaging technologies. Industrial and medical applications such as ultrasonography and elastography rely on the ultrasonic frequency range. On the other end of the spectrum, the lowest frequencies are known as the infrasonic range. These frequencies can be used to study geological phenomenon such as earthquakes. As stated in the introduction, the term ultrasonic is the name given to the study and application of sound waves having frequencies higher than those that can be heard by human ears. Ultrasonic nondestructive testing is the use of ultrasonic sound spectrum to examine or test materials or measure thickness without destroying the material. The testing frequencies range from 100,000 cycles per second (100 kHz) to 25,000,000 cycles per second (25 MHz). Ultrasonic testing does not give direct information about the exact nature of the discontinuity. This is deduced from a variety of information, such as materials property and its construction.
TABLE 3-6-1 Pressure Amplitude and Decibel Level Example of Common Sound
Pressure Amplitude 6
Pa
Decibel Level
Threshold of hearing
20*10
0 dB
Normal talking at 1 m
0.002 to 0.02 Pa
40 to 60 dB
Power lawnmower at 1 m
2 Pa
100 dB
Threshold of pain
200 Pa
134 dB
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THEORY OF SOUND Sound is the mechanical vibrations of particles in a medium. When a sound wave is introduced in a material, the particles in the material vibrate about a fixed point at the same frequency as the sound wave. The particles do not travel with the wave but react to the energy of the wave. It is the energy of the wave that moves through the material. The length of a particular sound wave is measured from through to trough, or from crest to crest. The distance is always the same. This distance is known as the wavelength (l). The time taken for the wave to travel a distance of one complete wavelength (l) is the same amount of time it takes for the source to execute one complete vibration. The velocity of sound (V) is given by the following equation: V¼lF where, l is the wavelength of the wave and F is the frequency of the wave. A number of sound waves travel through the solid matter; some of them are listed below. In a longitudinal wave, also called a compression wave, the particles vibrate back and forth in same direction as the motion of the sound. The ultrasonic vibrations in liquids and gases propagate in longitudinal waves only. This is because liquids and gases have no shear rigidity. In a shear wave, also called transverse wave, the particles vibrate back and forth in a direction that is at right angle to the motion of sound. It is also possible in some specific limits, to produce shear waves that travel along the free boundary or surface of a solid. These surface or Rayleigh waves penetrate the material to a depth of only a few particles. In solids, all the three modes of sound waves can propagate. The shortest ultrasonic wavelengths are of the order of the magnitude of the wavelength of visible light. Because of this, ultrasonic wave vibrations possess properties very similar to the light waves that are they can be reflected, focused, and refracted. High-frequency particle vibrations of sound waves are propagated in homogeneous solids in the same manner as directed light beams. Sound beams are reflected either partially or totally at any surface acting as a boundary between the object and the gas, liquid, or other type of solid. The ultrasonic pulses reflect from discontinuities, thereby enabling detection of their presence and location. Some of the terminology specially associated with ultrasonic testing methods is described next.
PIEZOELECTRICITY Piezoelectricity refers to the electricity produced by a vibrating crystal and its reversion back to vibrations of the crystal.
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When electric current is applied into the crystal, the crystal transforms the electrical energy to mechanical vibrations and transmits the vibrations through a coupling medium into the test material. These pulsed vibrations propagate through the object with a velocity that depends on the density and elasticity of the material.
SOUND BEAM REFLECTION High-frequency sound wave act in a similar way as light waves. If the wave is interrupted by an object, most of the sound beam is reflected. Crystals or transducers then pick up these sound beams and present them as vertical deflections of a horizontal trace or a line base on a cathode ray tube (CRT) or an oscilloscope. This type of presentation is called A-scan, other presentations being the B-scan, which presents a cross-sectional image of the discontinuity and the material being inspected. The C-scan presentation displays the discontinuity in plan-view.
SOUND BEAM FREQUENCIES Most ultrasonic testing is available within 400 kHz to 25 MHz. These vibrations are beyond the audible range and propagate in the test material as waves of particle vibrations. Sound beams of all frequencies can penetrate fine-grained material without difficulty. When using high frequencies in coarse-grained material, interpretation becomes difficult because interference in the form of scattering is noted. Depth of penetration is better achieved by lower frequencies. The selection of specific frequency for testing is mainly dependent on the material property and goal of the testing. Frequencies up to 1 MHz are generally a good choice, and they have better penetration and have less attenuation, and they scatter less by coarse grains and rough surfaces. The disadvantage of low frequencies is that they have large angle of divergence, so they cannot resolve small flaws. On the other side, the high-frequency transducers emit more concentrated beams with better resolving power, but they are more scattered by coarse grains and rough surfaces. Frequencies above 10 MHz are normally not used in contact testing because the higher frequency transducers are thinner and fragile. As the frequency of sound vibrations increases, the wavelength correspondingly decreases and approaches the dimension of the molecular or atomic structure. In immersion testing, however, all frequencies can be used because there is no physical contact between the transducer and the material being tested.
SOUND BEAM VELOCITIES Ultrasonic waves travel through solids and liquids at relatively high speed, but they are relatively rapidly attenuated or die down. The velocity of a
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TABLE 3-6-2 Sound Velocities in Various Mediums Longitudinal Velocity 3
cm/mSec
Material
Density g/cm
Air
0.001
0.033
738
Water
1.00
0.149
3,333
Plastic (Acrylic)
1.18
0.267
5,972
Aluminum
2.8
0.625
13,981
Steel
0.56
Cast Iron
0.35 to 0.56
Mercury
13.00
0.142
3,176
Beryllium
1.82
1.28
28,633
specific mode of sound is a constant through a given homogeneous material. The ultrasonic wave velocities through the various materials are given in Table 3-6-2. The difference in sound velocity is due to the difference in the density and elasticity of each material. Yet it may be noted that density alone is not able to account for the variations because it may be noted that beryllium has high sound velocity, although it is less dense than aluminum, and the acoustic velocity of water and mercury is nearly the same even though the density of mercury is 13 times greater than that of water. While discussing properties of sound, we introduced that the sound beams are refracted and subjected to mode conversion, resulting in a combination of shear and longitudinal waves. What principles govern the of mode transformation? When a longitudinal ultrasonic wave is directed from one medium into another of different acoustic properties at an angle other than normal to the interface between the two media, a wave mode transformation occurs. The resultant transformation depends on the incident angle in the first medium and on the velocity of sound in the first and second media. In each transformation, there is an equal angle of reflection back into the first medium. Snell’s law is used to calculate angle transformations based on the sound path angles and the sound velocities of the two media (Figure 3-6-1).
SNELL’S LAW OF REFLECTION AND REFRACTION Figure 3-6-1 shows an illustration of Snell’s law of reflection and refraction. We note that the sine of the incident angle a is to the sine of d (longitudinal) or e (shear) refracted angle as sound velocity of the
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(a) b
a
c
Second medium Air Snell’s Law of Reflection
(b)
b
a
c
First medium Second medium
d
e
Legend: Longitudinal wave Shear wave
Snell’s Law of Refraction FIGURE 3-6-1 (a) and (b), Snell’s law of reflection and refraction.
incident medium 1 is to the sound velocity of the refracted medium 2. The same relationship exists when mode conversion occurs within the same medium, using the sound velocities of different waves in the equation. This can be used to calculate refracted and reflected angles. For part A of the sketch, these equations can be written as: sin a=sin d ¼ Longitudinal velocity in medium 1=Longitudinal velocity in medium 2 sin a=sin e ¼ Longitudinal velocity in medium 1=Shear velocity in medium 2
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or sin a ðlongÞ=sin c ðshearÞ ¼ Longitudinal velocity in medium 1=Shear velocity in medium 1 In B part of Figure 3-6-1, the equation will be: sin a ðshearÞ=sin c ðlongÞ ¼ Shear velocity in medium 2=Longitudinal velocity in medium 2 We note that as the incident angle is increased from normal, this results in only the longitudinal wave. This longitudinal angle d also increases until it reaches 90 degrees. At this point, no more longitudinal wave is entering the second medium. This angle of the medium is called the first critical angle. As the incident angle is further increased, the shear angle e also increases until it becomes 90 degrees. At this point, the entire shear wave in the second medium is transformed into the surface wave. This is called second critical angle. These calculations use simple centerline of the beam as input. The actual application, however, is more complex because the sound beam has width and divergence. The amplitude of the sound is also higher at its center line, and it gradually dies down on the outer edges.
UNDERSTANDING THE VARIABLES ASSOCIATED WITH ULTRASONIC TESTING The sound velocity through a given material is the distance that sound energy will propagate in that material in a given time and is a function of material density, the material’s acoustic impedance, and the material’s temperature. Because sound velocities are relatively high, the most expression is in meters or feet per second. The sound velocity of a shear wave in a given material is usually one half of a longitudinal and about 1.1 times that of a surface wave. Specific velocities are tabulated in handbooks and are used for calculations to determine the angle transformations. The effect of temperature on sound velocity is normally not very significant in most metals but must be considered when calculating angles in plastics if they are used as wedges for shear wave search units. The frequency of ultrasound used for testing is usually between 1 and 6 MHz. The most common frequency for weld inspection is 2.25 MHz. The transducer element, when excited, resonates at its natural frequency. The resulting frequency is not a single frequency but a relatively narrow band of frequencies. Of these, one or more respond with highest amplitude. The frequency is related to the thickness of transducer element. Frequency is decreased as the thickness of the element is increased.
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The piezoelectric property of the transducer is affected by the natural frequency of the element and must be considered to obtain maximum sound amplitude. It may be noted that broadband pulse generation is usually effective and used with portable equipment. As explained earlier in discussion of the theory of sound, the wavelength (l) is the function of velocity (V) and frequency (F): V¼lF The expected minimum size of reflector (flaw) detectable with ultrasonic sound is about one half wavelength (l/2) as measured in a direction perpendicular to the direction of sound propagation.
SELECTION OF TEST EQUIPMENT A variety of ultrasonic test applications have prompted industry to develop specialized test equipment. The enormous development of electronics has allowed manufacturers to add more sophisticated functional features to their equipment, increasing portability, leading to better results, and taking away several calculations from the hands of the operators and adding them to the machine as features. The equipment with miniaturization in size and higher speed is very common features of this new equipment. Other developments include: l l l l l l
l l l l l l
Typically, real-time 320 240 pixels (QVGA) A-trace (40-Hz update) display Distance amplitude flaw gating (DAG) Weld trig Freeze and insta-freeze for spot weld applications Precision thickness measurement capability, combined flaw & thickness mode, 2 MB of memory for substantial storage capability and USB connectivity High-speed scrolling and encoded B-scan IP and IF gating Adjustable damping Multi-color liquid crystal display (LCD) screens Other features such as SplitView, SplitScan, AutoTrack, and Quarter VGA resolution
But in its very basic form, the ultrasonic principles are the same, and the equipment available is based on those principles. Longitudinal wave ultrasound is generally limited in use to detecting inclusions and lamellar types of discontinuities in the base material. Shear waves are most valuable in detection of weld discontinuities because of their ability to furnish three-dimensional coordinates for discontinuities. As stated earlier in this section, the sensitivity of shear wave is about twice that of longitudinal wave, the frequency and search unit size being constant.
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0.080 Hole 0.080
0.6
0.6 0.4
15
3.6
55 100
1.5
2.2 4
Type 1 R=1
0.125 6.6
Type 2
0.92 1
30
50
165
35
4
Type 2
300 60° 70°
R=4 6
Type 1 R = 25
3
23 25 2
60° 70° 2
9
91
12
0.080
15
40° 50° 60°
40° 50° 60°
1.2 1.42
2
8
80°
8 4 US CUSTOMARY DIMENSIONS
5
10
R = 100 15
20
80°
200 100 SI DIMENSIONS (MM)
Notes:
1. The dimensional tolerance between all surfaces involved in retaining or calibrating shall be within +0.005 inch (0.13 millimeter) of detailed dimension. 2. The surface finish of all surfaces to which sound is applied or reflected from shall have a maximum of 125 µin. r.m.s. 3. All materials shall be ASTM A36 or acoustically equivalent. 4. All holes shall have a smooth internal finish and shall be drilled 90 degrees to the material surface. 5. Degree lines and identification marking shall be indented into the material surface so that permanent orientation can be maintained. 6. Other approved reference blocks with slightly different dimensions or distance calibration slots are permissable.
FIGURE 3-6-2 Calibration block.
Shear wave angles are measured in the test material from a line perpendicular to the test surface. The search unit’s angle selection is based on the expected flaw orientation. Usually, it is a good practice to use more than one search angle to ensure proper detection of flaws. Three of the most common angles used for shear wave testing are 70-, 60-, and 45-degree probes (Figure 3-6-2).
A-Scan Equipment In A-scan systems, the data are presented as returned signal from the material under test. The data are presented on an oscilloscope. The horizontal base line on the oscilloscope screen indicates from left to right the elapsed time, and the vertical deflection shows signal amplitude. For a given velocity in the specimen, sweep can be calibrated directly across the screen in terms of distance or depth of penetration into the sample. Conversely, when the dimension (thickness) of the specimen is known, the sweep time may be used to
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determine ultrasonic velocities. The height of the indications or “pips” represents the intensities of the reflected sound beams. These are used to determine the size of the discontinuity, the depth, or distance to discontinuity from any given surface the sound beam is either entering or reflecting back. The main advantage of this type of presentation is that it gives the amplitude that can be used to determine the size and position of the discontinuity.
B-Scan Equipment B-scan is especially useful when the distribution and shape of large discontinuities within a sample cross section is of interest. In addition to the basic components of the A-scan equipment, B-scan provides the following functions. 1. Retains the image on the oscilloscope screen by use of a long persistence phosphor coating 2. Deflection of the image-tracing spot on the oscilloscope screen is synchronized with the motion of the transducer along the sample. 3. Image-tracing spot intensity modulation or brightness is in proportion to the amplitude of the signals received.
C-Scan Equipment C-scan equipment is intended to provide a permanent record of the test when high-speed automatic scanning is used in ultrasonic testing. C-scan equipment displays the discontinuities in a plan view. It does not give the depth or orientation of the discontinuity.
TESTING PROCEDURE Most of the testing is carried out as per the written procedures for specific work. These are based on the applicable code of construction. Hence, the discussion here is of general application, and specifics must be developed meeting the work requirements. Before testing with shear wave angle units, it is good practice and some codes mandate that the material is scanned with a longitudinal unit to ensure that the base material is free from such discontinuities that would interfere with shear wave evaluation of flaws. Some of the basic rules of testing are: 1. The sound path distance is basically limited to a specified limit generally up to 10 inches. 2. Three basic search unit angles used are 70, 60, and 45 degrees as measured from a line normal to the test surface of the material.
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3. It is assumed that any flaw will be normal to the test material surface and parallel to the weld axis. The flaw orientation would be the most serious direction for flaws in most welds. 4. The 70-degree search unit would provide the highest amplitude response from the type of flaw described earlier followed by the 60- and 45-degree search units. Hence, the order of preference in use shall be the same. 5. The relative amplitude response from the flaw is in direct proportion to its effect on the integrity of the material and weld. The generally accepted diminishing order of flaw severity in welds and material is: a. Cracks b. Incomplete fusion c. Incomplete penetration d. Inclusions (e.g., slag) e. Porosity 6. Ultrasonic indications are evaluated on a decibel amplitude basis. Each indication to be evaluated is adjusted with the calibrated decibel gain or attenuation control to produce a reference level height on the CRT, and the decibel setting number is recorded as indication level a. 7. The reference level b is attained from a reflector in an approved calibration block. The reflector indication is maximized with search unit movement and then adjusted with the gain or attenuation control to produce a reference-level indication. This decibel reading is the reference level. 8. The decibel attenuation factor c, used for weldments testing, is at the rate of two decibels per inch of sound path after the first inch. Example: A 6-inch sound path would produce an attenuation factor of (6 1) 2 ¼ 10. 9. Decibel rating d for flaw evaluation is in accordance with the construction code requirement, with gain control this is attained by applying the equations a b c ¼ d. However, for equipment with attenuation control, b a c ¼ d. Case studies associated with actual calibration testing of welds are included.
Role of Coupling in Testing The couplant material is used to maintain full contact of the transducer with the material surface. This allows the transfer of sound wave. It also helps full coverage of the test surface during testing. Coupling material should be hydraulic in nature and have good wetting properties to cover the material surface. The couplant materials used include water, oil, grease, glycerin, and cellulose gum powder mixed with water. The cellulose gum powder is most common material used for testing. It has significant advantage over others because it is low cost; its viscosity can be
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changed with the addition of more water; it is not a slipping hazard; it does not form a contaminant film on the material surface; and its residue is easily removable, so it is not an obstacle for any further work such as repair involving welding. Before testing, the equipment is checked for linearity and calibrated to cover the scope of work. The calibration blocks used for calibrating ultrasonic equipment before testing can be standard or very specific to the task at hand. A typical IIW block is shown in Figure 3-6-3. Both U.S. customary unit and metric unit calibration blocks are shown. Note that the block has the beam exit angle for correcting the angle of sound beam exit from the transducer. This is an important step in the accuracy of the test. Most standards and code mandate that before beginning calibration and testing, the beam exit angle is reestablished, and all future calculations are based on the correct angle of the beam path. This is one of the basic calibration steps; further detailed calibration steps are also involved in testing.
ATOMIZATION OF ULTRASONIC TESTING SYSTEMS At the beginning of chapter, we introduced some terms such as AUT, TOFD, pulse echo, and so on. We will discuss a few of these as an introduction because more and more ultrasonic inspections are now being done by automated systems. Most of the AUT systems are applied to and associated with girth welds, but that is not to assume that the technique is not used for longitudinal butt weds. Techniques used are to be based on zonal discrimination of the weld cross section, whereby the weld is divided into approximately equal vertical inspection sections (zones), each being assessed by a pair of ultrasonic transducers. These inspection zones are typically 2 to 3 mm (0.08e0.12 in) in height. For most applications, this requires the use of contact-focused transducers. This is essential to avoid excessive overlap with adjacent signals and interference with signals originating from off-axis geometric reflectors. The system is able to provide an adequate number of inspection channels to ensure the complete volumetric examination of the weld through thickness in one circumferential scan. The instrument is capable of providing a linear “A” scan presentation for each selected channel. The AUT inspection channels allow the volume of the weld scanned to be assessed in accordance with the inspection zones. It is important to have calibrated Instrument linearity within 5% of the ideal acceptable linearity for both linear and logarithmic amplifiers. Each inspection channel is suitable for selecting pulse-echo or through-transmission mode, gate position and length for a minimum of two gates, and gain. Recording thresholds is set so that the system can select display signals between 0% and 100% of full screen height for simple amplitude and transit time recording and from 0% to 100% for B-scan or “mapping” type recording
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of data. B-scan mapping is described later. Two recordable signal outputs per gate are available, in either analog or digital form, and they are representative of signal height and time of flight. These systems are made suitable for recording on a multichannel recorder or computer data acquisition software display.
THE RECORDING SYSTEM A marker circuit suitable for connection to the recorder or acquisition system is used. Its accuracy is very important for full and proper coverage of the weld, especially the circumferential welds. This acquisition is important as a means of electronically determining circumferential weld distance to an accuracy of at least 10 mm (0.4 in); often an optical encoder is used as a distance marker. The programmed scan lengths ensure that all probes cover the maximum circumferential distance required for a given circumference of the weld. The suitable correction factor is used for equipment that has the encoders traveling on a track or welding guide band to ensure that the circumferential distance recorded on the chart corresponds to the transducer position on the circumferential weld outer surface. Each transducer for weld discontinuities records for confirmation of the acoustic coupling arranged on the chart or display.
MAPPING (TIME OF FLIGHT DIFFRACTION) B-scan or “mapping” displays are used for volumetric flaw detection and characterizations, and TOFD techniques are often added to improve characterization and sizing. TOFD techniques are often used to augment pulse-echo techniques; however, a TOFD technique is not intended to replace the pulseecho techniques. When TOFD technique is used for mapping, the recording system should be capable of a 256-level greyscale display and be capable of recording full R-F waveforms for the TOFD transducer pairs. The above is only a brief a typical AUT system. The details of AUT inspection of welds should be assessed. Often an engineering critical assessment is carried out to assess the level of critical flaws that should be evaluated and accepted or rejected. The selection of specific equipment and its limits and advantages should be evaluated in terms of the specific quality level required for the specific weld.