Ultrasonic Flaw Detectionby Tom Nelligan Of all the applications of industrial ultrasonic testing, flaw detection is the oldest and the most common. Since the 1940s, the laws of physics that govern the propagation of sound waves through solid materials have been used to detect hidden cracks, voids, porosity, and other internal discontinuities in metals, composites, plastics, and ceramics. High frequency sound waves reflect from flaws in predictable ways, producing distinctive echo patterns that can be displayed and recorded by portable instruments. Ultrasonic testing is completely nondestructive and safe, and it is a well established test method in many basic manufacturing, process, and service industries, especially in applications involving welds and structural metals. This paper provides a brief introduction to the theory and practice of ultrasonic flaw detection. It is intended only as an overview of the topic. Additional detailed information may be found in the references listed at the end. 1. Basic Theory: Sound waves are simply organized mechanical vibrations traveling through a medium, which may be a solid, a liquid, or a gas. These waves will travel through a given medium at a specific speed or velocity, in a predictable direction, and when they encounter a boundary with a different medium they will be reflected or transmitted according to simple rules. This is the principle of physics that underlies ultrasonic flaw detection.

Frequency: All sound waves oscillate at a specific frequency, or number of vibrations or cycles per second, which we experience as pitch in the familiar range of audible sound. Human hearing extends to a maximum frequency of about 20,000 cycles per second (20 KHz), while the majority of ultrasonic flaw detection applications utilize frequencies between 500,000 and 10,000,000 cycles per second (500 KHz to 10 MHz). At frequencies in the megahertz range, sound energy does not travel efficiently through air or other gasses, but it travels freely through most liquids and common engineering materials.

Velocity: The speed of a sound wave varies depending on the medium through which it is traveling, affected by the medium's density and elastic properties. Different types of sound waves (see Modes of Propagation, below) will travel at different velocities.

Wavelength: Any type of wave will have an associated wavelength, which is the distance between any two corresponding points in the wave cycle as it travels through a medium. Wavelength is related to frequency and velocity by the simple equation

= c/f where = wavelength c = sound velocity f = frequency

Wavelength is a limiting factor that controls the amount of information that can be derived from the behavior of a wave. In ultrasonic flaw detection, the generally accepted lower limit of detection for a small flaw is one-half wavelength. Anything smaller than that will be invisible. In ultrasonic thickness gaging, the theoretical minimum measurable thickness one wavelength.

Modes of Propagation: Sound waves in solids can exist in various modes of propagation that are defined by the type of motion involved. Longitudinal waves and shear waves are the most common modes employed in ultrasonic flaw detection. Surface waves and plate waves are also used on occasion. - A longitudinal or compressional wave is characterized by particle motion in the same direction as wave propagation, as from a piston source. Audible sound exists as longitudinal waves. - A shear or transverse wave is characterized by particle motion perpendicular to the direction of wave propagation. - A surface or Rayleigh wave has an elliptical particle motion and it travels across the surface of a material, penetrating to a depth of approximately one wavelength. - A plate or Lamb wave is a complex mode of vibration in thin plates where material thickness is less than one wavelength and the wave fills the entire cross-section of the medium. Sound waves may be converted from one form to another. Most commonly, shear waves are generated in a test material by introducing longitudinal waves at a selected angle. This is discussion further under Angle Beam Testing in Section 4.

Variables Limiting Transmission of Sound Waves: The distance that a wave of a given frequency and energy level will travel depends on the material through which it is traveling. As a general rule, materials that are hard and homogeneous will transmit sound waves more efficiently than those that are soft and heterogeneous or granular. Three factors govern the distance a sound wave will travel in a given medium: beam spreading, attenuation, and scattering. As the beam travels, the leading edge becomes wider, the energy associated with the wave is spread over a larger area, and eventually the energy dissipates. Attenuation is energy loss associated with sound transmission through a medium, essentially the degree to which energy is absorbed as the wave front moves forward. Scattering is random reflection of sound energy from grain boundaries and similar microstructure. As frequency goes up, beam spreading increases but the effects of attenuation and scattering are reduced. For a given application, transducer frequency should be selected to optimize these variables.

Reflection at a Boundary: When sound energy traveling through a material encounters a boundary with another material, a portion of the energy will be reflected back and a portion will be transmitted through. The amount of energy reflected, or reflection coefficient, is related to the relative acoustic impedance of the two materials. Acoustic impedance in turn is a material property defined as density multiplied by the speed of sound in a given material. For any two materials, the reflection coefficient as a percentage of incident energy pressure may be calculated through the formula Z2 - Z1

R = ----------

Z2 + Z1

where R = reflection coefficient (percentage of energy reflected) Z1 = acoustic impedance of first material Z2 = acoustic impedance of second material For the metal/air boundaries commonly seen in ultrasonic flaw detection applications, the reflection coefficient approaches 100%. Virtually all of the sound energy is reflected from a crack or other discontinuity in the path of the wave. This is the fundamental principle that makes ultrasonic flaw detection possible.

Angle of Reflection and Refraction: Sound energy at ultrasonic frequencies is highly directional and the sound beams used for flaw detection are well defined. In situations where sound reflects off a boundary, the angle of reflection equals the angle of incidence. A sound beam that hits a surface at perpendicular incidence will reflect straight back. A sound beam that hits a surface at an angle will reflect forward at the same angle. Sound energy that is transmitted from one material to another bends in accordance with Snell's Law of refraction. Again, a beam that is traveling straight will continue in a straight direction, but a beam that strikes a boundary at an angle will be bent according to the formula: Sin 1V1

-------- = -----

Sin 2V2

where 1 = incident angle in first material 2= refracted angle in second material V1 = sound velocity in first material V2 = sound velocity in second material This relationship is an important factor in angle beam testing, which is discussed in Section 4.

2. Ultrasonic Transducers In the broadest sense, a transducer is a device that converts energy from one form to another. Ultrasonic transducers convert electrical energy into high frequency sound energy and vice versa.

Cross section of typical contact transducer Typical transducers for ultrasonic flaw detection utilize an active element made of a piezoelectric ceramic, composite, or polymer. When this element is excited by a high voltage electrical pulse, it vibrates across a specific spectrum of frequencies and generates a burst of sound waves. When it is vibrated by an incoming sound wave, it generates an electrical pulse. The front surface of the element is usually covered by a wear plate that protects it from damage, and the back surface is bonded to backing material that mechanically dampens vibrations once the sound generation process is complete. Because sound energy at ultrasonic frequencies does not travel efficiently through gasses, a thin layer of coupling liquid or gel is normally used between the transducer and the test piece. There are five types of ultrasonic transducers commonly used in flaw detection applications:

- Contact Transducers -- As the name implies, contact transducers are used in direct contact with the test piece. They introduce sound energy perpendicular to the surface, and are typically used for locating voids, porosity, and cracks or delaminations parallel to the outside surface of a part, as well as for measuring thickness.

- Angle Beam Transducers -- Angle beam transducers are used in conjunction with plastic or epoxy wedges (angle beams) to introduce shear waves or longitudinal waves into a test piece at a designated angle with respect to the surface. They are commonly used in weld inspection. - Delay Line Transducers - Delay line transducers incorporate a short plastic waveguide or delay line between the active element and the test piece. They are used to improve near surface resolution and also in high temperature testing, where the delay line protects the active element from thermal damage.

- Immersion Transducers - Immersion transducers are designed to couple sound energy into the test piece through a water column or water bath. They are used in automated scanning applications and also in situations where a sharply focused beam is needed to improve flaw resolution. - Dual Element Transducers - Dual element transducers utilize separate transmitter and receiver elements in a single assembly. They are often used in applications involving rough surfaces, coarse grained materials, detection of pitting or porosity, and they offer good high temperature tolerance as well.

Further details on the advantages of various transducer types, as well as the range of frequencies and diameters offered, may be found in the transducer section of our web site.

3. Ultrasonic Flaw Detectors Modern ultrasonic flaw detectors such as the Panametrics-NDT Epoch series are small, portable, microprocessor-based instruments suitable for both shop and field use. They generate and display an ultrasonic waveform that is interpreted by a trained operator, often with the aid of analysis software, to locate and categorize flaws in test pieces. They will typically include an ultrasonic pulser/receiver, hardware and software for signal capture and analysis, a waveform display, and a data logging module. While some analog-based flaw detectors are still manufactured, most contemporary instruments use digital signal processing for improved stability and precision. The pulser/receiver section is the ultrasonic front end of the flaw detector. It provides an excitation pulse to drive the transducer, and amplification and filtering for the returning echoes. Pulse amplitude, shape, and damping can be controlled to optimize transducer performance, and receiver gain and bandwidth can be adjusted to optimize signal-to-noise ratios. Modern flaw detectors typically capture a waveform digitally and then perform various measurement and analysis function on it. A clock or timer will be used to synchronize transducer pulses and provide distance calibration. Signal processing may be as simple as generation of a waveform display that shows signal amplitude versus time on a calibrated scale, or as complex as sophisticated digital processing algorithms that incorporate distance/amplitude correction and trigonometric calculations for angled sound paths. Alarm gates are often employed to monitor signal levels at selected points in the wave train to flag echoes from flaws. The display may be a CRT, a liquid crystal, or an electroluminescent display. The screen will typically be calibrated in units of depth or distance. Multicolor displays can be used to provide interpretive assistance. Internal data loggers can be used to record full waveform and setup information associated with each test, if required for documentation purposes, or selected information like echo amplitude, depth or distance readings, or presence or absence of alarm conditions.

4. Procedure Ultrasonic flaw detection is basically a comparative technique. Using appropriate reference standards along with a knowledge of sound wave propagation and generally accepted test procedures, a trained operator identifies specific echo patterns corresponding to the echo response from good parts and from representative flaws. The echo pattern from an test piece may then be compared to the patterns from these calibration standards to determine its condition. - Straight Beam Testing -- Straight beam testing utilizing contact, delay line, dual element, or immersion transducers is generally employed to find cracks or delaminations parallel to the surface of the test piece, as well as voids and porosity. It utilizes the basic principle that sound energy traveling through a medium will continue to propagate until it either disperses or reflects off a boundary with another material, such as the air surrounding a far wall or found inside a crack. In this type of test, the operator couples the transducer to the test piece and locates the echo returning from the far wall of the test piece, and then looks for any echoes that arrive ahead of that backwall echo, discounting grain scatter noise if present. An acoustically significant echo that precedes the backwall echo implies the presence of a laminar crack or void. Through further analysis, the depth, size, and shape of the structure producing the reflection can be determined.

Sound energy will travel to the far side of a part, but reflect earlier if a laminar crack or similar discontinuity is presented.

In some specialized cases, testing is performed in a through transmission mode, where sound energy travels between two transducers placed on opposite sides of the test piece. If a large flaw is present in the sound path, the beam will be obstructed and the sound pulse will not reach the receiver. - Angle Beam Testing - Cracks or other discontinuities perpendicular to the surface of a test piece, or tilted with respect to that surface, are usually invisible with straight beam test techniques because of their orientation with respect to the sound beam. Such defects can occur in welds, in structural metal parts, and many other critical components. To find them, angle beam techniques are used, employing either common angle beam (wedge) transducer assemblies or immersion transducers aligned so as to direct sound energy into the test piece at a selected angle. The use of angle beam testing is especially common in weld inspection. Typical angle beam assemblies make use of mode conversion and Snell's Law to generate a shear wave at a selected angle (most commonly 30, 45, 60, or 70 degrees) in the test piece. As the angle of an incident longitudinal wave with respect to a surface increases, an increasing portion of the sound energy is converted to a shear wave in the second material, and if the angle is high enough, all of the energy in the second material will be in the form of shear waves. There are two advantages to designing common angle beams to take advantage of this mode conversion phenomenon. First, energy transfer is more efficient at the incident angles that generate shear waves in steel and similar materials. Second, minimum flaw size resolution is improved through the use of shear waves, since at a given frequency, the wavelength of a shear wave is approximately 60% the wavelength of a comparable longitudinal wave.

Typical angle beam assembly

The angled sound beam is highly sensitive to cracks perpendicular to the far surface of the test piece (first leg test) or, after bouncing off the far side, to cracks perpendicular to the coupling surface (second leg test). A variety of specific beam angles and probe positions are used to accommodate different part geometries and flaw types, and these are described in detail in appropriate inspection codes and procedures such as ASTM E-164 and the AWS Structural Welding Code.

Ultrasonic Material Analysisby Tom Nelligan Ultrasonic nondestructive testing is a versatile technique that can be applied to a wide variety of material analysis applications. While ultrasonic NDT is perhaps better known in its more common applications for thickness gauging, flaw detection, and acoustic imaging, high frequency sound waves can also be used to discriminate and quantify some basic mechanical, structural, or compositional properties of solids and liquids. Ultrasonic material analysis is based on a simple principle of physics: the motion of any wave will be affected by the medium through which it travels. Thus, changes in one or more of four easily measurable parameters associated with the passage of a high frequency sound wave through a material-transit time, attenuation, scattering, and frequency content-can often be correlated with changes in physical properties such as hardness, elastic modulus, density, homogeneity, or grain structure.

Principles Ultrasonic NDT utilizes the range of frequencies from approximately 20 KHz to over 100 MHz, with most work being performed between 500 KHz and 20 MHz. Both longitudinal and shear (transverse) modes of vibration are commonly employed, as well as surface (Rayleigh) waves and plate (Lamb) waves in some specialized cases. Because shorter wavelengths are more responsive to changes in the medium through which they pass, many material analysis applications will benefit from using the highest frequency that the test piece will support. Sound pulses are normally generated and received by piezoelectric transducers that have been acoustically coupled to the test material. In most cases a single transducer coupled to one side of the test piece serves as both transmitter and receiver (pulse/echo mode), although in some situations involving highly attenuating or scattering materials separate transmitting and receiving transducers on opposite sides of the part are used (through transmission mode). A sound wave is launched by exciting the transducer with either a voltage spike or a continuous wave impulse. The sound wave travels through the test material, either reflecting off the far side to return to its point of origin (pulse/echo), or being received by another transducer at that point (through transmission). The received signal is then amplified and analyzed. A variety of commercial instrumentation is available for this purpose, utilizing both analog and digital signal processing. A significant advantage of ultrasonic testing over other material analysis methods is that it can often be performed in-process or on-line. High frequency sound waves can often be successfully transmitted into and out of moving materials without direct contact, through the use of a water bath or water stream as a coupling medium. Measurements can also be performed within closed containers by coupling sound energy through the wall. Because sound waves penetrate through the test specimen, material properties are measured in bulk rather than just on the surface. It is sometimes even possible, through the use of selective gating, to analyze just one layer of a multi-layer, multi-material fabrication. The relevant measurement parameters will typically be one or more of the following: 1. Sound velocity/pulse transit time: Sound velocity is usually the easiest ultrasonic parameter to measure. The speed of sound in a homogenous medium is directly related to both elastic modulus and density; thus changes in either elasticity or density will affect pulse transit time through a sample of a given thickness. Additionally, varying degrees of nonhomogeneity may have an effect on sound velocity. 2. Attenuation: Sound energy is absorbed or attenuated at different rates in different materials, governed in a complex fashion by interactive effects of density, hardness, viscosity, and molecular structure. Attenuation normally increases with frequency in a given material. 3. Scattering: Sound waves reflect from boundaries between dissimilar materials. Changes in grain structure, fiber orientation, porosity, particle concentration, and other microstructural variations can affect the amplitude, direction, and frequency content of scattered signals. Scatter effects can also be monitored indirectly by looking at changes in the amplitude of a backwall echo or a through-transmission signal. 4. Frequency (Spectrum) content: All materials tend to act to some degree as a low pass filter, attenuating or scattering the higher frequency components of a broadband sound wave more than the lower frequency components. Thus, analysis of changes in the remaining frequency content of a selected broadband pulse that has passed through the test material can track the combined effects of attenuation and scattering as described above. In some applications ultrasonic data such as velocity can be directly used to calculate properties such as elastic modulus. In other cases, ultrasonic testing is a comparative technique, where in order to establish a test protocol in a given application it will be necessary to experimentally evaluate reference standards representing the range of material conditions being quantified. From such standards it will be possible to record how sound transmission parameters vary with changes in specific material properties, and then from this baseline information it will be possible to identify or predict similar changes in test samples.

Equipment: A wide variety of ultrasonic instrumentation can be used in material analysis applications. Sound velocity can be measured with simple hand-held ultrasonic thickness gauges, while velocity, attenuation, and scattering effects can all be observed with modern digital flaw detectors. Pulser/receivers with appropriate auxiliary equipment and ultrasonic imaging systems with appropriate software can be used to quantify all of these properties, and to perform spectrum analysis (frequency content) testing as well. For information on both instrumentation and transducer recommendations for specific tests, contact us.

Applications: The following is a summary of some specific material analysis applications where ultrasonic techniques have been used and documented. Extensive discussion, as well as bibliographies on the subject, can be found in the texts by ASNT1 and Lynnworth2. Both books are recommended as a source of further detailed information regarding both test procedures and specific instrumentation requirements.

Elastic moduli: Young's modulus and shear modulus in homogenous, nondispersive materials can be calculated from longitudinal wave and shear wave velocity (along with material density). Use of waveguides often permits measurement at high temperatures.

Nodularity in cast iron: Both the concentration of graphite in cast iron and its shape and form can be quantified through velocity measurements.

Cure rate in epoxies and concrete: The speed of sound in these materials changes as they harden; thus sound velocity measurements can be correlated to the degree of curing. Concrete testing usually requires access to both sides for through-transmission coupling.

Liquid concentrations: The mixture ratio of two liquids with dissimilar sound velocities can be correlated to the sound velocity of the solution at a given temperature.

Density of slurries: The liquid/solid mix ratio of slurries such as drilling mud and paper slurry at a given temperature can be correlated to sound velocity and/or attenuation.

Density in ceramics: Uniformity of density in both green and fired ceramics can be verified by means of sound velocity measurements.

Food products: A wide variety of tests have been reported, including age of eggs and potatoes, ripeness of fruits, fat content in beef, and percent of solids in milk. Generally these tests are both nondestructive and non-contaminating.

Polymerization in plastics: In plastics and other polymers, variations in molecular structure such as length or orientation of polymer chains will often result in corresponding changes in sound velocity and/or attenuation.

Particle or porosity size and distribution: Changes in the size or distribution of particles or porosity in a solid or liquid medium will affect the amplitude and frequency of scattered ultrasound. Grain size in metals: Changes in grain size or orientation in steel, cast iron, titanium, and other metals will cause changes in the amplitude, direction, and/or frequency content of scattered ultrasound.

Anisotropy in solids: Variations in sound velocity, scattering, and/or attenuation across different axes of a solid can be used to identify and quantify anisotropy.

Case hardening depth in steel: High frequency shear wave backscatter techniques can be used to measure the depth of case hardening.

Temperature measurement: Ultrasonic thermometry has been used to measure very high temperatures (over 3,000 degrees Celsius) by monitoring changes in sound velocity in a reference medium.

For Further Reading: 1) American Society for Nondestructive Testing, Nondestructive Testing Handbook, Volume 7, Ultrasonic Testing (ASNT, 1991) 2) Lynnworth, Lawrence C., Ultrasonic Measurements for Process Control (Academic Press, 1989)

1. What is it? Ultrasonic nondestructive testing, also known as ultrasonic NDT or simply UT, is a method of characterizing the thickness or internal structure of a test piece through the use of high frequency sound waves. The frequencies, or pitch, used for ultrasonic testing are many times higher than the limit of human hearing, most commonly in the range from 500 KHz to 20 MHz. 2. How does it work? High frequency sound waves are very directional, and they will travel through a medium (like a piece of steel or plastic) until they encounter a boundary with another medium (like air), at which point they reflect back to their source. By analyzing these reflections it is possible to measure the thickness of a test piece, or find evidence of cracks or other hidden internal flaws. 3. What sort of materials can be tested? In industrial applications, ultrasonic testing is widely used on metals, plastics, composites, and ceramics. The only common engineering materials that are not suitable for ultrasonic testing with conventional equipment are wood and paper products. Ultrasonic technology is also widely used in the biomedical field for diagnostic imaging and medical research. 4. What are the advantages of ultrasonic testing? Ultrasonic testing is completely nondestructive. The test piece does not have to be cut, sectioned, or exposed to damaging chemicals. Access to only one side is required, unlike measurement with mechanical thickness tools like calipers and micrometers. There are no potential health hazards associated with ultrasonic testing, unlike radiography. When a test has been properly set up, results are highly repeatable and reliable. 5. What are the potential limitations of ultrasonic testing? Ultrasonic flaw detection requires a trained operator who can set up a test with the aid of appropriate reference standards and properly interpret the results. Inspection of some complex geometries may be challenging. Ultrasonic thickness gages must be calibrated with respect to the material being measured, and applications requiring a wide range of thickness measurement or measurement of acoustically diverse materials may require multiple setups. Ultrasonic thickness gages are more expensive than mechanical measurement devices. 6. What is an ultrasonic transducer? A transducer is any device that converts one form of energy into another. An ultrasonic transducer converts electrical energy into mechanical vibrations (sound waves), and sound waves into electrical energy. Typically, they are small, hand-held assemblies that come in a wide variety of frequencies and style to accommodate specific test needs. 7. What is an ultrasonic thickness gage? An ultrasonic thickness gage is an instrument that generates sound pulses in a test piece and very precisely measures the time interval until echoes are received. Having been programmed with the speed of sound in the test material, the gage utilizes that sound velocity information and the measured time interval to calculate thickness via the simple relationship [distance] equals [velocity] multiplied by [time]. 8. How accurate is ultrasonic thickness gaging? Under optimum conditions, commercial ultrasonic gages can achieve accuracies as high as +/- 0.001 mm (0.00004"), with accuracies of +/- 0.025 mm (0.001") or better possible in most common engineering materials. Factors affecting accuracy include the uniformity of sound velocity the test material, the degree of sound scattering or absorption, the surface condition, and the accuracy and care with which the instrument has been calibrated for the application at hand. 9. Who uses ultrasonic gages? A major use for ultrasonic gages is the measurement of remaining wall thickness in corroded pipes and tanks. The measurement can be made quickly and easily without needing access to the inside or requiring the pipe or tank to be emptied. Other important applications include measuring the thickness of molded plastic bottles and similar containers, turbine blades and other precision machined or cast parts, small diameter medical tubing, rubber tires and conveyor belts, fiberglass boat hulls, and even contact lenses. 10. What is an ultrasonic flaw detector? Sound waves traveling through a material will reflect in predictable ways off of flaws such as cracks and voids. An ultrasonic flaw detector is an instrument that generates and processes ultrasonic signals to create a waveform display that can be used by a trained operator to identify hidden flaws in a test piece. The operator identifies the characteristic reflection pattern from a good part, and then looks for changes in that reflection pattern that may indicate flaws. 11. What kind of flaws can you find with one? A wide variety of cracks, voids, disbonds, inclusions, and similar problems that affect structural integrity can all be located and measured with ultrasonic flaw detectors. The minimum detectable flaw size in a given application will depend on the type of material being tested and the type of flaw under consideration. 12. Who uses ultrasonic flaw detectors? Ultrasonic flaw detectors are widely used in critical safety-related and quality-related applications involving structural welds, steel beams, forgings, pipelines and tanks, aircraft engines and frames, automobile frames, railroad rails, power turbines and other heavy machinery, ship hulls, castings, and many other important applications. 13. What other types of instruments are available? Ultrasonic imaging systems are used to generate highly detailed pictures similar to x-rays, mapping the internal structure of a part with sound waves. Phased array technology originally developed for medical diagnostic imaging is used in industrial situations to create cross-sectional pictures. Large scanning systems are used by the aerospace industry and metalworking suppliers to check for hidden flaws in both raw materials and finished parts. Ultrasonic pulser/receivers and signal analyzers are used in a variety of materials research applications.

Elastic Modulus MeasurementApplication: Measurement on Young's Modulus and Shear Modulus of Elasticity, and Poisson's ratio, in nondispersive isotropic engineering materials. Background: Young's Modulus of Elasticity is defined as the ratio of stress (force per unit area) to corresponding strain (deformation) in a material under tension or compression.

Shear Modulus of Elasticity is similar to the ratio of stress to strain in a material subjected to shear stress.

Poisson's Ratio is the ratio of transverse strain to corresponding axial strain on a material stressed along one axis. These basic material properties, which are of interest in many manufacturing and research applications, can be determined through computations based on measured sound velocities and material density. Sound velocity can be easily measured using ultrasonic pulse-echo techniques with appropriate equipment. The general procedure outlined below is valid for any nondispersive material and sample geometry (i.e., velocity does not change with frequency). This includes most metals, ceramics, and glasses as long as cross sectional dimensions are not close to the test frequency wavelength. Rubber usually cannot be characterized ultrasonically because of its high dispersion and nonlinear elastic properties. In the case of anisotropic materials, elastic properties vary with direction, and so does longitudinal and/or shear wave sound velocity. Generation of a full matrix of elastic moduli in anisotropic specimens typically requires six different sets of ultrasonic measurements. Equipment: The technique requires ultrasonic pulser-receiver such as a 5072PR or 5077PR, an ultrasonic thickness gage such as a Model 35DL or 38DL PLUS, or a flaw detector with velocity measurement capability such as the EPOCH series instruments. It also requires two transducers appropriate to the material being tested, for pulse-echo sound velocity measurement in longitudinal and shear modes, Commonly used transducers include an M112 or V112 broadband longitudinal wave transducer (10 MHz) and a V156 normal incidence shear wave transducer (5 MHz). These work well for many common metal and fired ceramic samples. Different transducers will be required for very thick, thin, or highly attenuating samples. The test sample may be of any geometry that permits clean pulse/echo measurement of sound transit time through a section on thickness. Ideally this would be a sample at least 0.5 in. (12.5 mm) thick, with smooth parallel surfaces and a width or diameter greater than the diameter of the transducer being used. Caution must be used when testing narrow specimens due to possible edge effects that can affect measured pulse transit time. Procedure: Measure the longitudinal and shear wave sound velocity of the test piece using the appropriate transducers. A Model 35DL or 38DL PLUS thickness gage can provide a direct readout of material velocity based on an entered sample thickness, and an EPOCh series flaw detector can measure velocity through a velocity calibration procedure. In either case, follow the recommended procedure for velocity measurement as described in the instrument's operating manual. If using a pulser/receiver, simply record the round-trip transit time through an area of known thickness with both longitudinal and shear wave transducers, and compute:

Convert units as necessary to obtain velocities expressed as inches per second or centimeters per second. (Time will usually have been measured in microseconds, so multiply in/uS or cm/uS by 106 to obtain in/S or cm/s.) The velocities thus obtained may be inserted into the following equations.

Note on units:If sound velocity is expressed in cm/S and density in g/cm3, then Young's modulus will be expressed in units of dynes/cm2. If English units of in/S and lbs/in3 are used to compute modulus in pounds per square inch (PSI), remember the distinction between "pound" as a unit of force versus a unit of mass. Since modulus is expressed as a force per unit area, when calculating in English units it is necessary to multiply the solution of the above equation by a mass/force conversion constant of (1 / Acceleration of Gravity) to obtain modulus in PSI. Alternately, if the initial calculation is done in metric units, use the conversion factor 1 psi = 6.89 x 104 dynes/cm 2. Another alternative is to enter velocity in in/S, density in g/cm 3, and divide by a conversion constant of 1.07 x 104 to obtain modulus in PSI.

For shear modulus simply multiply the square of the shear wave velocity by the density. Again, use units of cm/S and g/cm 3to obtain modulus in dynes/cm 2or English units of in/S and lbs/in 3and multiply the result by the mass/force conversion constant.

Bibliography For further information on ultrasonic measurement of elastic modulus, consult the following: 1. Moore, P. (ed.), Nondestructive Testing Handbook, Volume 7, American Society for Nondestructive Testing, 2007, pp. 319-321. 2. Krautkramer, J., H. Krautkramer, Ultrasonic Testing of Materials, Berlin, Heidelberg, New York 1990 (Fourth Edition), pp. 13-14, 533-534. Teora del funcionamiento

Sound waves are all around us, as mechanical vibrations carried by a medium such as air or water. Ultrasonic testing involves frequencies beyond the upper limit of human hearing, higher than 20 KHz and most commonly in the range from 500 KHz to 20 MHz, although higher and lower frequencies are sometimes used as well. The exact test frequency will be selected with respect to the specific application at hand. All ultrasonic thickness gages work by very precisely measuring how long it takes for a sound pulse that has been generated by a probe called an ultrasonic transducer to travel through a test piece. Sound waves will reflect from boundaries between dissimilar materials, such as the air or liquid on the inside of a steel pipe wall, so this measurement can normally be made from one side in a "pulse/echo" mode.

The transducer contains a piezoelectric element which is excited by a short electrical impulse to generate a burst of ultrasonic waves. The sound waves are coupled into the test material and travels through it until they encounter a back wall or other boundary. The reflections then travel back to the transducer, which converts the sound energy back into electrical energy. In essence, the gage listens for the echo from the opposite side. Typically this time interval is only a few millionths of a second. The gage is programmed with the speed of sound in the test material, from which it can then calculate thickness using the simple mathematical relationshipT = (V) x (t/2) where T = the thickness of the part V = the velocity of sound in the test material t = the measured round-trip transit timeIn some cases a zero offset is also subtracted to account for fixed delays in the instrument and soundpath.It is important to note that the velocity of sound in the test material is an essential part of this calculation. Different materials transmit sound waves at different velocities, generally faster in hard materials and slower in soft materials, and sound velocity can change significantly with temperature. Thus it is always necessary to calibrate an ultrasonic thickness gage to the speed of sound in the material being measured, and accuracy can be only as good as this calibration. This is normally done with a reference standard whose thickness is precisely known. In the case of high temperature measurements, it is also necessary to remember that sound velocity decreases with temperature, so for optimum accuracy the reference standard should be at the same temperature as the test piece.Higher frequencies have a shorter associated wavelength, permitting measurement of thinner materials. Lower frequencies with a longer wavelength will penetrate farther and are used to test very thick samples, or for materials like fiberglass and coarse-grained cast metals that transit sound waves less efficiently. Selection of an optimum test frequency often involves balancing these requirements for resolution and penetration. In the ultrasonic frequency range, sound waves are highly directional, and while they will travel freely through typical metals, plastics, and ceramics, they will reflect from an air boundary such as an inner wall or a crack.Sound waves in the megahertz range do not travel efficiently through air, so a drop of coupling liquid is used between the transducer and the test piece in order to achieve good sound transmission. Common couplants are glycerin, propylene glycol, water, oil, and gel. Only a small amount is needed, just enough to fill the extremely thin air gap that would otherwise exist between the transducer and the target.A block diagram of a typical ultrasonic thickness gage is seen below. The pulser, under the control of the microprocessor, provides a voltage impulse to the transducer, generating the outgoing ultrasonic wave. Echoes returned from the test piece are received by the transducer and converted back into electrical signals, which in turn are fed into the receiver amplifier and then digitized. The microprocessor-based control and timing logic both synchronizes the pulser and selects the appropriate echoes that will be used for the time interval measurement.If echoes are detected, the timing circuit will precisely measure a time interval in one of the modes discussed in Section 3, and then typically repeat this process several times to obtain an averaged reading. The microprocessor then uses this time interval measurement along with programmed sound velocity and zero offset values to calculate thickness. Finally, the thickness is displayed and updated at a selected rate.