In the OSTI Collections: Infrasound and Ultrasound

Dr. Watson computer sleuthing scientist.

Article Acknowledgement:

Dr. William N. Watson, Physicist

DOE Office of Scientific and Technical Information

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Just as infrared and ultraviolet light are light waves whose frequencies are below and above the respective extremes of red and violet light, infrasound and ultrasound are sound waves whose frequencies are below and above the nominal range of human hearing (20 hertz to 20 kilohertz).  While ultrasound has widely known uses in medical imaging, it has also been used in other ways, some of them only recently.  Infrasound also has some rather different uses.  Reports available through OSTI’s SciTech Connect and DoE PAGES describe some of the recent research involving both. 

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Geophysical uses of infrasound

 

Sound waves, like propagating waves of any type, carry information about the events that produce them.  Since volcanic activity produces sound waves, including infrasound, that travel through the earth, the U. S. Geological Survey has developed infrasound sensors to enable the monitoring of volcanos in American mountain ranges.  Two new sensor designs with “novel and durable construction and compact size”, built at the Geological Survey’s Cascades Volcano Observatory, were recently tested at Sandia National Laboratories to see how well they met their design requirements.  “USGS VDP Infrasound Sensor Evaluation”[SciTech Connect] describes how three examples of each sensor type were checked for their sensitivity, self-generated noise, and output’s proportionality to input infrasound pressure.  Significant variation in most of these variables was found among the three sensors of one type (VDP100), with much less variation among the other three sensors (type VDP250).  

Model infrasound sensorsFigure 1.  Model infrasound sensors, three of type VDP100 (top, upper left, and lower left of picture) and three of type VDP250 (bottom, lower right, and upper right), placed in an infrasound chamber for testing.  Another already-calibrated infrasound sensor in the center was used as a reference.  (From “USGS VDP Infrasound Sensor Evaluation”[SciTech Connect], p. 12.) 

Infrasound can also be produced by explosions, including nuclear explosions conducted underground.  Some of the ongoing efforts to detect, locate, and characterize such explosions infrasonically[Wikipedia] are described in reports from Los Alamos National Laboratory, some about a series of experiments with nonnuclear explosions and infrasound sensors, and others about a set of algorithms for infrasound analysis. 

The experiment series, known as Source Physics Experiments[SciTech Connect], is intended to develop the ability to monitor compliance with the terms of the Comprehensive Nuclear-Test-Ban Treaty[Wikipedia] by understanding the acoustic effects of low-yield nuclear explosions, including their infrasound production.  The experiments themselves use nonnuclear explosions to study these effects.  Some of the more recent Source Physics Experiments, described in the slide presentations “SPE-5 Infrasound Predictions”[SciTech Connect] and “Tethered Pressure Measurement”[SciTech Connect], were based on the realization that sound waves from underground explosions should have larger amplitudes at high elevations than along the ground, and so involved sensors placed on tethered balloons near a nonnuclear explosion site.  “SPE-5 Infrasound Predictions” describes what the experimenters expected to find in Source Physics Experiment 5 based on the results of earlier experiments.  Apparently, though, this experiment wasn’t as successful as hoped; according to “Tethered Pressure Measurement”, the wind load on the balloon prevented its achieving suitable lift, so Source Physics Experiment 6 was planned to overcome this problem by replacing the balloon with a more aerodynamically stable Helikite[Wikipedia], a balloon-kite combination that would gain additional lift from the wind.  The data to be gathered from the experiments would be compared with related data from other experiments and with mathematical models of infrasound from explosions. 

Figure 2. Top left and top middle: Tethered balloon for holding infrasound sensor aloft in the fifth Source Physics Experiment (SPE-5), which turned out to have less lift than planned because of the wind load on it. To overcome this problem, a balloon-kite combination (a Helikite[Wikipedia]) with greater lift and aerodynamic stability, was to be used for SPE-6. Top right: A Helikite. Bottom: air-pressure variations expected in SPE-5 at 50 and 100 meters above ground level based on data from SPE-3. (Top left and top middle: from “SPE-5 Infrasound Predictions”[SciTech Connect], p. 9; top right: “Helikite Lifting Gyro-Stabilised Camera” (https://commons.wikimedia.org/wiki/File:Helikite_Lifting_Gyro-Stabilised_Camera.jpeg) from Wikimedia Commons, used under the Creative Commons Attribution-Share Alike 3.0 Unported License CC BY-SA 3.0; bottom: from “SPE-5 Infrasound Predictions”, p. 6.)

In principle, the locations of explosions or other energetic events can be found if the infrasound they generate is registered by widely separated detectors, since the infrasound is still detectable after travelling hundreds or thousands of kilometers through the atmosphere.  But how the infrasound propagates through the atmosphere depends on the atmosphere’s state, which is incompletely known.  Thus any estimate of an infrasound source’s location from the responses of widely separated detectors has to involve probabilities.  Los Alamos scientists have been developing an algorithm, which is based on a 2010 proposal to combine data from different infrasound detectors according to Bayes’ theorem in probability theory[OSTI], to work out where an infrasound source is likely to be, given the uncertainties in how the atmosphere is carrying the sound.  The underlying concepts are described in the Geophysical Journal International paper “Improved Bayesian Infrasonic Source Localization for regional infrasound”[DoE PAGES], while the algorithm, written in the Python computer language[Wikipedia], is described in the slide presentation “InfraPy: Python-Based Signal Analysis Tools for Infrasound”[SciTech Connect]


Figure 3. Top: Probability density functions assumed for various infrasound speeds v through different layers of the atmosphere (orange, blue, red) and through all media (green) between a seismic event and a distant detector. Middle and bottom: Likely locations of seismic events are inferred (with 95% and 99% confidence regions[Wikipedia]) from timing of infrasound received at multiple distant detectors and the assumed probability density function. (Top: From “Improved Bayesian Infrasonic Source Localization for regional infrasound”[DoE PAGES], p. 9 of 29; middle and bottom:  from “InfraPy: Python-Based Signal Analysis Tools for Infrasound”[SciTech Connect], slide 7 of 15.) 

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Ultrasound in nuclear technology

The uses of infrasound described above largely involve identifying an infrasound’s source with remotely located instruments.  Ultrasound uses, on the other hand, generally have less to do with where the ultrasound radiates from than what it’s directed toward.  Since ultrasound is a vibration generated by alternating stresses in a material, observing how a material vibrates when exposed to different frequencies of ultrasound tells something about its response to stress.  This actually tells us a lot, since the elastic response of many solids to stress—how much they stretch, contract, or bend under tension, compression, or shearing—is approximately proportional to the stress if it isn’t too large:  how the solid stretches &c. in response to the small stresses of ultrasonic waves indicates how much it will do so in response to much larger stresses.  And the ultrasound frequencies that a solid responds to most strongly can be determined quite precisely, since wave frequencies are among the most precisely measured of all physical quantities with present technology[SciTech Connect, p. 4]

Measuring the frequencies at which rocks and minerals resonate most strongly is one way that geophysicists determine their elastic properties.  Somewhat later, the same technique was independently thought of and applied to a material used in nuclear power plants and weapons:  plutonium.  Knowing the elastic properties of plutonium is essential to the successful design and maintenance of plutonium-based weapons and power plants, but those properties change as the plutonium ages.  A sample of plutonium steadily irradiates itself by its own gradual radioactive decay, so that whenever a plutonium atom well within the sample emits radiation, the radiation displaces many other atoms (and even heats them above plutonium’s melting point).  This alters how much strain a plutonium sample will undergo when a given stress is applied to it. 

These considerations, and findings from experiments they motivated, are described in slide presentations entitled “Resonant Ultrasound Spectroscopy”[SciTech Connect] and “Precision Plutonium Thermodynamics”[SciTech Connect].  (A solid’s mechanical response to stress, being affected by its temperature, counts as one of the solid’s thermal properties; hence the second presentation’s title.)  The experiments involved not only precise measurement of ultrasound frequencies, but precise information about the shape of the material sample being tested and an algorithm that accurately calculates the sample’s elastic parameters from the frequencies at which it resonates.  Such calculations have been made for four of plutonium’s different solid phases[Wikipedia], over temperatures ranging from a few kelvins above absolute zero[Wikipedia] to roughly twice room temperature (i.e., several hundred kelvins).  The experiments have revealed many phenomena related to plutonium aging, such as how a plutonium sample’s resonant frequency increases logarithmically[Wikipedia] with time for a few days and then increases linearly[Wikipedia] with time afterwards. 

Figure 4.  The resonant frequency of a self-irradiating sample of plutonium increased approximately logarithmically over a 12-day period, then increased approximately linearly thereafter. (From “Precision Plutonium Thermodynamics”[SciTech Connect], p. 19.) 

Plutonium isn’t the only nuclear-technology material that can be explored with ultrasound.  A report by researchers at the University of Minnesota and Oak Ridge National Laboratory describes first steps toward ultrasonic inspection of nuclear power plants’ concrete structures.  The problem of interest here is the slow reaction[Wikipedia] of the concrete’s alkaline cement paste[Wikipedia] with noncrystalline silica[Wikipedia] in aggregates[Wikipedia] when water is present, as it is for portions of the structure in groundwater. The reaction produces an expanding gel in the aggregates that, in turn, produces microcracks in the aggregates and adjacent cement paste which are detrimental to the concrete’s strength and can shorten the power plant’s operating years. The only techniques that have previously been available for detecting alkali-silica reactions are destructive to concrete. Experiments with a nondestructive method are described in “Linear Array Ultrasonic Test Results from Alkali-Silica Reaction (ASR) Specimens”[SciTech Connect]: four concrete slabs with varying alkali-silica reaction damage manifested that damage by their responses to ultrasound. The same study found that an algorithm developed to quantify concrete freeze-thaw damage was also useful for quantifying alkali-silica reaction damage. 



Figure 5. Top left: Ultrasonic scanner on a concrete slab of dimensions 0.9 m x 1.2 m x 0.2 m.  Top middle: North-to-south, east-to-west scan pattern.  Top right: Reconstructed intensity of incident and reflected ultrasound waves along the fifth-from-east scan line’s cross section, with the nonuniformity of the highest-intensity wave region (shown as red) indicating slight damage from alkali-silica reactions.  Note expansion of depth scale relative to length scale.  Center right: Surface cracks in sample highlighted in red.  Center left:  Close-up of sample’s southwest corner.  Bottom: Wave propagation in plain concrete specimen (left) and in specimen with three 12-millimeter vertical cracks on the surface (right).  (From “Linear Array Ultrasonic Test Results from Alkali-Silica Reaction (ASR) Specimens”[SciTech Connect], pp. 11, 17, 21, and 25.) 

Just as material samples can be inspected ultrasonically, so can complete devices and components.  Some nuclear reactors use liquid sodium flowing through heat exchangers to transfer thermal energy from the reactor cores to steam or some other fluid, which then uses the energy to drive turbines attached to electric generators and thus produce electric power.  If air were to get into the liquid sodium under certain conditions, sodium oxide might form and be deposited in the heat-exchanger channels, which could clog the channels up, thus slowing or stopping the sodium flow and its removal of reactor heat to the turbine-driving fluid.  This could not only slow or stop the generator, but could let the reactor core overheat.  The most serious effects would be likely in systems whose heat-exchange channels have diameters under 5 millimeters, such as some that are being designed to use “fast neutrons”—chain-reaction neutrons that move much faster than they would at the reactor core’s temperature[nuclearglossary]—and to also use carbon dioxide in a supercritical state[Wikipedia; Wikipedia] as the fluid that drives the generator turbines. 

A reliable mathematical model of sodium-oxide deposition in heat-exchanger channels requires input from measurements of a physical model exhibiting the actual phenomenon.  A six-month project of first steps toward acquiring such measurements is described in “Modeling and Validation of Sodium Plugging for Heat Exchangers in Sodium-cooled Fast Reactor Systems”[SciTech Connect].  In this project, prototype instrumentation was made for applying an ultrasound source to the location of a sodium oxide deposit and timing the echo from the deposit’s inner surface to measure its thickness.  (Other instrumentation was also made to detect growth in a sodium oxide deposit by the increase of its electrical resistivity.)  However, the instruments were only partially tested during the project period, because a testing facility intended to exhibit plugging couldn’t be operated under plugging conditions at the time, so the only confirmation of the instruments’ capability was their success in detecting similar material discontinuities in mockups of the testing facility.  Testing with sodium oxide plugging in the actual test facility was recommended by the report’s authors for future work. 

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Ultrasonic inspection of additively manufactured parts and related endeavors

Ultrasound is useful for more than nuclear technology.  Some different uses have been found in the field of additive manufacturing[OSTI], in which components are made by adding pieces of material together (the pieces being molecules or atoms in some cases) rather than subtracting unwanted portions from larger pieces of raw material. 

One of the best-known methods of additive manufacturing, the printing of successive material layers on top of already-printed ones to build a three-dimensional object, can be aided by ultrasonic inspection of parts while they are being built.  A common type of defect in 3-D printed plastic parts occurs when successive layers of plastic cool too much to bond together properly.  This results in a waste of material, a potential safety hazard if the parts’ users rely on their solidity—and a change in the parts’ resonant frequencies, which suggests that ultrasonic inspection of the parts could catch such defects as they are being made. 

However, ultrasonically inspecting 3-D printed parts (or any inhomogeneous parts) can be problematic.  Higher-frequency ultrasound would be strongly absorbed, and the parts’ internal structures would cause echoes.  Also, along the thermal gradient of a freshly deposited, cooling line of plastic, the plastic’s elasticity and sound-absorbing properties vary, which means its response to ultrasound would vary along that gradient as well, until the plastic cooled and the gradient disappeared.  These problems are suggested, in the Los Alamos slide presentation “In-Process Ultrasonic Inspection of Additively Manufactured Parts”[SciTech Connect], as possible reasons for the small amount of work being done thus far to use ultrasound to test additively manufactured parts. 

All the same, the authors of that presentation have advanced this method.  In their experiments, they exposed small plastic towers to ultrasound as they were being printed, some of which had a fault—an extra interlayer space—introduced halfway through their construction, and they observed how both whole and defective towers vibrated in response to the ultrasound.  The actual vibrational patterns measured were those of the base plate that the plastic towers were being built on.  These vibrations changed as the towers grew, but the changes for whole and defective towers diverged from each other past the halfway points of the towers’ construction (Figure 6, bottom). 



Figure 6.  Top: Within the red line on the table top, a 3-D printer with a printed rectangular plastic tower in the center of its build plate, with a single ultrasound transmitter and three ultrasound receivers around it. To the printer’s immediate left is a data acquisition device; an amplifier is at the far left of the table. Center: Schematic of the tower and the ultrasound transmitter and receivers displayed in the inset photo.  Sinusoidal ultrasonic chirp from the transmitter is distorted by interaction with the tower, with result received by the three receivers. Bottom: A tower’s frequency response changes as it’s built, but the nature of the change will differ according to whether the tower remains “healthy” or has a flaw introduced during its construction. Note how the frequency responses of towers built with normal and weak interlayer bonds halfway through their construction change differently for the second halves of their construction periods. (From “In-Process Ultrasonic Inspection of Additively Manufactured Parts”[SciTech Connect], pp. 8, 10, and 19.) 

A later slide presentation by another Los Alamos researcher (“2016 Research Final Presentation”[SciTech Connect]) illustrates some further experiments on more complex objects with deliberately introduced defects, with their vibrations measured more directly by laser light. The objects’ rapid back-and-forth oscillations induced by ultrasound would cause the frequency of a reflected laser beam to also oscillate by the Doppler effect[Wikipedia]; measuring the laser frequency’s oscillations thus indicated the amplitudes and frequencies of the object’s own vibrations[Wikipedia]. According to Section 1 of the presentation, both the amplitudes and frequencies of ultrasound vibrations were higher at the defect sites because those sites were less constrained by attachment to the larger mass and stiffness of the rest of the object.  Amplitude variations provided the most accurate indication of the size, shape, and nature of a given defect, while frequency variations could detect damage in later print layers. 

Figure 7.  Left:  A method of measuring the ultrasonic response of a 3-D-printed object differently from the method illustrated in Figure 6.  The printer’s base plate is ultrasonically vibrated by a transducer, but the object’s own induced vibrations are measured, with a Laser Doppler Vibrometer[Wikipedia], by their effect on a laser beam that scans the part’s surface.  Right:  As the object is built, amplitude and frequency variations indicate where defects occur; amplitudes and frequencies of ultrasound vibrations are higher there because those sites are less constrained by attachment to the larger mass and stiffness of the rest of the object.  (After “2016 Research Final Presentation”[SciTech Connect], pp. 7, 11, and 12 of 33.)

While the laser in the reported experiments was directed at a single point of the printed object, the report’s author noted an interest in seeing if a useful inspection could be made by splitting the laser with mirrors to scan the object simultaneously at different points on all sides.  The second and third sections of the presentation described similar success with ultrasound-based detection of damage or deviation in objects set at oblique angles to the laser—an arrangement hardly avoidable when inspecting curved objects—and in aerospace components made of composite materials.  One common phenomenon of wave physics was naturally illustrated by the experiments with composites:  smaller defects were more apparent when the composites were vibrated at higher frequencies that produced shorter-wavelength ultrasound waves.  In general, sound or light or any other type of wave will be disturbed more strongly by encounters with features at least as large as one wavelength than features much smaller than one wavelength.  On the other hand, more ultrasonic noise was found with higher frequencies, which obscured the defects. 

 

Applying the laser-and-ultrasound technique to measure a feature that isn’t a defect is the subject of another slide presentation, “A Study on Melt Pool Depth Monitoring of Direct Energy Additive Manufacturing Using Laser-Ultrasound”[SciTech Connect].  The “melt pool” mentioned in the title is where a piece of metallic 3-D “printing” is melted by a laser beam so the next layer of metal powder will fuse onto it.  The pool of melted metal needs to be just large enough to accomplish that purpose.  Too small, and the added powder won’t fuse into place; too large, and the part might malform.  With the technique described, the melt pool’s depth can be measured continually while the metal part is being built.  The melt-pool presentation includes a second slide presentation on a mathematical study of the technique’s ability to detect damage. 

Figure 8.  Schematic of a 3-D printing technique using a metal powder for “ink”.  A laser beam is used to melt the point at which powder is added (the “melt pool”) so the powder will bond to the portion of the object already printed; another laser is used to detect how the melt pool responds to ultrasonic vibrations induced by the Pb[ZrxTi1-x]O3 (PZT)[Wikipedia] actuator and so measure the melt pool’s depth.  (From “A Study on Melt Pool Depth Monitoring of Direct Energy Additive Manufacturing Using Laser-Ultrasound”[SciTech Connect], p. 4.) 

While the Doppler effect is observed on one laser beam to deduce the melt pool’s depth from the resonant ultrasound frequency, another laser beam is used to melt the part of the metal that additional powder is fused to.  Lasers thus constitute an energy source as well as an information channel.  Ultrasound can also perform both functions.  While ultrasound’s informational function is evident in the preceding reports, one further report mentions how ultrasound provides energy for a fabrication process. 

The process is another additive one, in which successive metal tape layers are welded onto already-welded layers as they are vibrated by a sonotrode—a solid piece of metal that resonates at an appropriate ultrasonic frequency.[Wikipedia]  The process allows even dissimilar metals to be welded together, which among other things allows sensors to be embedded in a metal substrate.  One problem with this method of ultrasonic welding is that foils of hard metal tend to form nuggets that stick to the sonotrode instead of the metal underneath, and require periodic removal for the sonotrode to keep working properly.  The report title describes its authors’ approach to solving the problem (“Development of coatings for ultrasonic additive manufacturing sonotrode using laser direct metal deposition process”[SciTech Connect]).   The authors developed a defect-free metal coating that adhered well to sonotrodes—a significant first step. 

Often it takes more than one breakthrough to completely solve a technical problem.  Accordingly, further steps were expected from the beginning, and could be defined once successful first steps had been made.  The sonotrode coating’s thickness still needed to be optimized so it could sufficiently resist nugget formation while the sonotrode kept a resonant frequency of 20 kilohertz (the nominal upper limit of typical human hearing and thus the lowest ultrasound frequency).  This would require tests under ultrasonic additive manufacturing conditions at various temperatures and loads.  Since the sonotrode undergoes significant local temperature increases during its fabrication, the coating’s inhibition of nugget formation might deteriorate.  If it did, the researchers would try enhancing the coating’s resistance by alloying it with a high-hardness ceramic. 

At the same time, the authors’ project exhibited another typical situation:  the potential of a solution to do more than solve the immediate problem.  Not only were benefits expected in the field of ultrasonic additive manufacturing if nugget formation could be prevented, but the embedding of sensors and claddings in functional components that this would facilitate were expected to be useful in the nuclear, automotive, and aerospace industries. 

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References

Wikipedia

Comprehensive Nuclear-Test-Ban TreatyMonitoring

Allsopp Helikite

Python (programming language)

Confidence interval

Allotropes of plutonium (Plutonium phases)

Kelvin

Logarithmic growth

Linearity

Alkali-silica reaction

Portland cement

Silicon dioxide (Silica)

Construction aggregate

Supercritical fluid

Supercritical carbon dioxide

Doppler effect

Laser Doppler vibrometer

Lead zirconate titanate

Sonotrode



Reports available from OSTI’s SciTech Connect

“USGS VDP Infrasound Sensor Evaluation” [Metadata]

2016-10-01

Sandia National Laboratories



“SPE-5 Infrasound Predictions” [Metadata]

2016-04-05

Los Alamos National Laboratory

Sandia National Laboratories



“Tethered Pressure Mesurement” [Metadata]

2016-09-15

Los Alamos National Laboratory

Air Resources Laboratory/Special Operations and Research Division, National Oceanic and Atmospheric Administration



“InfraPy: Python-Based Signal Analysis Tools for Infrasound” [Metadata]

2016-05-31

Los Alamos National Laboratory



“Resonant Ultrasound Spectroscopy” [Metadata]

2016-04-27

Los Alamos National Laboratory



“Precision Plutonium Thermodynamics” [Metadata]

2016-04-04

Los Alamos National Laboratory



“Linear Array Ultrasonic Test Results from Alkali-Silica Reaction (ASR) Specimens” [Metadata]

2016-04-01

Oak Ridge National Laboratory

University of Minnesota



“Modeling and Validation of Sodium Plugging for Heat Exchangers in Sodium-cooled Fast Reactor Systems” [Metadata]

2016-06-29

Westinghouse Electric Company LLC

Argonne National Laboratory



“In-Process Ultrasonic Inspection of Additively Manufactured Parts” [Metadata]

2016-01-27

Society of Experimental Mechanics, 34th Annual International Modal Analysis Conference, January 25-28, 2016

Los Alamos National Laboratory

Michigan Technological University

Stanford University

Tennessee Technological University



“2016 Research Final Presentation” [Metadata]

2016-08-16

Los Alamos National Laboratory



“A Study on Melt Pool Depth Monitoring of Direct Energy Additive Manufacturing Using Laser-Ultrasound” [Metadata]

Presentation at departmental seminars at LANL and at LANL-CNU Engineering Institute, Korea.

2016-03-14

Los Alamos National Laboratory

Chonbuk National University [English] [Chinese] [Japanese] [French]



“Development of coatings for ultrasonic additive manufacturing sonotrode using laser direct metal deposition process” [Metadata]

2016-10-01

Oak Ridge National Laboratory



Report available from DoE PAGES

“Improved Bayesian Infrasonic Source Localization for regional infrasound” [Metadata]

2015-10-20

Geophysical Journal International, Volume 203, Issue 3, pp. 1682-1693

Los Alamos National Laboratory

Sandia National Laboratories



Additional references

Cascades Volcano Observatory, U. S. Geological Survey

The Language of the Nucleusfast neutron

In the OSTI Collections:  Bayesian Inference

In the OSTI Collections:  3-D Printing and Other Additive Manufacturing Technologies

Time-lapse video of an additively manufactured metal part[YouTube] (cited by “A Study on Melt Pool Depth Monitoring of Direct Energy Additive Manufacturing Using Laser-Ultrasound”[SciTech Connect])

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