1aSAb4 – Seismic isolation in Advanced Virgo gravitational wave detector – Valerio Boschi

Seismic isolation in Advanced Virgo gravitational wave detector

Valerio Boschi – valerio.boschi@ego-gw.it

European Gravitational Observatory
Istituto Nazionale di Fisica Nucleare
Sezione di Pisa
Largo B. Pontecorvo, 3
56127 Pisa, Italy

 

Popular version of paper 1aSAb4

Presented Monday morning, May 13th, 2019

177th ASA Meeting, Louisville, KY

 

Imagine to drop a glass of water in the ocean. Due to that the global level of all the seas on the Earth will increase by an extremely small amount. A rough estimate would lead you to this amazingly tiny displacement: 10-18 m !! This length is equivalent to the sensitivity of current gravitational wave (GW) detectors.

GWs are ripples of space-time, produced by the collapse of extremely dense astrophysical objects, like black holes or neutron stars. Those signals induce on the matter small variation of length (less than 10-18 m at 100 Hz) that can be detected only by the world most precise rulers, the interferometers.

Second generation gravitational wave interferometers like the Advanced Virgo experiment, shown in fig. 1, which is based in Cascina, Italy and the two US-based Advanced LIGO detectors, are collecting GW signals since 2015 opening the doors of the so-called multi-messenger astronomy.

In order reach the required level of sensitivity of current interferometers many disturbances need to be strongly reduced. Seismic noise if not attenuated would represent the main limitation of current detectors. In facts, even in the absence of local or remote earthquakes, ground moves by mm in the frequency region between 0.3 and 0.4 Hz. This motion, called microseism, is caused by the continuous excitation of the Earth crust produced by the sea waves.

In this conference contribution we will present an overview of the seismic isolation systems used in Advanced Virgo GW interferometer. We will concentrate on the so-called super-attenuator, the seismic isolator used for all the detector main optical components, shown in fig. 2. This complex mechanical device is able to provide more than 12 orders of magnitude of attenuation above a few Hz. We will also describe its high-performance digital control system and the control algorithms implemented with it. Thanks to the performance and reliability of this system the current duty cycle of Advanced Virgo, is almost 90 %.

Figure 1 Aerial View of Advanced Virgo (EGO/Virgo collaboration)

 

Figure 2 Inside view of a super-attenuator

 

 

 

 

1pSA8 – Thermoacoustics of solids – Can heat generate sound in solids? – Haitian Hao

Acoustical balance between the singers and the orchestra in the Teatro Colón of Buenos Aires

Haitian Hao – haoh@purdue.edu
Mech. Eng., Purdue Univ.
Herrick Labs,
177 S. Russell St.
Rm.1007
West Lafayette, IN 47906

Carlo Scalo
Mech. Eng., Purdue Univ.
Herrick Labs,
177 S. Russell St.
Rm.1007
West Lafayette, IN 47906

Mihir Sen
Aerosp. and Mech. Eng.
Univ. of Notre Dame,
Notre Dame, IN

Fabio Semperlotti
Mech. Eng.
Purdue Univ.
West Lafayette, IN

Popular version of paper 1pSA8, “Thermoacoustic instability in solid media” – Haitian Hao, Carlo Scalo, Mihir Sen, and Fabio Semperlotti
Presented Monday, May 07, 2018, 2:45pm – 3:00 PM, Greenway C
175th ASA Meeting, Minneapolis

Many centuries ago glass blowers observed that sound could be generated when blowing through a hot bulb from the cold end of a narrow tube. This phenomenon is a result of thermoacoustic oscillations: a pressure wave propagating in a compressible fluid (e.g. air) can sustain or amplify itself when being provided heat. To date, thermoacoustic engines and refrigerators have had remarkable impacts on many industrial applications.

After many centuries of thermoacoustic science in fluids, it seems natural to wonder if such a mechanism could also exist in solids. Is it reasonable to conceive thermoacoustics of solids? Can a metal bar start vibrating when provided heat?

The study of the effects of heat on the dynamics of solids has a long and distinguished history. The theory of thermoelasticity, which explains the mutual interaction between elastic and thermal waves, has been an active field of research since the 1950s. However, the classical theory of thermoelasticity does not address instability phenomena that can arise when considering the motion of a solid in the presence of a thermal gradient. In an analogous way to fluids, a solid element contracts when it cools down and expands when it is heated up. If the solid contracts less when cooled and expands more when heated, the resulting motion will grow with time. In other terms, self-sustained vibratory response of a solid could be achieved due to the application of heat. Such a phenomenon would represent the exact counterpart in solids of the well-known thermoacoustic effect in fluids.

By using theoretical models and numerical simulations, our study indicates that a small mechanical perturbation in a thin metal rod can give rise to sustained vibrations if a small segment of the rod is subject to a controlled temperature gradient. The existence of this physical phenomenon in solids is quite remarkable, so one might ask why it was not observed before despite the science of thermoacoustics have been known for centuries.

“Figure 1. The sketch of the solid-state thermoacoustic device and the plot of the self-amplifying vibratory response.”

It appears that, under the same conditions of mechanical excitation and temperature, a solid tends to be more “stable” than a fluid. The combination of smaller pressure oscillations and higher dissipative effects (due to structural damping) in solids tends to suppress the dynamic instability that is at the origin of the thermoacoustic response. Our study shows that, with a proper design of the thermoacoustic device, these adverse conditions can be overcome and a self-sustained response can be obtained. The interface conditions are also more complicated to achieve in a solid device and dictates a more elaborate design.

Nonetheless, this study shows clear theoretical evidence of the existence of the thermoacoustic oscillations in solids and suggests that applications of solid-state engines and refrigerators could be in reach within the next few years.

2pSA – Seismic-infrasound-acoustic-meteorological sensors to dynami-cally monitor the natural frequencies of concrete dams – Henry Diaz – Alvarez

2pSA – Seismic-infrasound-acoustic-meteorological sensors to dynami-cally monitor the natural frequencies of concrete dams

Henry Diaz – Alvarez – henry.diaz-alvarez@usace.army.mil

Luis De Jesus-Diaz – Luis.A.DeJesus-Diaz@erdc.dren.mil

Vincent P. Chiarito – Vincent.P.Chiarito@usace.army.mil

Chris P. Simpson – Christopher.P.Simpson@usace.army.mil

Mihan H. McKenna – Mihan.H.McKenna@usace.army.mil

 

U.S. Army Engineer Research and Development Center

Geotechnical and Structures Laboratory

3909 Halls Ferry Road,

BLDG 5014Vicksburg, MS 39180

 

Popular version of paper 2pSA, “Seismic-Infrasound-Acoustic-Meteorological Sensors to Dynamically Monitor the Natural Frequencies of Concrete Dams”

Presented Tuesday afternoon, May 8, 2018, 1:00-3:45 PM

175th ASA Meeting, Minneapolis

 

The U.S. Army Engineer Research and Development Center (ERDC) is leading research using seismic-infrasound-acoustic-meteorological (SIAM) arrays to determine structural characteristics of critical infrastructure. Fundamental, vibrational modes of motion for large structures, such as dams, are usually in the sub-audible, infrasound frequency range. Infrasound is low-frequency, sub-audible sound, traditionally defined to be between 0.1 to 20 Hz and below the range of human hearing from 20 Hz to 20,000 Hz [1]. To validate the concept and its potential use for monitoring flood control structures, a structural evaluation was conducted at the Portugues Dam in Ponce, Puerto Rico.

The dam’s dynamic properties were studied prior to the deployment of SIAM arrays using detailed finite element models (FEM) assembled in COMSOL Multiphysics software [2].  The natural frequencies of 4.8 Hz and 6.7 Hz, respectively, were determined for the lower modes of vibrations, shown in Figure 1[3].

Figure 1. Modal analysis of the Portugues dam using COMSOL  multiphysisc software. Vibration mode 1 (a) and vibration mode 2 (b)

To validate the results from the FEM dynamic analysis, Performance Based Testing (PBT) was conducted at the dam.  The PBT consisted of measuring the crest input and output response to an ambient excitation using an array of accelerometers along each monolith.

Power Spectra Density (PSD) analysis of the data from accelerometers was used to confirm the natural resonance frequencies in the dam (Figure 2), and was also used to develop an estimate of the response shape associated with the fundamental modes of vibration developed in the FEM (Figure 1).

Figure 2. Power Spectra Density (PSD) analysis from accelerometers gages due to ambient excitation of the dam.

Instrumentation for a SIAM array consists of five IML infrasound sensors each with four porous hose wind filters (Figure 3), three audible microphones, a 1 Hz triaxial seismometer, and two RefTek 130s digitizers. To triangulate the specific source location of the infrasound, at least three SIAM arrays are required during the field data collecton. Typically one array in deployment also utilizas a bi-level meterorogical station.

Figure 3.Example of one SIAM array used during test in the Cerrillo area.

A total of three SIAM arrays were used to monitor the dam at distances of 0.46 km Upstream (CPBBR), 0.2 km Downstream (Gazebo), and 6.0 km (Cerrillo) from the dam as shown in Figure 4.

Figure 4. Illustration of the SIAM array location during the data collection.

An example time-series from a single infrasound sensor at the downstream array with ambient excitation highlighted is shown in Figure 5. The PSD analysis for ambient excitation in Figure 6. shows correlated energy at frequencies 4.3 Hz and 6.0 Hz, which align with the vibrations modes measured on structure with acelerometers. Results from both the FEM using COMSOL Multiphysics agree with the infrasound field experimental data and were used to validate to SIAM array data collected.

 

 

Figure 5. Raw data from a single infrasound sensor located at the downstream array

Figure 6. PSD analysis from infrasound sensors, located at the Downstream array, ambien excitation.

Performing an infrasound survey of Portugues Dam provides an opportunity to validate whether infrasound’s can be used to remotely determine the fundamental frequencies of vibration of large structures. Infrasound waves are capable of propagating at a significant standoff distance from the source structure. Potential benefits of infrasound monitoring include the determination of a structure’s health without a physical inspection and also passive monitoring of several structures of interest using relatively few SIAM arrays.

 

 

[1] P. Campus, D. R. Christie, “Worldwide observations of infrasonic waves” in Infrasound Monitoring for Atmospheric Studies, edited by A. Le Pichon, E. Blanc, A. Hauchecorne (Springer, Dordrecht, 2010), pp. 185–234.

[2] COMSOL Multiphysics® v. 5.2. www.comsol.com. COMSOL AB, Stockholm, Sweden

[3] H. Diaz-Alvarez, V.P Chiarito, S. McComas, and M.H McKenna. (2015). Infrasound Assessment of the Roller Compacted Concrete Dam: Case Study of the Portugues Dam in Ponce, PR. COMSOL conference 2015, Newton, MA. (2015)

1aSA – On a Fire Extinguisher with Sound-wind for the Beginning Stage of Fire – Myung-Sook Kim

On a Fire Extinguisher with Sound-wind for the Beginning Stage of Fire

Myung-Jin Bae, mjbae@ssu.ac.kr
Myung-Sook Kim, kimm@ssu.ac.kr
Soongil University, 369 Sangdo-ro, Dongjak-gu, 06978 Seoul Korea

Popular version of paper , 1aSA “On a fire extinguisher using sound winds”
Presented 10:30 AM – 12:00 PM., November 28, 2016.
172nd ASA Meeting, Honolulu, U.S.A.

There are a variety of fire extinguishers available on the market with differing extinguishing methods, including powder-dispersers, fluid-dispersers, gas-dispersers and water-dispersers. There has been little advancement in the technology of fire extinguishers in the past 50 years. Yet, issues may arise when using any of these types of extinguishers during an emergency that hinder its smooth implementation. For example, powder, fluid, or gas can solidify and become stuck inside of containers; or batteries can discharge due to neglected management. This leaves a need for developing a new kind of fire extinguisher that will operated reliably at the beginning stage of fire without risk of faulting. The answer may be the sound fire extinguisher.

The sound fire extinguisher has been in development since the DAPRA, Defense Advanced Research Projects Agency of the United States, publicized the result of its project in 2012, suggesting that a fire can be put out by surrounding it with two large sound speakers. Speakers were enormously large in size then because they needed to create enough sound power to extinguish fire. As a follow-up, in 2015 American graduate students introduced a portable sound extinguisher and demonstrated it with a video posted on YouTube. But it still required heavy equipment, weighing 9 kilograms, was relatively weak in power and had long cables. In August of 2015, we, the Sori Sound Engineering Research Institute (SSERI), introduced an improved device, a sound extinguisher using a sound lens in a speaker to produce more focused power of sound, roughly 10 times stronger in its power than the device presented in the YouTube video.

Our device still exhibited problems, such as its heavy weight over 2.5 kilograms, and its obligatory vicinity to the flame. Here we introduces a further improved sound extinguisher in order to increase the efficiency rate of the device by utilizing the sound-wind. As illustrated in Figures 1 and 2 below, the sound fire extinguishers do not use any water or chemical fluids as do conventional extinguishers, only emitting sound. When the sound extinguisher produces low frequency sound of 100 Hz, its vibration energy touches the flame, scatters its membrane, and blocks the influx of oxygen and subdues the flame.

The first version of the extinguisher, where a sound lens in a speaker produced roughly 10 times more power with focusing, introduced by the research team of SSERI is shown in Figure 1. It was relatively light, weighing only 2.5 kilograms and 1/3 the weight of previous ones, and thus could be carried around with one hand without any connecting cables. It was also small in size measuring 40 centimeters (a little more than 1 feet) in length. With an easy on-off switch, it is trivial to operate up to 1 or 2 meters (about 1 yard) distance from the flame. It can be continuously used for one hour when fully charged.

The further improved version of the sound fire extinguisher is shown in Figure 2. The most important improvement to be found in our new fire extinguisher is the utilization of wind. As we blow out candles using the air from our mouth, similarly the fire can be put out by wind if its speed is over 5 meters/second when it reaches the flame. In order to acquire the power and speed required to put out the fire, we developed a way to increase the speed of wind by using low-powered speakers: a method of magnifying the power of sound wind.

 

1EMB00001e242b2b    EMB00001e242b37
Figure 1. The first sound fire extinguisher by SSERI: the mop type.

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Figure 2. The improved extinguisher by SSERI: the portable type

Wind generally creates white noise, but we covered wind with particular sound frequencies. When wind acquires certain sound frequency, namely, its resonance frequency, its amplitude magnifies it and creates a larger sound-wind. Figure 3 below illustrates the mechanism of a fire extinguisher with sound-wind amplifier. A speaker produces the low frequency sound (100 Hz and below) and creates sound-wind, resonates it by utilizing the horn-effect to magnify and produce 15 times more power. The magnified sound-wind touches the flame and instantly put out the fire.

In summary, with these improvements, the sound-wind extinguisher is fit best for the beginning stage of a fire. It can be used at home, at work, on board in aircrafts, vessels, and cars. In the future, we will continue efforts to further improve the functions of the sound-wind fire extinguisher so that it can be available for a popular use.

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Figure 3: The mechanism of a sound-wind fire extinguisher

References
[1] DAPRA Demonstration, https://www.youtube.com/watch?v=DanOeC2EpeA
[2] American graduate students (George Mason Univ.), https://www.youtube.com/watch?v=uPVQMZ4ikvM
[3] Park, S.Y., Yeo, K.S., Bae, M.J. “On a Detection of Optimal Frequency for Candle Fire-extinguishing,” ASK, Proceedings of 2015 Fall Conference of ASK, Vol. 34, No. 2(s), pp. 32, No. 13, Nov. 2015.
[4] Ik-Soo Ahn, Hyung-Woo Park, Seong-Geon Bae, Myung-Jin Bae,“ A Study on a sound fire extinguisher using special sound lens,” Acoustical Society of America, Journal of ASA, Vol.139, No.4, pp.2077, April 2016.

*Video file attached: sound-wind extinguisher V2

2pSAa8 – A Study on a Sound Extinguisher Using Sound Lens – Ik-Soo Ahn

Ik-Soo Ahn, aisbestman@naver.com
Hyung-Woo Park pphw@ssu.ac.kr
Myung-Jin Bae, mjbae@ssu.ac.kr

Soongil University
369 Sangdo-ro, Dongjak-gu
Seoul, Korea 06978

 

2Seong-Geon Bae sgbae@kangnam.ac.kr
2Kangnam University
111, Gugal-dong, Giheung-gu, Yongin-si, Gyeonggi-do Korea 16979

 

Popular version of paper 2pSAa8“A study on a sound fire extinguisher using special sound lens”

Presented Tuesday afternoon, May 24, 2016, 3:10 A in Salon E

171st ASA Meeting, Salt Lake City

 

In 2012, DARPA, Defense Advanced Research Projects Agency of the United States, demonstrated that fire can be put out by surrounding it with two large sound speakers. This verified the possibility of a fire extinguisher utilizing sound. Since then, many people have tried to develop a more efficient sound extinguisher, recognizing its future value. For example, in 2015 a couple of American graduate students introduced a portable sound extinguisher and demonstrated it on YouTube, but it was too heavy and too weak with long cables. The basic mechanism for a sound extinguisher can be summarized as follows: When the sound extinguisher produces low frequency sound of 100Hz, its vibration energy touches the flame, scatters its membrane, and then blocks the influx of oxygen, so the flame goes down.

Picture 1 Fire with strong flame

Picture 1 Fire with strong flame

Picture 2 Applying the extinguisher

Picture 2 Applying the extinguisher

Picture 3 The result

Picture 3 The result

Recently, a research team of SSERI, the Sori Sound Engineering Research Institute, introduced an improved device, a “sound-wind extinguisher,” by installing a sound lens in a speaker to produce more focused power of sound, roughly 10 times stronger in its power than the previous one. This sound-wind extinguisher is very light, weighting only about 2 kg, 1/3 of the previous one, and can be carried around with one hand without any connecting cable. It is also small in size measuring 40cm in length. With an easy on-off switch, you can use it anywhere, up to 1~2m distance from the flame.

The most important improvement to be found in our sound extinguisher from the previous one is the installation of a sound lens. If you use the sound in a usual way with a normal speaker, it scatters into the air without displaying any effect on the flame. On the other hand, when the sound lens is used with a speaker, the lens concentrates the sound generated from the speaker into one place and makes it possible to reach the fire more directly. In other words, it amplifies sound to maximize its efficiency without losing the power of sound which might be caused by the interference of the air. air. The team also succeeded in reducing the size and weight of the extinguisher, so that anyone can carry it anywhere at any time, improving its portability with an easy on-off switch. The experimental sound extinguisher is shown in the following pictures and video clip.

The following figure illustrates how and where to install a sound lens inside of the sound extinguisher.
Int Structure

 

We believe that the sound-wind extinguisher is fit best for the beginning stage of a fire. It can be used at home, at work, on board in aircrafts, vessels, and cars.

 

References

[1] DAPRA Demonstration, https://www.youtube.com/watch?v=DanOeC2EpeA

 

 

[2] American graduate students (George Mason Univ.), https://www.youtube.com/watch?v=uPVQMZ4ikvM

 

 

[3] Ahn, I.S., Bae, M.J. “On a Compact Extinguisher Using Sound Lens,” KICS, Proceedings of 2016 Conference of KICS, Vol. 32, No. 1, pp. 10C-13-1~2. Jan. 20-22, 2016.

 

 

[4] Lee, E.Y., Bae, M.J. “On a Focused Transducer for Fire-extinguishing,” ASK, Proceedings of 2015 Fall Conference of ASK, Vol. 34, No.2(s), pp. 35, No. 13, 2015.

 

 

[5] Park, S.Y., Yeo, K.S., Bae, M.J. “On a Detection of Optimal Frequency for Candle Fire-extinguishing,” ASK, Proceedings of 2015 Fall Conference of ASK, Vol. 34, No. 2(s), pp. 32, No. 13, 2015.

 

 

[6] Yeo, K.S., Park, S.Y., Bae, M.J. “On an Extinguisher with Sound and Wind,” ASK, Proceedings of 2015 KSCSPC, Vol. 32, No. 1, pp. 170-171, Aug. 14, 2015.

 

 

 

 

3aSA7 – Characterizing defects with nonlinear acoustics – Pierre-Yves Le Bas, Brian E. Anderson, Marcel Remillieux, Lukasz Pieczonka, TJ Ulrich

Characterizing defects with nonlinear acoustics

 

Pierre-Yves Le Bas, pylb@lanl.gov1,  Brian E. Anderson1,2, Marcel Remillieux1, Lukasz Pieczonka3, TJ Ulrich1

1Geophysics group EES-17, Los Alamos National Laboratory, Los Alamos, NM 87545, USA

2Department of Physics and Astronomy, Brigham Young University, N377 Eyring Science Center, Provo, UT 84601, USA

3AGH University of Science and Technology, Krakow, Poland

 

Popular version of paper 3aSA7, “Elasticity Nonlinear Diagnostic method for crack detection and depth estimation”
Presented Wednesday morning, November 4, 2015, 10:20 AM, Daytona room
170th ASA Meeting, Jacksonville

 

One common problem in industry is to detect and characterize defects, especially at an early stage. Indeed, small cracks are difficult to detect with current techniques and, as a result, it is customary to replace parts after an estimated lifetime instead of keeping them in service until they are effectively approaching failure. Being able to detect early stage damage before it becomes structurally dangerous is a challenging problem of great economic importance. This is where nonlinear acoustics can help. Nonlinear acoustics is extremely sensitive to tiny cracks and thus early damage. The principle of nonlinear acoustics is easily understood if you consider a bell. If the bell is intact, it will ring with an agreeable tone determine by the geometry of the bell. If the bell is cracked, one will hear a dissonant sound, which is due to nonlinear phenomena. Thus, if an object is struck it is possible to determine, by listening to the tone(s) produced, whether or not it is damaged. Here the same principle is used but in a more quantitative way and, usually, at ultrasonic frequencies. Ideally, one would also like to know where the damage is and what its orientation is. Indeed, a crack growing thru an object could be more important to detect as it could lead to the object splitting in half, but in other circumstances, chipping might be more important, so knowing the orientation of a crack is critical in the health assessment of a part.

To localize and characterize a defect, time reversal is a useful technique. Time reversal is a technique that can be used to localize vibration in a known direction, i.e., a sample can be made to vibrate perpendicularly to the surface of the object or parallel to it, which are referred to as out-of-plane and in-plane motions, respectively. The movie below shows how time reversal is used to focus energy: a source broadcasts a wave from the back of a plate and signals are recorded on the edges using other transducers. The signals from this initial phase are then flipped in time and broadcast from all the edge receivers. Time reversal then dictates that these waves focus at the initial source location.

1

Time reversal can also be more that the simple example in the video. Making use of the reciprocity principle, i.e., that a signal traveling from A to B is identical to the same signal traveling from B to A, the source in the back of the plate can be replaced by a receiver and the initial broadcast can be done from the side, meaning TR can focus energy anywhere a signal can be recorded; and with a laser as receiver, this means anywhere on the surface of an object.

In addition, the dominant vibration direction, e.g., in-plane or out-of plane, of the focus can be specified by recording specific directions of motion of the initial signals. If during the first step of the time reversal process, the receiver is set to record in-plane vibration, the focus will be primarily in that in-plane direction; similarly if the receiver records the out-of-plane vibration in the first step of the process, the focus will be essentially in the out-of-plane direction. This is important as the nonlinear response of a crack depends on the orientation of the vibration that makes it vibrate. To fully characterize a sample in terms of crack presence and orientation TR is used to focus energy at defined locations and at each point the nonlinear response is quantified.  This can be done for any orientation of the focused wave. To cover all possibilities, three scans are usually done in three orthogonal directions.

Figure 2 shows three scans on x, y and z directions of the same sample composed of a glass plate glued on an aluminum plate. The sample has 2 defects, one delamination due to a lack of glue between the 2 plates (in the (x,y) plane) at the top of the scan area and one crack perpendicular to the surface in the glass plate in the (x,z) plane in the middle of the scan area.

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Figure 2. Nonlinear component of the time reversal focus at each point of a scan grid with wave focused in the x, y and z direction (from left to right)

As can be seen on those scans, the delamination in the (x,y) plane is visible only when the wave is focused in the Z direction while the crack in the (x,z) plane is visible only in the Y scan. This means that cracks have a strong nonlinear behavior when excited in a direction perpendicular to their main orientation. So by scanning with three different orientations of the focused vibration one should be able to recreate the orientation of a crack.

Another feature of the time reversal focus is that its spatial extent is about a wavelength of the focus wave. Which means the higher the frequency, the smaller the spot size, i.e., the area of the focused energy. One can then think that the higher the frequency the better the resolution and thus higher frequency is always best. However, the extent of the focus is also the depth that this technique can probe; so lower frequency means a deeper investigation and thus a more complete characterization of the sample. Therefore there is a tradeoff between depth of investigation and resolution. However, by doing several scans at different frequencies, one can extract additional information about a crack. For example, Figure 3 shows 2 scans done on a metallic sample with the only difference being the frequency of the focused wave.

 

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Figure 3. From left to right: Nonlinear component of the time reversal focus at each point of a scan grid at 200kHz and 100kHz and photography of the sample from its side.

 

At 200kHz, it looks like there is only a thin crack while at 100kHz the extent of this crack is larger toward the bottom of the scan and more than double so there is more than just a resolution issue. At 200kHz the depth of investigation is about 5mm; at 100kHz it is about 10mm. Looking on the side of the sample in the right panel of figure 3, the crack is seen to be perpendicular to the surface for about 6mm and then dip severely. At 200kHz, the scan is only sensitive to the part perpendicular to the surface while at 100kHz, the scan will also show the dipping part. So doing several scans at different frequencies can give some information on the depth profile of the crack.

In conclusion, using time reversal to focus energy in several directions and at different frequencies and studying the nonlinear component of this focus can lead to a characterization of a crack, its orientation and depth profile, something that is currently only available using techniques, like X-ray CT, which are not as easily deployable as ultrasonic ones.