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We have presented an overview of our team’s (at Stevens Institute of Technology) contribution in development of resonance and nonlinear Seismo-Acoustic Mine Detection (SAMD) techniques. Among our major accomplishments are the discovery and quantitative characterization of mine resonances; the discovery of a very strong nonlinear dynamics of the buried mines manifesting itself through the combination and intermodulation frequencies; the development of a physical model describing the linear and nonlinear dynamics of the soil-mine systems; the development and field validation of the nonlinear detection technique including data collection and processing algorithms. The developed model analytically describes linear and nonlinear responses of the coupled soil-mine system providing an improved understanding of the effects of the soil and mine parameters on detection performance, explaining and predicting many laboratory and field experimental observations of soil-mine vibration responses. Since 1988, SAMD has matured from just ideas and a few laboratory experiments into a well understood and highly promising technique proven during numerous field tests with real (live) mines. The primary advantages of resonance and nonlinear techniques as compared to conventional electromagnetic methods, such as metal detectors and GPRs, are very low false alarms and a high detection contrast for buried AT and AP mines, especially for non-metal mines. The main drawback is that SAMD is relatively slow as compared to electromagnetic methods. As it currently stands, these capabilities make SAMD an excellent confirmatory detection method: after the electromagnetic detection systems provide fast scan of the large area identifying suspicious locations, SAMD scans only the suspected areas confirming or rejecting the alarms. SAMD is still an evolving technology and many improvements and implementation could be developed making SAMD faster, more robust, and affordable, especially for humanitarian demining. Thus, there are ongoing efforts to develop SAMD-dedicated sensors and sensor systems, such as an array of contact accelerometers (Martin, et.al., 2005), a multi-beam LDV (Lal, et.al., 2003), and a seismic and acoustic vibration imaging (SAVi) recently initiated by DARPA (www.darpa.mil/sto/underground/savi.html). Other developmental efforts involve various advance signal processing approaches, the use of directional acoustic sources, fusion of SAMD with electromagnetic methods, etc. All of these and other related technological advances eventually will bring SAMD onto the landmine fields as a capable and robust detection tool.

The increasing probability of collapse in defective structures owing to aging is one of the major issues in transportation. Therefore, different methodologies that are capable of monitoring structural components have been used to identify defects and predict failure. Among these methods, the non-linear vibro-acoustic modulation (VAM) technique has been implemented for many years in a variety of industries, such as aerospace. This method utilizes the effect of nonlinear interactions between a high frequency ultrasonic wave (carrier signal) and a much lower frequency structural vibration (modulating signal). This interaction takes place at nonlinear interfaces (cracks, bolted connections, delaminations, etc.) manifesting itself in the spectrum as side-band components around the carrier frequency. In this study, the VAM method was investigated as a non-destructive evaluation (NDE) method for fracture critical members (FCMs) in steel bridges. The results of the experimental studies revealed that using the VAM technique on test specimens during the tension only fatigue tests would provide some useful information on the existence of micro-cracks and on failure prediction. The use of the VAM technique for center-notched rectangular test specimens of structural steel under low-amplitude fatigue loading at a frequency of 10 Hz is capable of predicting the failure at 70–80% of the fatigue lifetime of the specimen. Moreover, in this investigation utilizing fatigue cycling as a modulating signal was successfully substituted for the conventional utilization of resonance structural bending vibrations.

Dedicated to Prof. Dr. Eng. Akimitsu Kurita on his 70th birthday, in honour of his scientific achievements, guidance and the education of his students. This paper explores a new interdisciplinary method for internal damage detection and tracking in composite materials using thermo-chemical sensing. A micro-sized network of strings is interwoven into the composites. Each string consists of a pair of tubes containing one of two different non-polar reactants. A local defect within the composites causes straining and cracking of the tube shell, resulting in direct contact between the two non-polar reactants. The latter undergo a chemical reaction resulting in a polar product. When exposed to a microwave energy source, a polar product heats up dramatically within seconds in comparison to the surrounding composite material or the non-polar reactants. This localized thermal signature can be rendered visible by an infrared camera. This study summarizes the findings of an in-depth computational and experimental study of this sensing technology which is expected to be applicable across industries using composites, among them aerospace, automotive, offshore and bridge engineering. Potential applications in steel offshore or steel bridge engineering involve using composite sensing patches to cover fatigue fracture-critical components. Defects initiating on the steel substrate surface are expected to be sensed on demand with this proposed sensing technology.

Vibro-Acoustic Modulation method (VAM) utilizes effect of the nonlinear interaction between higher frequency ultrasonic wave (carrier signal) and much lower frequency structural vibration (modulating signal). This interaction is taken place at the nonlinear interfaces (cracks, bolted connections, delaminations, etc.) manifesting itself in the spectrum as side-band components around the carrier. There are numerous studies applying VAM for nondestructive testing and structural health monitoring. Most of them utilize resonance structural bending vibrations as the modulating signal and measure a ratio of sideband to carrier spectral components defined as Modulation Index (MI). The present VAM study utilizes in-plane non-resonance very low frequency (10 Hz) tensile oscillations for monitoring fatigue and stress-corrosion damage evolution in steel. Experiments consistently demonstrated significant increase in MI during 70% - 80% of the fatigue life. Additionally, newly developed algorithm separates Amplitude and Frequency Modulations during the damage evolution demonstrating FM dominance at initial micro-crack growth stages and transition to AM dominance during macro-crack formation.

Vibro-Acoustic Modulation method for detection and characterization of various structural and material flaws has been actively researched for the last two decades. Most of the studies focused on detection and monitoring of macro-cracks requiring well established baseline (no-damage) value of the modulation index. The baseline value is specific for a particular structure, measuring setup, and other factors and can't be established in many practical situations without a long term monitoring looking for a relative change in the Modulation Index. In this work, we propose and investigate a baseline-free Vibro-Acoustic Modulation method, which does not require monitoring of relative Modulation Index change, unlike conventional approach. It was hypothesized that the nonlinear mechanisms (and respective nonlinear response) of a structure are different for undamaged and damaged material. For example: material without damage or at early stages of fatigue have classic elastic or hysteretic/dissipative nonlinearity while damaged (cracked) material may exhibit contact bi-linear or Hertzian nonlinear mechanisms. These mechanisms yield different power law dependencies of Modulation Index (MI) as function of applied vibration amplitude, B: MI ~ Bβ. Thus, quadratic nonlinearity yields linear dependence, β =1, and Hertzian nonlinearity results in β

The Vibro-Acoustic Modulation (VAM) method utilizes nonlinear interaction of a high frequency ultrasonic wave (carrier signal) with frequency ω and a low frequency modulating vibration with frequency Ω << ω, in the presence of various flaws such as fatigue and stress-corrosion cracks, disbonds, etc. Most of the reported VAM studies correlate flaw presence and growth with the increase in the Modulation Index (MI) defined in the spectral domain as the ratio of the side-band spectral components at frequencies ω±Ω to the amplitude of the carrier. This approach, however, does not differentiate between amplitude, AM, or frequency, FM, modulations contributing to the MI. It has been assumed that the prevailing modulation is AM due to contact-type nonlinear mechanisms. However, there could be other mechanisms leading to phase/frequency modulation. The present study aims to develop an algorithm of AM/FM separation specifically for the VAM method. It is shown that the commonly used Hilbert Transform (HT) separation may not work for a typical VAM scenario. The developed In-phase/Quadrature Homodyne Separation algorithm addresses HT shortcomings. The algorithm has been tested both numerically and experimentally (for fatigue crack evolution) showing FM dominance at initial micro-crack growth stages and transition to AM dominance during macro-crack formation.

Acoustic Velocity Horns (AVHs) are acoustically small funnels open to incident acoustic waves from mouth and throat (for single horns) or both mouths for double horns. Unlike traditional pressure horns terminated at the throat, AVHs yield appreciable amplification of the particle velocity across a wide frequency range starting from extremely low infrasound frequencies. Such horns can be utilized to enhance the performance of conventional vector and acoustic intensity sensors. The present paper includes derivation of directional properties and gains for acoustic velocity as well as pressure for horns of various configurations: single and symmetrical double-horns, and symmetrical horns with inserts of various profiles. The study reveals that the conical double-horns provide the highest velocity gain as compared to horns with an exponential profile or with inserts. The maximum gain cannot exceed a horn's mouth-to-throat radii ratio. In addition to the velocity gain, AVHs offer dipole directionality for the particle velocity and omnidirectional response with no gain for acoustic pressure. These findings were experimentally validated using a water-submerged conical double-horn.

A design of actively controlled metamaterial is proposed and discussed. The metamaterial consists of layers of electrically charged nano or micro particles exposed to external magnetic field. The particles are also attached to compliant layers in a way that the designed structure exhibits two resonances: mechanical spring-mass resonance and electro-magnetic cyclotron resonance. It is shown that if the cyclotron frequency is greater than the mechanical resonance frequency, the designed structure could be highly attenuative (40-60 dB) for vibration and sound waves in very broad frequency range even for wavelength much greater than the thickness of the metamaterial. The approach opens up wide range of opportunities for design of adaptively controlled acoustic metamaterials by controlling magnetic field and/or electrical charges.

Preliminary measurements and numerical predictions reveal that simple, and relatively small, horns generate remarkable amplification of acoustic particle velocity. For example, below 2 kHz, a 2.5 cm conical horn has a uniform velocity amplification ratio (throat-to-mouth) factor of approximately 3, or, in terms of a decibel level, 9.5 dB. It is shown that the velocity amplification factor depends on the horn's mouth-to-throat ratio as well as, though to a lesser degree, the horn's flare rate. A double horn configuration provides limited additional gain, approximately an increase of up to 25%.

Subjected to in-service and environmental loads, even relatively new structural components may reveal signs of microscopic deterioration. Very often, this initial damage further progresses into meso- and macroscales leading to development of one or several macrocracks that cause ultimate structural failure. Although the onset of macroscale cracking can be reliably detected by modern NDE methodologies, there is an increasing need for inspection technologies that may allow for assessing structural damage at a wide range of scales, i.e., from micro to macro. This article explores application of the nonlinear acoustic vibro-modulation technique (VMT) to incipient damage detection and monitoring. The nonlinear acoustic detection of the macroscopic damage is illustrated with examples: inspection of the cast aluminum automotive parts and testing of the aging aircraft fuselage. The microscale damage assessment is realized by real-time monitoring of the acoustic nonlinearity in the strain controlled three-point-be...

The motivation for this research is a lack of accurate, efficient and costeffective methodology to detect internal defects in composite plates. A micro-size network of strings is interwoven into the composites. Each string consists of a pair of tubes, containing one of two different non-polar reactants. A local defect within the composites causes straining and cracking of the tube shell, resulting in the direct contact of the two non-polar reactants. The latter undergo a chemical reaction resulting in a polar product. Our preliminary investigation shows that a polar product, when exposed to a microwave energy source, heats up dramatically in comparison to the ambient composite material or the non-polar reactants. Our proposed structure-health monitoring approach builds upon this finding by using a short term low-power microwave exposure, causing a local high-thermal signature along potential internal defects. The elevated temperature regions are visualized with an infrared camera. The research presented in this paper has the following objectives: (a) We introduce adequate non-polar reactants and quantify the temperature sensitivity of the polar product. (b) We investigate the optimized microstructure and material of the double-cell tube and fine-tune the design to enable fracturing of the cell walls under certain strain and ease embedment into the composites. (c) The paper sheds light on potential manufacturing processes of the micro-size sensing network per se as well as the embedment into the composites. The clear advantages of this methodology over others are that it provides large area coverage, has no requirement for an internal power source and wiring and hence does not compromise the structural integrity of the composites. doi: 10.12783/SHM2015/214

Buried landmines exhibit complex structural vibrations, which are dependent on interaction between soil and mines as well as on their respective properties. This paper presents experimental and theoretical studies of multimodal vibrations of buried mines and discusses the effects of burial depth and soil properties on dynamics of the soil-mine system. The two-dimensional model of the soil-mine system that accounts for soil-coupled mine’s multiple vibration modes and spatial distribution of vibrations over the soil surface is introduced. The model was tested using experiments with the plastic mine simulant. The study reveals that the soil shear stiffness is one of the key governing parameters determining the resonance vibration frequency and the amplitude of the soil-mine system. Burial depth, soil moisture, and consolidation are among factors leading to the increase of the soil shear stiffness, therefore effectively influencing modal vibrations of buried mines.

The paper introduces a “soft table” for the natural frequencies and modal parameters of uniform circular plates with elastic edge support. In contrast to conventional tabulations on paper, the “soft table” allows to save space and display parameters for required number of vibration modes. The “soft table” is realized in a collection of Matlab® codes that calculate natural frequencies and mode shape parameters for a given set of translational and rotational constraints and modifiable Poisson ratio. Calculations of natural frequencies and modal parameters for a wide range of constraints revealed distinctive regions where the frequencies and parameters could change considerably. Knowing position and extension of these regions could be essential for design and vibration control of structures incorporating circular plates.

Small wavelength size acoustic velocity horns (AVH) were recently introduced, [1], as particle velocity amplifiers having flat amplitude and phase frequency responses below their first resonance. AVH predicted amplification characteristics have been experimentally verified, [2,3], demonstrating interesting opportunities for vector sensors (VS) sensitivity enhancement. Present work provides enhanced analysis of amplification and characteristics of complex shape horns. Additionally, we address another AVH feature: turbulence flow noise reduction due to turbulence field spatial averaging across horn's mouth area. Numerical analysis demonstrated up to 25 dB convective turbulent pressure and velocity reduction at the horn throat.

Nonlinear acoustic technique has been recently introduced as a new tool for nondestructive inspection and evaluation of fatigued, defective, and fractured materials. Various defects such as cracks, debonding, fatigue, etc. lead to anomalous high levels of nonlinearity as compared with flawless structures. One of the acoustic manifestations of such nonlinearity is the modulation of ultrasound by low frequency vibration. Two methods employing the nonlinear interaction of ultrasound and vibration were developed, namely vibromodulation (VM) and impactmodulation (IM) methods. VM method employs forced harmonic vibration of a structure tested, while IM method uses impact excitation of structure natural modes of vibration. The feasibility tests were carried out for different objects and demonstrated high sensitivity of the methods for detection of cracks in steel pipes and pins, bonding quality in titanium and thermoplastic plates used for airspace applications, cracks in combustion engine, adhesion flaws in bonded composite structures, and cracks and corrosion in reinforced concrete. The model of the crack allowing to describe the modulation of sound by vibration is discussed. The developed nonlinear technique demonstrated certain advantages as compared with the conventional linear acoustic technique, specifically discrimination capabilities, sensitivity, and applicability to highly inhomogeneous structures.

The proposed class of acoustic metamaterials utilizes clouds of electrically charged nano or micro particles exposed to an external magnetic field. The particles are also elastically supported or embedded into elastically compliant medium in a way that the designed structure exhibits two resonances: mechanical spring-mass resonance and electro-magnetic cyclotron resonance. It is shown that if the cyclotron frequency is greater than the mechanical resonance frequency, the designed structure could be highly attenuative (40–60 dB) for vibration and sound waves in a very broad frequency range covering low and very low frequencies. The approach opens up wide range of opportunities for design of adaptive acoustic metamaterials by controlling magnetic field and/or electrical charges.

Existing methods of dynamic viscoelastic properties measurements have some limitation with respect to softness of material, its spatial anisotropy, and frequency range. Specifically, there are very limited capabilities to measure dynamic shear moduli of very soft (real shear modulus value is below 104 Pa) and anisotropic materials such as biological tissue and some polymers. The proposed technique is intended to fill this niche. The method utilizes the resonance approach, in which the specimen with certain orientation is treated as a massless spring attached to a massive rigid plate. The plate is excited with a one‐dimensional forced vibration in such a way that the specimen has only shear deformation. The resonance oscillation of the plate is analyzed with an impedance head and a spectral analyzer. Once the resonance frequency and the quality factor of the resonance are obtained, the complex shear modulus of the specimen can be calculated. An experimental setup utilizing the proposed approach was built a...

The nonlinear dynamic equations introduced by Biot to model poroelastic media have not been implemented to describe nonlinear acoustic waves in such media. The equations of the semilinear Biot model are revised and a mathematical model depicting the physical nonlinearity is established. A perturbation technique is then applied to find solutions to the nonlinear Biot equations. The computational results are carried out in details for a one‐dimensional model in which the contributions of the fast and slow compressional waves into second harmonic wave are analyzed. The correlation between second and third‐order Biot coefficients and measurable nonlinear parameters is also presented along with a parameter analysis.

The freshwater bivalve mollusk, Dreissena polymorpha – better known as the Zebra Mussel, since its introduction into the Great Lakes in 1985 has been quickly spreading throughout the waterways of both the United States and Canada causing serious economic and environmental consequences. The present study has been directed toward developing acoustic techniques to control Zebra Mussel infestation in various water intake and storage facilities. The effects of ultrasonic and hydrodynamic cavitation, low‐frequency sound and vibration on various life stage mussels (from eggs and larvae to adults) have been studied. It was found that cavitation can be used as a control measure for Zebra Mussel veligers. The efficiency of the ultrasonic and hydrodynamic cavitation treatments were measured as an output energy rate to achieve 100% mortality. Laboratory experiments demonstrated the effective use of low‐frequency waterborne sound to prevent Zebra Mussels from settling and translocating and to reduce reproduction abili...

Underwater sonar imaging relies upon the information contained in time correlation delays across numerous channels. Designing higher resolution imaging systems currently requires increasing the array aperture to wavelength ratio (L / λ), where L is the effective array length and λ is the acoustic wavelength in the medium. This fundamental constraint leads engineers down the path of adding a significant number of channels to a sonar design, which in turn increases array complexity and cost. Our research in bio-inspired sonar has revealed an approach that circumvents this constraint by exploiting bandwidth in addition to time-delay for the angular imaging process. Presented will be the results of a 2-channel underwater prototype tested in an acoustic tank and design work toward a 3-channel prototype for extending imaging to elevation angles. This work represents ongoing efforts toward developing a compact, low-cost broadband underwater sonar for near- to mid-range imaging and classification. Future integration with an autonomous underwater vehicle will demonstrate this technology for simple obstacle detection and avoidance of complex objects. [Work supported by ONR and internal investments by NUWC Division Newport.]Underwater sonar imaging relies upon the information contained in time correlation delays across numerous channels. Designing higher resolution imaging systems currently requires increasing the array aperture to wavelength ratio (L / λ), where L is the effective array length and λ is the acoustic wavelength in the medium. This fundamental constraint leads engineers down the path of adding a significant number of channels to a sonar design, which in turn increases array complexity and cost. Our research in bio-inspired sonar has revealed an approach that circumvents this constraint by exploiting bandwidth in addition to time-delay for the angular imaging process. Presented will be the results of a 2-channel underwater prototype tested in an acoustic tank and design work toward a 3-channel prototype for extending imaging to elevation angles. This work represents ongoing efforts toward developing a compact, low-cost broadband underwater sonar for near- to mid-range imaging and classification. Future integrat...

Conventional low frequency (from 10 Hz) velocity vector sensors utilize inertial approach using neutrally buoyant body with inertial sensor (accelerometer or moving coil geophone) inside. In order to achieve high sensitivity at the low frequencies these sensors must have relatively high inertia (mass). High mass leads to increased size (such as soccer ball) of the sensor in order to satisfy the condition of neutral buoyancy. The developed alternative non-inertial sensor utilizes torsional frictionless and extremely compliant suspension enabling very light and small, yet sensitive with low resonance sensor.

The paper considers receiving acoustic horns designed for particle velocity amplification and suitable for use in vector sensing applications. Unlike conventional horns, designed for acoustic pressure amplification, acoustic velocity horns (AVHs) deliver significant velocity amplification even when the overall size of the horn is much less than an acoustic wavelength. An AVH requires an open-ended configuration, as compared to pressure horns which are terminated at the throat. The appropriate formulation, based on Webster's one-dimensional horn equation, is derived and analyzed for single conical and exponential horns as well as for double-horn configurations. Predicted horn amplification factors (ratio of mouth-to-throat radii) were verified using numerical modeling. It is shown that three independent geometrical parameters principally control a horn's performance: length l, throat radius R(1), and flare rate. Below a predicted resonance region, velocity amplification is practically independent of frequency. Acoustic velocity horns are naturally directional, providing maximum velocity amplification along the boresight.

A new concept of a low‐frequency (

A non-inertial sensing approach for an Acoustic Vector Sensor (AVS), which utilizes eddy-current displacement sensors and operates well at Ultra-Low Frequencies (ULF), is described here. In the past, most ULF measurements (from mHertz to approximately 10 Hertz) have been conducted using heavy geophones or seismometers that must be installed on the seafloor; these sensors are not suitable for water column measurements. Currently, there are no readily available compact and affordable underwater AVS that operate within this frequency region. Test results have confirmed the validity of the proposed eddy-current AVS design and have demonstrated high acoustic sensitivity.

Knowledge of the variability of the acoustic emission characteristics from SCUBA divers is critically important for designing and operating a passive acoustic SCUBA characterization system. Using modeling and experimental measurements in a controlled environment, we identified key source factors influencing the variability of the acoustic emission parameters including Source Band Level (SBL), Spectral Power Density (SPD), and breathing periodicity or emission modulation frequency. The key factors are: equipment, specifically, the design of the first stage (high pressure) regulator and its service life; diver's experience and training; and, finally, operating conditions of the equipment and diver, i.e. tank air pressure and the diver's motion activity. We found, for example, that the SBL could vary as much as 16 dB depending on the equipment used and up to 15 dB depending on the diver's intensity of motion.

Assessment of structural deterioration due to in‐service and environmental loads is an essential element in ensuring safety, operability and long life of various structures and structural components. We applied vibro‐acoustic modulation technique to monitor material degradation at the micro/meso scale before the onset of the macro‐scale fracture. The technique explores nonlinear acoustic interaction of high frequency ultrasound and low frequency structural vibration at the site of the incipient damage. It is shown that micro/meso scale degradation increases the material nonlinearity leading to modulation of the high frequency ultrasonic signal by low frequency vibration, quantified by the nonlinear acoustic Damage Index (DI). Numerous tests with hundreds of test coupons proved high sensitivity of the developed technique to micro/meso scale fatigue damage accumulation. The study further demonstrated that the remaining life of the fatigued material can be predicted using phenomenological damage accumulation rule expressed through measured Damage Index.

Assessment of structural deterioration due to in-service and environmental loads is an essential element in ensuring safety, operability and long life of various structures and structural components. There is an increasing need for technologies that could allow for monitoring material degradation at the micro/meso scale before the onset of the macro-scale fracture. In this contribution, we report results of the real-time monitoring of the material micro/meso scale degradation using the nonlinear acoustic vibro-modulation technique. The technique explores nonlinear acoustic interaction of high frequency ultrasound and low frequency structural vibration at the site of the incipient damage. It is shown that micro/meso scale degradation increases the material nonlinearity leading to modulation of the high frequency ultrasonic signal by low frequency vibration, quantified by the nonlinear acoustic damage index (DI). The correlation of DI measured for hundreds of test coupons with material degradation was confirmed using traditional ultrasonic and SEM examinations.Copyright © 2007 by ASME

Buried in soil, landmines exhibit distinguishable nonlinear dynamic characteristics. These characteristics have been successfully used for nonlinear acoustic/seismic detection of both antipersonnel and antitank landmines. Despite a high potential of the nonlinear acoustic landmine detection technique, its utility is currently limited by a relatively high noise level of the LDV at frequencies typically used for landmine detection. To mitigate this limitation, we propose a modulation approach that exploits a nonlinear interaction of the low frequency resonance vibrations and higher frequency sound waves. The result of the modulation is manifested in a high frequency range as additional spectral components at the combination frequencies. The nonlinear response of the soil-mine dynamic system measured at the combination frequencies is used for the detection of the buried landmine. Exploring the higher frequency range has another benefit of using a directional high frequency sound source.

Structural health monitoring (SHM) is an essential technology for maintaining the safety, high performance and operational readiness of civilian and military aircraft. Passive and active SHM methodologies incorporating fiber optic, piezoelectric, dielectric and other types of sensors have been explored for the detection of structural damage resulting from excessive mechanical and/or environmental (e.g. corrosion) loads. One of the promising SHM techniques is embedded ultrasonics in which information on structural health is inferred from records of the elastic waves scattered by structural inhomogeneities or cracks. This approach works well when the damage size exceeds a wavelength of the interrogation signal. As a result, small-scale incipient damage may be difficult to detect using conventional embedded ultrasonics practices. In addition, in complex structures such as the aircraft skin, a large number of structural features (e.g. holes, ribs, etc) opens the possibility of misclassification if features and damage scattered signals exhibit comparable characteristics. To address these limitations of the ultrasonic SHM, we propose to utilize the damageinduced nonlinearity of structural material for discriminating different stages of material deterioration and damage development. A concept of a nonlinear acoustic SHM is introduced that is based on the effect of the nonlinear interaction of elastic waves at the damage interface. This nonlinear interaction results in additional spectral components at the combination frequencies noticeable in the spectrum of the received signal. Amplitudes of the additional spectral components indicate the presence and severity of structural damage. We demonstrate the applicability of the nonlinear acoustic SHM for the detection of material degradation at various scales: from micro/meso damage accumulation to macro scale cracks. Examples of macro-scale damage detection are presented and damage characterization capabilities of the method are highlighted. To determine the sensitivity of the method to the micro/meso scale damage accumulation before the material fracture, we conducted a series of strain-controlled fatigue tests in which deterioration of structural material (aluminum) was monitored simultaneously by assessing the sample’s mechanical characteristics and by measuring the nonlinear acoustic manifestation of structural damage at the combination frequencies. A stable increase of the material nonlinearity was observed during the test, which correlated with the respective decrease of the specimen’s stiffness. Scanning acoustic microscopy and SEM analyses of the experimental samples further support the feasibility of nonlinear acoustic SHM for monitoring material degradation at extremely early stages before the onset of macro-scale disintegration and fracture.

Monitoring the incipient damage at the earliest possible stage is essential for predicting structural performance and remaining life of structural components. Existing prognostic methodologies incorporate conventional SHM and NDE techniques responsive to cracks and delaminations resulted from the irreversible material fracture and disintegration at the macro-scale. There is an increasing need for technologies that could allow for monitoring material degradation at the micro/meso scale before the onset of the macro-scale fracture. In this contribution, we report results of the real-time monitoring of the material micro/meso scale degradation using the nonlinear acoustic vibro-modulation technique. The technique explores nonlinear acoustic interaction of high frequency ultrasound and low frequency structural vibration at the site of the incipient damage. The indicator of the damage severity, nonlinear acoustic damage index (DI), was measured in real time during the strain-controlled three-point bending fatigue test of aluminum and steel specimens. Nondestructively, degradation of the specimen was revealed through the increase in the DI, which correlated well with the respective decrease in the specimen's stiffness. Destructive SEM examination confirmed sensitivity of the DI to the incipient micro/meso scale damage and advocated for utilizing the vibro-modulation approach for assessment of material degradation before fracture.

The seismo-acoustic method is one of the most promising emerging techniques for the detection of landmines. Numerous field tests have demonstrated that buried landmines manifest themselves at the surface through linear and nonlinear responses to acoustic/seismic excitation. The present paper describes modeling of the nonlinear response in the framework of the mass–spring model of the soil–mine system. The perturbation method used in the model allows for the derivation of an analytical solution describing both quadratic and cubic acoustic interactions at the soil–mine interface. This solution has been compared with actual field measurements to obtain nonlinear parameters of the buried mines. These parameters have been analyzed with respect to mine types and burial depths. It was found that the cubic nonlinearity could be a significant contributor to the nonlinear response. This effect has led to the development of a new intermodulation detection algorithm based on dual-frequency excitation. Both quadratic and intermodulation nonlinear algorithms were evaluated at the U.S. Army outdoor testing facilities. The algorithms appear to complement each other in improving the overall detection performance.

Resonance behavior of many types of landmines was first experimentally discovered in 2000 (Donskoy et al . in Proceedings of SPIE Vol. 4394, pp. 575-582, 2001). Laboratory studies and field tests have shown that mine’s resonance response is a complex phenomenon dependent upon interaction between soil and mines and their respective properties. Although the resonance effect was successfully used by various research teams for detection of landmines, there were no thorough studies on various factors influencing buried mine's resonance response. This paper presents results of theoretical and experimental investigation of this problem including multi-modal structure of mine's vibration response, effect of burial depth and soil condition. In the modeling efforts we considered multiple modes of vibration of mine casing and represented them as oscillators with effective parameters. This approach allowed for simplification of analysis and expanding existing lump-element model to account for multiple vibration modes. The experimental tests were focused on studying the effects of burial depth and soil moisture content on resonance behavior of soil-mine system. The tests have shown that a resonance frequency initially decreases with burial depth, as expected. However, an anomalous resonance frequency increase was observed at greater depths; soil moisture even further increases the resonance frequency.

In recent years, an innovative vibro‐modulation technique has been introduced for the detection of contact‐type interfaces such as cracks, debondings, and delaminations. The technique utilizes the effect of nonlinear interaction of ultrasound and vibrations at the interface of the defect. Vibration varies the contact area of the interface, modulating a passing ultrasonic wave. The modulation manifests itself as additional side‐band spectral components with the combination frequencies in the spectrum of the received signal. The presence of these components allows for the detection and differentiation of the contact‐type defects from other structural and material inhomogeneities. The vibro‐modulation technique has been implemented in the N‐SCAN® damage detection system providing a cost effective solution for the complex NDT problems. N‐SCAN® proved to be very effective for damage detection and characterization in structures and structural components of simple and complex geometries made of steel, aluminum, composites, and other materials. Examples include 24 foot‐long gun barrels, stainless steel pipes used in nuclear power plants, aluminum automotive parts, steel train couplers, etc. This paper describes the basic principles of the nonlinear vibro‐modulation NDE technique, some theoretical background for nonlinear interaction, and justification of signal processing algorithms. The laboratory experiment is presented for a set of specimens with the calibrated cracks and the quantitative characterization of fatigue damage is given in terms of a modulation index. The paper also discusses examples of practical implementation and application of the technique.

The paper presents results of the field test of the nonlinear seismo-acoustic technique for detection and discrimination of land mines. The tests were conducted in summer 2001 at the U.S. Army's outdoor testing facilities. Plastic antitank mines (M19, VS1.6, VS2.2) and plastic antipersonnel mines (M14, VS50, TS50) were confidently detected at their maximum burial depths in both gravel and dirt lanes. Mine M14 is one of the smallest mines and is very difficult to detect by other techniques. The test proved that the nonlinear seismo-acoustic detection algorithm is very sensitive to AT and AP mines, while completely insensitive to false targets, such as rocks, chunks of metal or wood, thus promising to deliver high probability of detection with low false alarm rate. The results of the tests are in good agreement with the developed physical model of the seismo-acoustic detection.© (2002) COPYRIGHT SPIE--The International Society for Optical Engineering. Downloading of the abstract is permitted for personal use only.

A novel technique for detection and discrimination of artificial objects, such as land mines, pipes, containers, etc., buried in the ground, has been developed and tested. The developed approach utilizes vibration (using seismic or airborne acoustic waves) of buried objects, remote measurements of soil surface vibration (using laser or microwave vibrometers), and processing of the measured vibration to extract mine’s “vibration signatures.” The technique does not depend upon the material from which the mine is fabricated whether it be metal, plastic, wood, or any other material. It depends upon the fact that a mine is a “container” whose purpose is to contain explosive materials and associated detonation apparatus. The mine container is in contact with the soil in which it is buried. The container is an acoustically compliant article, whose compliance is notably different from the compliance of the surrounding soil. Dynamic interaction of the compliant container and soil on top of it leads to specific linear and nonlinear effects used for mine detection and discrimination. The mass of the soil on top of a compliant container creates a classical mass–spring system with a well-defined resonance response. Besides, the connection between mass (soil) and spring (mine) is not elastic (linear) but rather nonlinear, due to the separation of the soil/mine interface in the tensile phase of applied dynamic stress. These two effects, constituting the mine’s vibration signature have been measured in numerous laboratory and field tests, which proved that the resonance and nonlinear responses of a mine/soil system can be used for detection and discrimination of buried mines. Thus, the fact that the mine is buried is turned into a detection advantage. Because the seismo-acoustic technique intrinsically detects buried containers, it can discriminate mines from noncompliant false targets such as rocks, tree roots, chunks of metal, bricks, etc. This was also confirmed experimentally in laboratory and field tests.

In-water calibration of Acoustic Vector Sensors (AVS) operating at very low frequencies (fraction of Hz to hundreds of Hz) presents a set of unique challenges as the acoustic wavelengths are much longer than any existing laboratory calibration facilities. The developed calibration approach utilizes existing Naval Undersea Warfare Center’s pressurized horizontal calibrating steel tube equipped with two independently controlled sound sources located at the opposite ends of the tube. Controlling the phase and amplitude of these sources allows for creating of pressure or velocity fields inside the tube. Respective pressure and particle velocity complex amplitudes are measured and calculated, respectively, with two reference hydrophones. Experimental results of this calibration approach is presented for a newly developed very low frequency AVS comprising of pressure and non-inertial velocity sensors built into an acoustic velocity horn.

Small wavelength size acoustic velocity horns (AVH) were recently introduced [J. Acoust. Soc. Am. 131(5), 3883–3890 (2012)] as particle velocity amplifiers having flat amplitude and phase frequency responses below their first resonance. AVH predicted amplification characteristics have been experimentally verified demonstrating interesting opportunities for vector sensors (VS) sensitivity enhancement. Present work provides enhanced analysis of amplification and characteristics of complex shape horns. Additionally, we address another AVH feature: turbulence flow noise reduction due to turbulence field spatial averaging across horn’s mouth area. Numerical analysis demonstrated up to 25 dB convective turbulent pressure and velocity reduction at the horn throat.

Acoustic metamaterials can be made out of micro/nano size structures employing various structural elements such as cantilever oscillators [JASA, 132(4), 2866–2872]. Natural frequencies and eigenmodes depend on stiffness and mass distribution which should be reflected in macro as well as micro (nano) structure analysis. This study comprises three parallel approaches to model nano cantilever beams, i.e., the analytical method, the finite element analysis, and the molecular/atomic dynamics analysis, to identify how material imperfections influence the dynamic response of the nanocantilever. The study explores to what extent and under what circumstances macrostructure mechanics differs from nanostructure molecular/atomistic mechanics and how the built-in imperfection can be used to control structural dynamic properties at various scales.

The objective of this grant is to purchase a Scanning Laser-Doppler Vibrometer System PSV-300-H from Polytec PI. The system was acquired for the total cost of $166,450 of which $40,000 was provided by this DURIP grant. The rest of the funds came from research contract and matching contribution ($50,000) from Stevens Institute of Technology. The system was delivered in Spring of 2000 and was extensively utilized in ARO projects DAAG-98-1-0402 and DAAD19-00-1-0039 related to the Nonlinear Seismo-Acoustic Land Mine Detection and Discrimination Technique. Thus, the Scanning Laser-Doppler Vibrometer System became major measuring tool in both laboratory and field tests. The utilization of the system also contributed to education and outreach efforts. A number of graduate students had used the system while working for the ARO and other projects. The system was extensively used for numerous demonstration of the developed land mine detection technique to potential industrial partners, investors, as well as to groups of high school and undergraduate students.

The nonlinear dynamic equations introduced by Biot to model porous media have not been implemented to describe nonlinear acoustic waves in such media. In this work the equations are revised and a mathematical model depicting the physical nonlinearity is established. A perturbation technique is then applied to find solutions to the nonlinear Biot equations. An important feature of the developed model is the introduction of the dependence of the structural parameters of the medium on its porosity. The model establishes a correlation between the measurable effective nonlinear parameter and structural parameters of the porous medium. This suggests employing nonlinear measurements as a diagnostic tool for porous media.

This paper is further development of the nonlinear vibro- acoustic technique, first presented at the previous SPIE conference on Detection and Remediation Technologies for Mines and Mineline Targets, Orlando '98. The present paper discusses the physical/mathematical model and experimental result of detection and discrimination of buried land mines. The mathematical model based on simplified 'mass-spring' approach. The effective spring has a nonlinear stiffness due to a nonlinear boundary condition at the soil-mine interface. Resulting nonlinear equation of motion and its solution in a good agreement with experimental observations. It has been demonstrated numerically and experimentally, that dynamically compliant mine cases exhibit strong nonlinear acoustic response, while less compliant false targets, such as rocks, solid pieces of wood and steel, and etc., behave as dynamically linear systems. The discovered nonlinear phenomenon has been used to develop the nonlinear vibro-acoustic method for land mine detection and discrimination. The experimental studies were performed with real inert plastic and wooden mines under laboratory and field conditions. First, the detection was performed with a contact sensor. Later, the method was tested using remote senors, such as a laser-doppler vibrometer and specially developed microwave vibrometers. These remote sensor demonstrated applicability for the developed nonlinear technique.

This investigation is a preliminary study on the possible implementation of a novel nonlinear vibro-acoustic nondestructive testing technique to low corrosion rate concrete-steel systems. The method is extremely sensitive to the presence of integrity reducing flaws such as steel-concrete debondings and cracks developed as a result of volume expansions associated with corrosion products. The acoustical method was compared with tradition electrochemical techniques such as Linear Polarization and Electrochemical Impedance Spectroscopy so as to observe acoustical behavior changes in active and passive systems. The nonlinear vibro-acoustic method showed a significant increase in nonlinear interaction for low corrosion rates. These results suggest that the proposed concept is indeed capable of detecting the early stage of degradation.

Conventional active acoustic methods of NDT are based on the principles of linear acoustics1,2. These include effects of reflection, scattering, transmission, absorption of probe acoustic energy. The presence of a defect leads to phase and/or amplitude variation of received signals while the frequencies of the received signals are the same as of emitted probe signals.

A new concept of a low-frequency (< 1000-Hz) underwater sound source has been proposed and tested (Donskoy and Blue, 1994). The present work is a further development of the source. A full scale reaction force driver to power the source was developed, built, and tested. In order to extend frequency band of the source, a double resonance approach (resonating reaction mass and resonating radiation piston) was employed. This approach allows for a significant extension (up to 400%) of the frequency band without an increase in a vibromotive force. The driver consists of a brushless servomotor, a brushless resolver for feedback, a Digital Signal Processor (DSP) based servo amplifier, and an interface with a PC. Vibromotive force is created with an eccentric weight mounted to a resonating mechanical structure. The driver can generate up to 8.000 lbs force, it has a programmable frequency control in the range up to 117 Hz, high power output (3.3 kW), compact size, low weight, and relatively low cost.

Previously [Donskoy and Cray, J. Acoust. Soc. Am., 129(4), Pt. 2, 2644 (2011)] the authors numerically investigated an acoustic particle velocity amplification effect with conical open-ended horns. Here, using Webster's approach, an analytical solution for the particle velocity response of conical horns (single and double) is derived and analyzed. The solutions are verified by comparison with direct numerical computations and supported with experimental measurements. It is shown that small horns, compared to the acoustic wavelength, are capable of providing substantial particle velocity amplification. For example, a 20 cm horn, in-water, can deliver nearly 10 dB of amplification over a very broad frequency range (from zero to 1000 Hz) without significant amplitude and phase distortion. Another unique feature of the velocity horn is its dipole directionality. The paper presents a thorough analysis of horn's amplification versus geometrical parameters. [This work is supported in part by the ONR summer fellowship program].