Dimitri Donskoy

Vibro-Acoustic Modulation Nondestructive Evaluation Technique

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.

Micro- and Macroscale Damage Detection Using the Nonlinear Acoustic Vibro-Modulation Technique

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-bending fatigue test. In the experiment, a stable increase of the nonlinear response during specimen fatigue was observed indicating early damage accumulation before the macroscopic fracture.

Nonlinear acoustic waves in porous media in the context of Biot’s theory

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.

Low frequency underwater acoustic radiator

Low frequency underwater acoustic radiator apparatus includes an air-trapping chamber with a lower portion open to the water, characterized by a mean density higher than that of water. A closed, rigid float element having a mean density equal to or lower than that of water is disposed with a selected loose fit within the chamber. When the chamber and float combination are submerged in water with a substantially vertical orientation of the chamber, the float element floats within the chamber, with the trapped air functioning as a spring coupling. A vibromotive element is coupled to the float element or the chamber element to induce oscillation of the coupled element. The coupling of the chamber and the float by the air spring converts the dipole oscillation of the oscillating element into a monopole pulsation suitable for a low frequency underwater sound radiator. The resonant frequency of the assembly can be changed by altering the volume of air in the chamber, or by changing the depth at which the device is submerged.

Structural vibrations of buried land mines

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.

Nonlinear vibrations of buried landmines.

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.

Nonlinear Vibro-Acoustic Nondestructive Testing Technique

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.

Electro-magnetically controlled acoustic metamaterials with adaptive properties.

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.

Acoustic particle velocity horns

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 Websters 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 horns 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.

Variability of SCUBA diver's acoustic emission

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; divers experience and training; and, finally, operating conditions of the equipment and diver, i.e. tank air pressure and the divers 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 divers intensity of motion.