Liudas Mažeika

Ultrasonic method for the whole blood coagulation analysis

The developed ultrasonic method is based on the experimentally established fact that the ultrasound velocity in a blood sample is changing in a specific way during the blood clotting process. For ultrasound velocity measurements pulse echo method was selected. Implementation of this approach has a few problems caused by small dimensions of the measurement cell. All elements of the cell contacting with the blood are made of biologically compatible materials. The length of Pd coated chamber is 5 mm, volume 0.2 ml. Data of measurements are stored and processed by PC. The ultrasound velocity in a sample is displayed as it changes in time during the experiment; the temperature is monitored as well. Application of the digital filtering allows to smooth the coagulation curve and reach the sensitivity up to +/-3 cm/s. The coagulation curves were obtained at frequency 5 MHz and they represent peculiar stages of blood clotting characterised by their duration and ultrasound velocity differences. Fine structure of ultrasonic velocity changes is registered from the very beginning of the clotting to lysis. Such experiments were carried out using blood samples taken from a few hundred volunteers. For dynamic calibration and periodic checking of the measuring system the liquid medium in which acrylamide polymerisation reaction takes place is proposed as a reference liquid. Such a liquid mimics clotting blood from the point of view of ultrasonic velocity changes.

Ultrasonic guided wave tomography for the inspection of the fuel tanks floor

A novel ultrasonic non-destructive technique (NDT) based on application of a transmission tomography of guided ultrasonic waves is proposed for floor inspection of large storage tanks and detection of non-uniformities, such as corrosion. The technique needs access only to the outer edge of the tank floor and does not require emptying the tank. Theoretical estimations have been verified by laboratory experiments using a scaled physical model of the tank. Estimation of the attenuation of different wave modes propagating in steel plates and determination of the losses in the lap welds showed that most suitable is S0 Lamb wave mode which possesses smallest losses and consequently enables investigation of tank floors up to average diameter 20–30 m. The in situ experiments carried out in a real 8 m diameter tank demonstrated that the developed technique could be used for reconstruction of the spatial distribution of the non-uniformities in a tank floor.

Ultrasonic technique for the investigation of structural properties of biological fluids

Biological fluids are specific objects for acoustical investigation due to the wide spectra of relaxation processes, especially nonstationary fluids such as blood during its coagulation process. The proposed method combines measurement of ultrasound attenuation over a frequency range and, ultrasound velocity dispersion being negligible, precise measurement of velocity variations at fixed frequency. In the dynamic spectroscopy method the wideband ultrasonic signal transmitted through the media is digitized with a sample rate of 200 MHz, averaged, and processed by a PC. Ultrasound absorption frequency dependency with intervals of 1 min is determined from amplitude spectra. Using small volume (1 ml) cell with multiple reflections, in the range of 2–17‐MHz, diffraction corrections and ultrasonic attenuation were determined in low‐absorptive standard liquid, conservative, and native coagulating blood. The clot formation process in the native blood is also monitored at frequencies 5 or 10 MHz using the time‐of‐...

Investigation of accurate imaging of the defects in composite materials using ultrasonic air-coupled technique

In this paper air-coupled ultrasonic investigation of delamination type defects in GLARE3-3/2 composite material is presented. An air-coupled ultrasonic measurement technique has a great potential for investigation of those materials in manufacturing and maintenance. An important advantage of this technique is that investigations can be performed without couplants and therefore various materials that can absorb liquids (honeycombs, porous) can be tested. The objective of the work was to investigate the interaction of the ultrasonic wave with delamination type defect in GLARE3-3/2 composite material and to determine influence of interaction effects on accuracy of the defect size measurement. Numerical and experimental investigation was carried-out in the through transmission mode. Results of numerical investigation and experimental verification of interaction of the ultrasonic wave are presented. Estimation of accuracy of the measurement of the delamination defect sizes is also presented.

Evaluation of diffraction errors in precise pulse-echo measurements of ultrasound velocity in chambers with waveguide.

Ultrasound velocity measurements in medicine and biology usually are performed using relatively small measurement chambers. When the pulse-echo method is used, the presence of the reflector close to the transducer can cause essential diffraction errors. These errors may be reduced using an additional buffer rod as a waveguide between the transducer and the measurement chamber. The objective of the presented work was analysis of diffraction errors in measurement chambers with a buffer rod. The work was performed in two steps. In the first stage propagation of transient ultrasonic waves in a buffer rod was analysed using an axisymmetric finite element model. This approach enables all dimensions of the measurement chamber and the waveguide to be taken into account, but is less accurate in the time domain. In the second step the absolute values of diffraction errors were evaluated using a mixed analytic-numeric disk shaped transducer diffraction model. In this case only the dimensions of the waveguide and measurement chamber along the wave propagation direction were taken into account. Diffraction errors were calculated by simulating small changes of ultrasound velocity in the liquid under investigation. The simulation performed allowed optimisation of the dimensions of the measurement chamber and a buffer rod thus minimising measurement errors.

Determination of spatial position of multiple targets by ultrasonic binaural method

The binaural technique used in ultrasonic sonars has a higher performance speed in comparison with a mechanically scanned ultrasonic beam, however, in presence of multiple targets meets a very serious ambiguity problem. In this case the number of targets detected exceeds the actual number of targets, e.g., there are additional non-existing targets found. Objective of this research was development of a simulation tool enabling to model multi-channel sonar in the environment with multiple targets of arbitrary geometry and development of robust signal processing procedures, suitable for detection of multiple targets from the data collected using the binaural method. The developed software enables to simulate operation of multi-channel sonar in an environment with multiple reflectors and predict a time history of the reflected signals. The novel algorithm for separation of real targets from the virtual ones in presence of multiple targets has been developed. Performance of the proposed algorithm was investigated both in the simulated and a real environment. The results obtained indicate a significant improvement of the sonar performance.

Application of ultrasound reflection tomography for nondestructive evaluation of objects with a complex geometry

In the case of solid objects with complex geometry, conventional ultrasound tomography techniques tend not to be efficient enough. The objective of the paper is the development and experimental verification of an ultrasound tomography approach suitable for objects with non‐flat boundaries. The reconstructed images in ultrasound reflection tomography usually are imperfect because of ultrasound wavefront curvature, which is significantly affected by wave refraction and mode conversion phenomena at the boundaries. In order to achieve better resolution of reconstructed images in the ultrasound reflection tomography, the curvature of the wavefronts has to be known and taken into account performing backprojection. Ultrasonic wavefronts were reconstructed from measured ultrasonic wave parameters. Obtained results were taken into account performing backprojection in the ultrasound refection tomography. When the curvature of the wavefront is known, the shape and the size of the defects can be reconstructed with gr...

Ultrasonic measurement of zirconium tubes used in channel-type nuclear reactors

Abstract An ultrasonic technique for the determination of the inner and outer diameters and wall thickness of zirconium tubes used in channel-type nuclear reactors has been developed. The method is based on the time-of-flight technique of ultrasonic waves reflected by the front and back walls of the tube. The measurements are performed around the circumference and along the axis of the tubes. The influence of eccentricity of the measuring probe is studied both theoretically and experimentally. It is shown that the absolute measurement error is less than 100 μtm for the diameters and 50 μm for the wall thickness. In situ tests carried out at the Ignalina state nuclear power plan:, Lithuania, proved the good performance of the system in a harsh environment.

Influence of the Spatial Dimensions of Ultrasonic Transducers on the Frequency Spectrum of Guided Waves

Ultrasonic guided wave (UGW)-based condition monitoring has shown great promise in detecting, localizing, and characterizing damage in complex systems. However, the application of guided waves for damage detection is challenging due to the existence of multiple modes and dispersion. This results in distorted wave packets with limited resolution and the interference of multiple reflected modes. To develop reliable inspection systems, either the transducers have to be optimized to generate a desired single mode of guided waves with known dispersive properties, or the frequency responses of all modes present in the structure must be known to predict wave interaction. Currently, there is a lack of methods to predict the response spectrum of guided wave modes, especially in cases when multiple modes are being excited simultaneously. Such methods are of vital importance for further understanding wave propagation within the structures as well as wave-damage interaction. In this study, a novel method to predict the response spectrum of guided wave modes was proposed based on Fourier analysis of the particle velocity distribution on the excitation area. The method proposed in this study estimates an excitability function based on the spatial dimensions of the transducer, type of vibration, and dispersive properties of the medium. As a result, the response amplitude as a function of frequency for each guided wave mode present in the structure can be separately obtained. The method was validated with numerical simulations on the aluminum and glass fiber composite samples. The key findings showed that it can be applied to estimate the response spectrum of a guided wave mode on any type of material (either isotropic structures, or multi layered anisotropic composites) and under any type of excitation if the phase velocity dispersion curve and the particle velocity distribution of the wave source was known initially. Thus, the proposed method may be a beneficial tool to explain and predict the response spectrum of guided waves throughout the development of any structural health monitoring system.

Simulation of ultrasonic fields radiated by a circular source through a layer with nonparallel boundaries

Active elements of ultrasonic transducers are separated from the medium by thick protective layers, which may have nonparallel front and back surfaces. The objective of this work was to develop a simple method suitable for fast calculation of ultrasonic fields radiated through a layer with parallel or nonparallel boundaries and enabling one to take into account multiple reflections inside the layer. The main presumption of the proposed method is the following: after refraction at the boundary between two media, the ultrasonic field consists of plane and edge waves as before refraction. The proposed simulation method is based on transformation of a multi-layered medium into a virtual one without internal boundaries and equivalent to the actual medium from the point of a view of the times of flight of the direct plane and edge waves. The method enables simulation of radiated ultrasonic fields even after multiple reflections within the layer with parallel or nonparallel boundaries. The examples of simulated ...