Rymantas Kažys

Ultrasonic detection and characterization of delaminations in thin composite plates using signal processing techniques

Abstract For the detection of delaminations in thin multilayered composite plates, two different techniques based on the processing of band-limited ultrasonic pulses reflected from the sample under investigation are presented. The first technique exploits the differences between the signals reflected from the flawless and delaminated regions, which are revealed using the modified iterative L1 norm deconvolution. From filtered A-scans, two-dimensional C-type scans are produced in two different ways. The second technique is based on a spectroscopic analysis of the differential signature of a sample, which is obtained by subtracting from the reference signal, reflected by a flawless region, the pulse received at any arbitrary point and presentation of two-dimensional images at various frequencies. The experimental results indicate that both techniques enable one to determine between which layers delamination took place.

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 non-destructive on-line estimation of the tensile stiffness of a running paper web

Abstract A new method for measuring the elastic properties of paper on-line in a paper machine by using ultrasound has been developed. The unique feature of the method is that a continuous stochastic ultrasonic signal is used for measurements, which is generated by means of a dry friction between a running paper web and a rigidly fixed friction head. The signals from the web are picked up by an array of air-coupled ultrasonic receivers, which have no mechanical contact with the paper web. The ultrasound velocity in the web is obtained by means of the correlation processing. By using advanced digital signal processing, the tensile stiffness index (TSI) of the running web is determined. From the sensor we get the TSI values both in the machine direction and the cross-machine direction. The on-line ultrasonic sensors have been installed on two different paper machines for more than two years. This paper describes the background of the method, as well as some experiences and applications from two mill installations.

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.

Analysis of adaptive imaging algorithms for ultrasonic non-destructive testing

Abstract An adaptive imaging algorithm for ultrasonic non-destructive testing (NDT), which enables one to compress the dynamic range of the received ultrasonic signals, is presented. For this purpose, the data obtained in previous steps during B or C-scans are exploited to predict the system gain necessary at the next scanning position. Such an approach allows one to collect data during a single scan without primary knowledge about the object under investigation. This increases the reliability and the speed of NDT. The reduced dynamic range allows one to obtain high quality acoustic images with lower resolution high-speed analogue-to-digital (A/D) converters because the initial full-range signal can be restored after A/D conversion using collected gain values. The results of an experimental investigation of the measurement errors for various gain adjustment methods are given. The method described has additional advances in sophisticated signal post-processing. The images obtained using the synthetic aperture focussing technique (SAFT) are presented to demonstrate the differences in image quality. The sources of possible artefacts are analysed.

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.

Preoperative assessment of skin tumor thickness and structure using 14-MHz ultrasound

OBJECTIVE The aim of this study was to compare the relationship between skin tumor thickness and homogeneity and to evaluate the accuracy of 14-MHz ultrasound while measuring the thickness of different skin tumors. MATERIAL AND METHODS The ultrasonographic and histological analysis of 72 skin tumors was performed. Preoperative vertical tumor thickness (T) and structure of 12 melanomas, 34 melanocytic nevi and 26 basal cell carcinomas was assessed by 14-MHz ultrasonography. After the tumors were excised the vertical thickness measurement (Breslow index, pT) was performed by pathologist. According to the histological thickness all skin tumors were divided to thin (≤1mm) and thick (>1mm). The accuracy of the 14-MHz ultrasound measurements and correlation between the ultrasonographic and histological tumor thickness were estimated. RESULTS Homogeneous structure was assessed for all thin (≤1mm) and the majority (81.3%) of thick (>1mm) melanocytic skin tumors. Nonhomogeneous structure was estimated in thin and thick basal cell carcinomas, accordingly 42.9% and 31.9%. Measurements of T and pT correlated moderately in thick (>1mm) tumors (r=0.694), while in thin (≤1mm) tumors correlation was low (r=0.336). Moderate correlation between ultrasonographic and histological thickness was computed for melanocytic skin tumors as well as for basal cell carcinomas (r=0.564 and r=0.690). CONCLUSIONS Medium frequency ultrasound is not a reliable tool for the precise measurement of thin (≤1mm) skin tumors. Ultrasonography using a 14-MHz frequency transducer enables more precisely to measure the thickness of basal cell carcinoma than melanocytic skin tumors. The 14-MHz ultrasound is support tool to suggest the morphologic type of skin tumor.

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.