V. G. Kartashev

APPLICATION OF SIGNAL PROCESSING METHODS TO ULTRASONIC NON-DESTRUCTIVE TESTING OF ARTICLES WITH HIGH STRUCTURAL NOISE

This article describes the methodology of signal processing in ultrasonic nondestructive testing of materials with structural heterogeneities. Reflections of probing signal from these heterogeneities entail appearance of a peculiar interference a structural noise witb the same spectral composition as the probing signal. Therefore, it is hard to overcome these interferences. The article also discusses the methodology of recognising the signal on the background of structural noise in ultrasonic non-destructive testing using spatially dispersed transducers, focusing antennae, antennae with synthesised aperture, and the most efficient method spatialtemporal signal processing, which suggests multichannel reception of probing signal combined with optimal signal filtration in each channel.

Problems of extracting ultrasonic signals from structural noise during inspection of articles produced from materials with complex structures

Problems of ultrasonic testing of materials with complex structures are considered. Such problems arise owing to a correlated interference—structural noise (SN). The simplest models describing the formation of SN are analyzed, and static noise characteristics are determined for them. The main concepts that define the parameters of SN are introduced. The requirements imposed on the procedure of discriminating echo signals from a flaw against a correlated-noise background are determined.

The Fundamentals of the Theory of Spatiotemporal Signal Processing as Applied to Problems of Ultrasonic Flaw Detection of Articles from Complexly Structured Materials

The basic propositions of spatiotemporal signal processing as applied to the problems of ultrasonic testing of complex-structure articles with a high attenuation of acoustic signals are considered. Different algorithms of extracting informational echo signals from white and structural noise during multichannel spatiotemporal signal processing are studied.

Ultrasonic structural probing of products based on materials with a complex structure by analyzing the statistical characteristics of the structural noise

It is shown that during ultrasonic control of products that are made of materials with a complex structure (cast iron, concrete, and bronze) the use of ultrasonic signals with a wavelength that is comparable to the sizes of the acoustic irregularities in a material induces the combined reflection of an ultrasonic probing signal from structural irregularities, which masks the reflections from the initial defects of the product. The combined reflected signal is correlated with the probing signal; in ultrasonic defectoscopy of products it has to be considered as interference that masks the echo signals from the initial defects of the product. At the same time, it provides information on the structural state of a studied material. In this work we consider the model of a complex material with a random structural irregularity distribution, for which the statistical characteristics of the combined reflected signal, viz., the density of the structural-noise probability, dispersion, and energy spectrum, are determined. We propose a new method for defining the structural state of a complex material by analyzing the statistical characteristics (energy spectrum and dispersion) of the structural noise.The experimental results on the analysis of the statistical characteristics of the structural noise are discussed for cast-iron products, allowing the structural state to be detected and monitored during their operation.

Choosing parameters of space-time signal processing in ultrasonic structuroscopy of cast-iron molds subjected to thermal cycling

It is shown that in order to improve the accuracy of measuring the speed of ultrasonic vibrations in cast-iron products by echo-pulse method, one should use noise-immune space-time echosignal processing methods that improve signal–to–pattern-noise ratio. Signal parameters should also be adapted for characteristics of the test object. Requirements for the choice of probing-signal parameters have been established as well as optimum displacement step of a double-crystal transducer. The value of the speed of ultrasound at a time moment that precedes the destruction of cast-iron mold has been determined.

Synthesis of low-frequency broadband ultrasonic mosaic transducers with nondistorting spatiotemporal characteristics

It is shown that the spatial distribution of the acoustic field of a broadband transducer may influence the degree of distortion of ultrasonic signals. The principle for forming a uniform acoustic-field distribution within the directivity characteristic of a low-frequency broadband transducer based on the chosen topology of a mosaic transducer is proposed. The comparative spatiotemporal characteristics of mosaic broadband transducers with optimized and nonoptimized mosaic topologies are obtained as a result of simulation. It is shown that for an optimized topology uniform field distributions can be obtained in both far- and near-field zones of the broadband transducers.

Choosing Phased Array Pitch in Ultrasonic Tomography of Materials with Inhomogeneous Structure

The influence of phased array (PA) pitch on signal/pattern-noise ratio when probing objects with inhomogeneous structure by low-aperture transducers of transverse acoustic waves is considered. A saturation phenomenon has been discovered for small PA pitches. The phenomenon is due to the influence of positive cross correlation between pattern-noise instances at neighboring receiving transducers. The optimum PA pitch has been determined to be approximately 0.5 of the wavelength when probing the object area across the PA and 0.4 of the wavelength when probing areas at the side of the PA. Experimental research has been carried out and proved the validity of the theoretical results.

Pattern-Noise in Ultrasonic Inspection of Articles Made of Complexly Structured Materials

Pattern-noise (PN) that occurs due to combined reflection of an ultrasonic probing signal from acoustic inhomogeneities in the material structure is an interference that conceals echo-signals from flaws. The most efficient method for extracting echo-signals from PN is multichannel spacetime signal processing (STSP). Different inspection purposes dictate different configurations of STSP schemes with the parameters that are determined by PN correlation characteristics. A theoretical model has been used to determine mutual PN correlation characteristics for the main types of transducers. The theoretical results have been confirmed experimentally.

Applying Special Spatiotemporal Signal Processing Algorithms to Ultrasonic Testing of Large Antique Cast Bells

Original spatiotemporal signal processing algorithms have been developed to improve the reliability of ultrasonic testing of unique antique cast bronze articles. In particular, to inspect large bells on the grounds of the Moscow Kremlin, with numerous relief images that limit the surface available for mounting piezoelectric transducers, various spatiotemporal signal processing algorithms have been designed, including the ones based on rotating a double-crystal transducer with the summation of partial defectograms.

Application of synchronous detection for ultrasonic thickness gauging of concrete Articles with a nonuniform Structure

Some problems that are encountered in the ultrasonic pulse-echo thickness gauging of large concrete building structures with high frequency-dependent ultrasound attenuation and a complicated nonuniform structure are considered. It has been shown that ultrasonic signals with complex modulation with further optimal filtering of echo signals should be used in single-channel testing to increase the sensitivity during the testing of concrete articles and to enhance echo signals in the presence of white noise. Synchronous detection of echo signals is advisable to increase the precision of measurements. Multichannel spatial–temporal signal processing via thickness measurements at several neighboring points of an article and further coherent summation of partial measurement results should be used to enhance ultrasonic echo signals in the presence of structural noise. However, it is recommended that the multichannel testing of concrete articles with a nonuniform volumetric distribution of physicomechanical properties be performed with the summation of partial echo-signal envelopes after their synchronous detection due to the broad spread of ultrasonic signal velocities in neighboring regions.