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No 4 (2025)
Acoustic methods
Cepstral Analysis of Ultrasonic Echoes Measured by an Antenna Array in Order to Obtain Super-Resolution Images of Reflectors
Abstract
The digital focusing aperture (DFA) method is widely used to image reflectors during ultrasonic inspection. The reliability of inspection is determined by the quality of the DFA image — resolution and signal-to-noise ratio. To achieve super-resolution of echo signals, which will lead to radial super-resolution of CFA-image of reflectors, various methods are used: maximum entropy method, Bernoulli—Gauss deconvolution, Lucy—Richardson deconvolution, methods of recognition with compression (CS), methods of construction of autoregressive models of signals, etc. To apply these methods, we need to know the impulse response of the ultrasonic inspection system. It can be measured, but you can use methods of “blind” deconvolution, which are used in image and signal processing. For example: the method of eliminating camera blur at its random displacement, maximum correlated kurtosis deconvolution (MCKD), cepstral analysis, etc. In this paper, a cepstral analysis method for super-resolution or for obtaining information about the impulse response of the system is considered to construct an AR spectrum model to obtain the radial super-resolution of CFA images. The performance of the proposed method is confirmed by model experiments.
3-15
Ultrasonic Evaluation of Residual Stresses in AISI 316Ti Steel Specimen after Laser Shock Peening
Abstract
Residual stresses induced by laser shock peening in the near-surface layer in AISI 316Ti austenitic stainless steel specimen were measured by ultrasonic technique using critically refracted longitudinal waves. The results of ultrasonic measurements were compared with the results obtained by hole drilling method. The values of the residual stresses induced by laser shock peening, the initial residual stresses in the rolled sheet and the yield strength of the material were compared. The thermal stability of laser-induced residual stresses after annealing the specimen for 5 hours at the temperature of 200 °C and re-annealing for 5 hours at the temperature of 280 °C was investigated. The results of study were analyzed taking into account the accepted assumptions, limitations and uncertainties. The structure near the untreated and laser-treated surface was studied using optical and scanning electron microscopes. The directions of further studies for the development of nondestructive technique for ultrasonic evaluation of residual stresses induced by laser shock peening of the surface were proposed.
16-28
Evaluation of the Non-Uniformity of Acoustic and Elastic Properties of Compression Coil Springs
Abstract
The paper is devoted to the investigation of the acoustic and elastic properties of automotive and railway springs manufactured by cold coiling and high-temperature machining, respectively. The mirror-shadow method of multiple reflections based on measuring the velocities of longitudinal and transverse waves propagating along the rod diameter of the spring is used to evaluate the non-uniformity of the acoustic properties. Specially designed pass-through electromagnetic-acoustic transducers of transverse waves of axial polarisation and transducers of longitudinal waves on the basis of flexible piezo film of polyvinylidene fluoride provide multiple reflection of volume waves along the cross-section of the coiled coil of the spring. The elasticity, shear moduli and Poisson’s ratio are calculated from the results of wave velocity measurements. It was established that the non-uniformity of acoustic and elastic properties along the length of the bar differs for automotive and railway springs. It was found that there are differences in the non-uniformity of acoustic and elastic properties along the length of the coiled bar for automotive and railway springs.
29-41
Thermal methods
Infrared Diagnostics of Turbulence in the Front of Wildland Fire and the Formation of Induced Atmospheric Turbulence
Abstract
An infrared diagnostic of the scale of turbulence in the front of a natural fire is presented, as well as a comparison with the scale of turbulence in the air near the combustion source for a model grassland and crown fire. An analysis of the flame of a grassland fire reveals smaller turbulence scales than the flame of a crown fire. A fire-induced atmospheric turbulence at a height of 10 m is observed with the corresponding frequency of air temperature pulsation (0.1-6 Hz for a steppe fire and 0.1-3 Hz for a crown fire). The values of the structural functions of refractive index and temperature fluctuations are significantly higher than the background values and can be used for remote fire detection.
42-51
Infrared Thermal Imaging Detection and Image Segmentation of Micro-Crack Defects in Semiconductor Silicon Wafer Scanned by Laser
Abstract
Mono-crystalline silicon wafers play a key role in photovoltaic technology and microelectronics manufacturing due to their good semiconductor characteristics. In order to meet the demand of high-tech industries, the production technology of silicon wafer is supposed to meet the high-precision standard, and if the micro-cracks produced during grinding are not detected on time, the yield of a useful product will be reduced. In order to achieve more efficient detection of micro-cracks in silicon wafers, a scanning laser thermal nondestructive testing system was developed. Using the pseudo static matrix reconstruction algorithm, the experimental data has been converted into static images to provide easier defect detection and evaluation. The influence of geometric characteristics (length, width and depth) of micro-cracks and laser excitation power on surface temperature signals in the laser scanning tests has been studied. The image enhancement techniques, such as linear gray scale transformation, basic function transformation and histogram equalization have been compared. The effectiveness of using super-pixel segmentation, dual threshold segmentation, iterative threshold segmentation and UNet3+ network for improving micro-crack detection efficiency has been explored. Common segmentation techniques have not proven to be useful in the image enhancement because of the presence of noise. Better results in image segmentation have been achieved by using a UNet3+ network, which ensured identification accuracy of about 90 % in the segmentation of micro-crack defects.
52-68
Defectoscopy Stories
69-72