R. Gr. Maev

Air gas dynamic spraying of powder mixtures : Theory and application

The radial injection gas dynamic spray (RIGDS) technology of powder coatings deposition was considered for this work. A coating was created by injecting powders with variable compositions into a supersonic air jet and depositing powder on the substrate. This study describes the preliminary analysis of an air gas dynamic spray method realized by a portable RIGDS apparatus with a radial injection of powder. Attention was given to shock compaction processes during the coating structure formation and examples of powder mixtures utilization in RIGDS. It was shown that the operational parameters of supersonic powder-gas jet have a significant influence on the coatings microstructure, thus defining the high performance of the coating. Compaction and bonding of particles were analyzed.

High Resolution Ultrasonic Method for 3D Fingerprint Representation in Biometrics

Biometrics is an important field which studies different possible ways of personal identification. Among a number of existing biometric techniques fingerprint recognition stands alone – because very large database of fingerprints has already been acquired. Also, fingerprints are an important evidence that can be collected at a crime scene. Therefore, of all automated biometric techniques, especially in the field of law enforcement, fingerprint identification seems to be the most promising. Ultrasonic method of fingerprint imaging was originally introduced over a decade as the mapping of the reflection coefficient at the interface between the finger and a covering plate and has shown very good reliability and free from imperfections of previous two methods. This work introduces a newer development of the ultrasonic fingerprint imaging, focusing on the imaging of the internal structures of fingerprints (including sweat pores) with raw acoustic resolution of about 500 dpi (0.05 mm) using a scanning acoustic microscope to obtain images and acoustic data in the form of 3D data array. C-scans from different depths inside the fingerprint area of fingers of several volunteers were obtained and showed good contrast of ridges-and-valleys patterns and practically exact correspondence to the standard ink-and-paper prints of the same areas. Important feature reveled on the acoustic images was the clear appearance of the sweat pores, which could provide additional means of identification.

Air-Coupled Imaging Method Applied to the Study and Conservation of Paintings

An air-coupled acoustical imaging method for the study of wooden panel paintings is presented. After a brief overview of the state of the art, an introduction is given regarding the production technique of the art object under investigation. The technology employed is described in detail, as well as the proposed method. After a feasibility campaign of experiments, real ancient paintings have been investigated by means of a through-transmission and a single-sided lay-out. Defects were imaged in all the objects examined and in both the configurations adopted

Inverse Problem Solution to Find Real‐Time Temperature Distribution Inside the Spot Weld Medium Using Ultrasound Time of Flight Methods

The scope of this paper is to find temperature distributions in the spot weld at different stages of welding using ultrasonic through transmitted waves. The finite difference modeling provides the temperature distributions in the spot weld. For the given welding schedule the delay of the signal passing through the heated area is unique for every temperature distribution. Based on the experimentally obtained TOF curve it is possible to determine the temperature at any point of the weld.

Monitoring of pulsed ultrasonic waves’ interaction with metal continuously heated to the melting point

A resistance spot welder has been equipped with embedded ultrasonic transducers. Using this setup for ultrasonic data acquisition during welding and a feature analysis promising results are reported. The technology discovers a large potential for nondestructive investigation of thermoelastic behavior of welds.

Locating an acoustic point source scattered by a skull phantom via time reversal matched filtering

This paper examines the utilization of the time reversal matched filtering method to resolve the location of an acoustic point source beneath a skull phantom (variable thickness layer), without the removal of this layer. This acoustical process is examined experimentally in a water tank immersion system containing an acoustic source, a custom-made skull phantom, and a receiving transducer in a pitch-catch arrangement. The phantom is designed to approximately model the acoustic properties of an average human skull bone (minus the diploe layer), while the variable thickness of the phantom introduces a variable time delay to the acoustic wave, relative to its entry point on the phantom. This variable delay is measured and corrected for, and a matched filtering time reversed process is used to determine the location of the point source. The results of the experiment are examined for various positions of the acoustic source behind the phantom and compared to the reference cases with no phantom present. The average distance between these two cases is found to be 4.36 mm, and within the expected deviation in results due to not accounting for the effects of refraction.

Localisation of deformation in cold gas dynamic spraying

One of the main specific features of the cold spray process is localisation during particle impact deformation. The resultant adiabatic shear instability is the phenomenon that is known to play a major role in the particle/substrate and particle/particle bonding and coating structure formation during the cold spray process. In this article, the cold spray process is numerically simulated as an impact between a single spherical particle with the substrate, and comparison with experimental results for Armco iron and stainless steel particles is made. To describe a deformation behavior of the two materials at high strain rates, the Johnson–Cook model has been chosen. The results indicate that the model accurately captures the grain distortion in the impact zone for both materials. This approach provides valuable information on the characterisation of the strain localisation at the particle/substrate interface.

Method of quantitative evaluation of elastic properties of metals at elevated temperatures

A resistance spot welder has been used to evaluate the sound velocity in various materials at elevated temperatures. The method is indirect and its accuracy in the full temperature range strongly depends on the accuracy of the modeling. It provides a relatively simple setup to evaluating the elastic properties of metals, their alloys, and compounds in the molten state.

Grain Size Measurement of Copper Spot Welding Caps Via Ultrasonic Attenuation and Scattering Experiments

During ultrasonic testing of resistance spot welds in real time, the probe sends the sound waves through the thickness of the copper electrode cap into the materials being welded. Characteristics of the reflected waves from the weld interfaces allow a reliable decision to be made on the quality of the joint. Transmission of high frequency sound waves through the relatively thick copper welding cap cause the signal to be greatly attenuated due to grain scattering. For this reason, close monitoring of the copper cap properties prior to installation is essential for adequate performance. Finding copper alloys with a small average grain size is essential in order to minimize the attenuating effects. The conducted backscatter and attenuation experiments indicate correlation between the ultrasonically measured parameters and the optically found copper grain size. This correlation suggests that the attenuation or backscatter technique could be used alone in order to validate the proper copper alloy to be used in spot weld probes. Using nondestructive testing techniques for this purpose greatly reduces the time and cost involved compared to optical techniques.

REAL‐TIME ULTRASONIC EXPULSION DETECTION AND INDENTATION MEASUREMENT IN RESISTANCE SPOT WELDS

The thickness of a resistance spot weld resulting from electrode indentation is an important quality control parameter for estimating the final nugget diameter and identifying the occurrence of expulsion. An ultrasound transducer has been installed in a welding electrode allowing for evaluation of a spot weld during welding, using echo pulses. In such a system, the unknown increase in temperature during welding increases time of flight through the weld, while geometric contraction of the plates decreases it. These opposing effects often give false representations of the true weld thickness. This paper proposes a novel method of separating these effects. This is done using signal and image processing of successive A‐scans to identify the solid‐liquid nugget interface, where the melting temperature of the material is known. This paper demonstrates how the final weld thickness can be determined very accurately at the point in time when the nugget completely solidifies. It can also be shown that excessive ind...