Ultrasonic flaw detection has developed methods for recording and analyzing echo signals to determine the type of reflector and its dimensions. The method of digital focusing with an antenna (CFA) allows you to restore the image of the entire discontinuity boundary using echo signals reflected from the bottom of the control object, taking into account the transformation of the wave type. However, this approach is not always applicable in practice, since the shape of the bottom of the object of control may be unknown. Using the features of the behavior of the reflection coefficient for different types of waves, it is possible to draw a conclusion about the type of reflector from images only on a direct beam. Numerical and model experiments have confirmed the operability of the proposed approach.
Image restoration of reflectors by digital focusing antenna (CFA), along with such advantages as high resolution over the entire area of image restoration of reflectors, the ability to obtain images taking into account the reflection and transformation of the wave type from the boundaries of the object of control, has several disadvantages: a large volume of measured echo signals, a long image recovery time and insufficiently high energy of ultrasonic waves, entered into the object of control. The Plane Wave Imaging (PWI) method allows you to combine the advantages of phased array antenna technology (FAR) and CFA technology. In PWI mode, when a plane wave is emitted, all elements of the antenna array (AR) work (as in the HEADLIGHT mode), which allows you to increase the energy entered into the control object, and echo signals are recorded by all elements of the AR (as in the CFA mode). The images of the reflectors are restored by the combination SAFT method. To obtain an image, the number of emitted plane waves can be used less than the number of antenna array elements, which reduces the volume of measured echo signals. The transfer of calculations to the area of spatial sectors makes it possible to increase the speed of restoration of the presentation of reflectors. Model experiments have shown the positive and negative sides of obtaining images of reflectors by the PWI method in comparison with the CFA method both for the case of using a prism and without a prism.
The TOFD method, widely used in ultrasonic flaw detection, allows to distinguish a crack from a volumetric reflector by the phase of the echo signals and to determine its height with high accuracy. However, the TOFD method without scanning with piezoelectric converters across the welded joint does not allow to determine the displacement of the reflector from the center of the seam, which is very important when evaluating the control results. The scanning devices used for this purpose have a complex design, their price is higher than that of one—dimensional sanitizing devices, and, most importantly, the control time increases significantly. If we use echo signals reflected from the bottom of the object of control, taking into account the change in the type of wave, then a combined image of the reflector can be obtained from a set of partial images recovered by the digital focusing antenna (CFA) method. If the echo signals measured in the combined mode for each piezoelectric transducer are used, it is possible to estimate the displacement of the reflector across the welded joint with an accuracy of ± 1.5 mm. Numerical and model experiments have confirmed the operability of the proposed approach.
There is a concept of technology maturity levels (technology readiness level — TRL), which describes the technology transition from the TRL 1 level. “The fundamental concept of technology has been formulated” to the TRL 9 level. “The product meets all requirements: engineering, production, operational, quality and reliability.” So the Total Focusing Method technology, aka digital antenna focusing (CFA), aka combinational SAFT, aka IWEX, aka Sampled Phased Arrays (SPA) has turned from an exotic and contested algorithm into a standardized one.
In the first part of the article, it is shown that in order to restore a high-quality image of reflectors in a welded joint of a pipeline of the Du800 type, it is not enough to use the CFA method in a variant designed for the propagation of ultrasound in a homogeneous isotropic medium. To further improve the image quality in the CFA method, it is necessary to: determine the elastic properties of the surfacing of the Dn800 pipeline and take into account its anisotropy during ultrasound propagation, as well as the surface profile. To increase the resolution and reduce the noise level, the CFR-H and maximum entropy (ME) methods were applied. Comparison of the results of ultrasound and X-ray controls allowed us to conclude that the first of them is more informative for detecting crack-like reflectors.
The application of the digital image focusing (DFA) method to determine the type of detected reflectors is considered. To do this, two antenna arrays are used, located on opposite sides of the reflector, with the help of which echo signals are recorded through three acoustic channels in the double scanning mode.
When conducting ultrasonic testing, a situation may arise when the values of the recorded echo signals will be greater than the dynamic range of the receiving amplifier and the analog-to-digital converter of the flaw detector. This will cause the echo signals of pulses of large amplitude to undergo a cut-off operation (clipping) and reduce their amplitude, which may lead to an error when estimating the size of the reflector. A declipping method based on the Gershberg algorithm is proposed―Papulis, and its comparison with the declipping method using the least squares method is carried out. Numerical and model experiments have shown that the Gershberg method―Papulisa works more steadily than the least squares method for noisy echo signals and in the case of a rough step of their sampling.
The Plane Wave Imaging (PHI) method allows you to combine the advantages of phased array antenna technology (FAR) and CFA technology. In PWI mode, when emitting a plane wave, all elements of the antenna array (AR) work (as in the HEADLIGHT mode), which allows you to increase the energy entered into the control object, and echo signals are recorded by all elements of the AR (as in the CFA mode).
Practically any welded joint cannot be considered as a homogeneous isotropic medium for control by ultrasonic (ultrasonic) waves. If the phase changes during the propagation of the ultrasonic wave are less than 180 degrees, then the medium can be considered as isotropic and homogeneous. Otherwise, restoring the image of the reflectors using simple algorithms will cause the glare of the reflectors to shift from their true positions, and the shape of the glare will be distorted. Moreover, distortions can lead to the fact that instead of one glare, two or more with a smaller amplitude will appear. As a result, the glare amplitude of the large reflector may not reach the rejection level and the defect will be missed. One of such objects is repair welding in welded joints of pipelines DN800, which have anisotropy. The situation is complicated by the presence of anticorrosive surfacing on the inner surface of the pipe, which also has pronounced anisotropic properties, which does not allow working on a single reflected beam, without taking into account its anisotropic properties.
Practically any welded joint cannot be considered as a homogeneous isotropic medium for control by ultrasonic (ultrasonic) waves. If the phase changes during the propagation of the ultrasonic wave are less than 180 degrees, then the medium can be considered as isotropic and homogeneous. Otherwise, the restoration of the image of the reflectors using simple algorithms will lead to the displacement of the reflector blocks from their true positions, and the shape of the glare will be distorted. Moreover, distortions can lead to the fact that instead of one block there will be two or more with a smaller amplitude. As a result, the glare amplitude of the large reflector may not reach the rejection level and the defect will be missed.