Ultrasonic waves are mechanical vibrations excited in an elastic medium by a piezoelectric transducer under the action of an electric voltage. Typical ultrasonic wave frequencies range from 0.1MHz to 50MHz. In industry, frequencies from 0.5MHz to 15MHz are most often used.
Ultrasonic testing typically uses single-element transducers to create a divergent sound field. The ultrasonic beam is distributed along the acoustic axis with a small divergence. Beam divergence helps to detect and measure the size of defects at an angle to the beam.
Let us imagine that a piezoelectric element is cut into a set of identical elements, the width of each of which is many times less than its length (e < W). Each of these elements can be considered as a source of a cylindrical wave.
Wave fronts from a set of narrow piezoelectric elements will interfere, creating a total wave front.
These wave fronts can be delayed in time and synchronized in phase and amplitude to create a focused and controllable ultrasonic beam.
The main feature of the ultrasonic phased array technology is the computer-controlled amplitude and excitation pulse phase of individual piezoelectric elements in the multi-element transducer. Piezoelectric elements can be excited in such a way that it is possible to control the parameters of the ultrasonic beam, for example, the angle, the focal length, the size of the focal spot through a computer program. This allows you to detect defects, differently oriented with respect to the acoustic axis. A simple single-element transducer with a high probability may miss defects located at a high angle to the acoustic axis of the transducer, or away from the ultrasonic beam.
Detection of differently oriented defects by single-element (left) and multi-element transducer (right). The beam from the transducer of the phased array type can be directed at different angles and focused, providing the detection of different defects.
In order to create the beam at the required angle and the required focus, the individual elements are excited at slightly different times. As shown in Fig. 1.2, the echo signal from a certain point comes to the individual elements of the transducer at different times that can be calculated. The echoes on each element are delayed in time, and then summed. Their sum is reflected on the A-scan, which reflects the amplified echo signal from the required focal point and the attenuated signal from all other points on the beam path.
• When emitting, the acquisition unit sends the clock pulse to thephased array unit. The latter generates a high voltage pulse of a given duration and with a given delay determined by the focal law. Each element of the array receives one delayed pulse. The sum of the waves emitted by each element is a beam propagating at a certain angle and focused at a certain distance. This beam is reflected from the defect.
• When receiving, the signal is received by each element of the array and then delayed in time according to a given focal law. The delayed pulses are summed and form a single pulse that enters the acquisition unit.
Main components of the system for phased array operation
The main units required for the construction of the simplest system for working with phased arrays are shown in Fig. 1.9.
The chart shows:
the computer with TomoViewspftware, TomoScan III PAspecial flaw detector, the MCDU-02 scanner control unit(top row), the inspected sample, the transducer – phased array, scanner (lower row).
Even better images can be obtained by combining linear and sector scanning. This combination of methods provides well-recognizable images of defects. Rice. 1.11 shows examples of detection of various artificial defects and differences in their images in the B-scan.
Extensive diffusion of the root and body of the pop. seam. WT – 5.95 мм.