How does it work?
Phased array systems offer the possibility of performing inspections with ultrasonic beams of various angles and focal lengths using a single array of transducers. Software control over beam angle and focusing is achieved by application of precisely controlled delays to both the emission pulse and received signal for each element in an array of transducers, hence the term "Phased Array".
While the term "phased array" implies handling the many signals from multi-element transducers, the resulting A-Scan responses are comparable with those obtained using a fixed angle probe with a conventional pulse-echo imaging system. Therefore, imaging and image interpretation also remain the same as for a conventional pulse-echo system.
As with other UT imaging systems, the A-Scan data can be processed to provide top, side and end view images of the inspected volume of material. In addition to standard imaging, Phased Array systems can produce sectorial scans (S-Scans), a feature unique to this technology. S-Scans are real-time side view images generated from a single inspection point without any physical movement of the transducer.
Multiplexing also allows motionless scanning. Here a focused beam is created using a few of the many elements contained in a Phased Array probe (up to 128). The beam is then multiplexed to the other elements to allow a high speed scan of the component with no transducer movement along that axis. In addition to specifying "individual" pulse-echo probes for an inspection, it is also possible to programme the use of a TOFD pair or a transmit-receive configuration.
Signal to noise is frequently improved with focussed probe techniques of which phased array is one example. Flaw detection and sizing ability is only limited by the beam width which, in theory, can be less than 1mm (0.04"), depending on the excitation frequency. This ability to narrowly focus the beam at a given range provides enhanced spatial resolution of flaw signals with respect to imaging systems that use standard ultrasonic probes. Although the calibration procedure can be complex and time consuming, once completed, scanning speeds of up to 100mm (4") per second can be achieved.
What will it find?
Manufacturing flaws (lack of sidewall fusion, lack of root penetration, lack of root fusion, porosity, etc.), in-service flaws (fatigue cracking, stress corrosion cracking, corrosion, erosion, etc.) and parent material flaws (inclusions and laminations).
Where is it used?
A particular application of phased arrays is pipe girth weld inspection where it replaces systems with many probes. This technique is also used for the rapid inspection of thick section pressure vessel welds (including nozzles), turbine disks, shafts and blade roots, welds in coarse grained materials and steam generator tubes.