3.3 UT Phased Array
Phased Array ultrasonic technique has been used last years in industrial area mainly at energy sector (nuclear and electric power stations), recent developments give chance to apply this technique as a quality control tool in pipeline construction.
Industry has applied massive investment to extent equipments life. Life extension or any equipment integrity studies need multi-discipline information about service condition history, fracture mechanic concepts and a perfect knowing about discontinuities that exists. Actual tolerable discontinuities dimensions are higher than that written in codes, because of safety incertitude multipliers and NDT associated errors. An approach different from fabrication quality assurance must be considered in equipment life extension and integrity calculation with respect to NDT. Later subject needs an NDT that is precise and reliable with respect to dimensioning significant structural discontinuities.
UT Phased Array application on FPSOs turret weld discontinuities has had the objective to give a precise and reliable characteristic to the weld in service inspection.
Phased array ultrasonic technology moved from the medical field to industrial sector at the beginning of the 1980s. By the mid-1980s, piezocomposite materials were developed and made available to manufacture complex-sharped phased array probes.
Advances in piezocomposite technology, micro-machining, microelectronics, and computing power (including simulation packages for probe design and beam-component interaction), contributed to the revolutionary development of phased array technology. Most conventional ultrasonic inspection use monocrystal probes with divergent beams. The ultrasonic field propagates along an acoustic axis with a single refracted angle. The divergence of this beam is the only “additional” angle, which might contribute to detection and sizing of misoriented small discontinuities.
Assume the monoblock is cut in many identical elements, each with a width much smaller than its length. Each small crystal may be considered a line source of cylindrical waves. The wave front of the new acoustic block will interfere, generating an overall wave front. The small wave fronts can be time-delayed and synchronized for phase and amplitude, in such a way as to create an ultrasonic focused beam with steering capability.
The main feature of phased array ultrasonic technology is the computer-controlled excitation (amplitude and delay) of individual elements in a multi-element probe. The excitation of piezocomposite elements can generate an ultrasonic focused beam with the possibility of modifying the beam parameters such as angle, focal distance, and focal spot size through software. The sweeping beam is focused and can detect in specular mode the misoriented discontinuities.
To generate a beam in phase and with a constructive interference, the .various active probe elements are pulsed at slightly different times. The echo from the desired focal point hits the various transducer elements with a computable time shift. The echo signals received at each transducer element are time-shifted before being summed together. The resulting sum is an A-scan that emphasizes the response from the desired focal point and attenuates various other echoes from other points in the material.
There are three major computer-controlled beam scanning patterns: Electronic scanning: the same focal law and de-lay is multiplexed across a group of active elements (see Figure 12); scanning is performed at a constant angle and along the phased array probe length (aperture). This is equivalent to a conventional ultra-sonic transducer performing a raster scan for corrosion mapping or shear wave inspection. If an angled wedge is used, the focal laws compensate for different time delays inside the wedge. Generally this scanning pattern is used at “in line“ fabrication inspection of plates, strips, bars and tubes. It could be used also in welding inspection.