The Phased Array Technique is aprocess wherein UT data are generated by controlled incremental variation ofthe ultrasonic beam angle in the azimuthal or lateral direction while scanning theobject under examination. This process offers an advantage over processes usingconventional search units with fixed beam angles because it acquiresconsiderably more information about the
reflecting object by using moreaspect angles in direct impingement. Each phased array search unit consists ofa series of individually wired transducer elements on a wedge that areactivated separately using a pre-selectable time delay pattern. With a lineardelay time between the transmitter pulses, an inclined sound field isgenerated. Varying the angle of refraction requires a variation of the lineardistribution of the delay time. Depending on the search unit design, it ispossible to electronically vary either the angle of incidence or the lateral skewangle. In the receiving mode, acoustic energy is received by the elements andthe signals undergo a summation process utilizing the same time delay patternas was used during transmission.
Flaw sizing is normallyperformed by measuring the vertical extent (in the case of cracks) or thecross-sectional distance (in the case of volumetric/planar flaws) at the 6 dBlevels once the flaw has been isolated and the image
normalized. Tandem sizing andanalysis uses techniques similar to pulse-echo but provides images that areeasier to interpret since specular reflection is used for defects orientedperpendicular to the surface. For cracks and planar
defects, the result should beverified using crack-tip-diffraction signals from the upper and lower ends ofthe flaw, since the phased array approach with tomographic reconstruction ismost sensitive to flaw tip indications and is
able to give a clearreconstruction image of these refraction phenomena. As with other techniques,the phased array process assumes isotropic and homogeneous material whoseacoustic velocity is constant and accurately known.
Sectorial scans (S-scans) withphased array provides a fan-like series of beam angles from a single emissionpoint that can cover part or all of a weld, depending on search unit size,joint geometry, and section thickness. Such a series of beam angles candemonstrate good detectability of side-drilled holes because they areomni-directional reflectors. This is not necessarily the case for planarreflectors (e.g., lack of fusion and cracks) when utilizing line
scanning techniques where thebeam could be misoriented to the point they cannot be detected. This isparticularly true for thicker sections when using single line scanning techniques.
ITE has expert manpower withyears of expertise in performing PAUT of various components including Welding,Castings, Pipes, Tubes, Plates, etc.
As a recent addition to theportfolio of our services we have started performing Phased Arrau UltrasonicTesting of Boiler Tubes as well with diameter as low as .75 inch to 4.5 inches.This is done used a specialised attachment called Cobra scanner. Cobra scanneris used with Olympus Omniscan MX2 withother necessary accessories to perform reliable scanning of Boiler Tubes weldedjoints. With Cobra Scanner, we have been able to remove various challenges andlimitations faced by customers in the power sector. One of them being reduceddependence of Radiography Testing, since PAUT using cobra scanner give betterand faster result that Radiography Testing.
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