NDE and inspection
A new phase for a sound examination technique15 April 2009
A new system uses multiple flexible phased arrays to perform ultrasonic testing on complex structures, such as welds on PWR pressurizer nozzles, in a single pass. Guided by 3D computer models that help focus the beams, the system can examine rough or uneven surfaces accurately. After a number of experimental trials the device is now being evaluated in industrial conditions. By Guy Maes and Daniel Richard
Phased array ultrasonic testing (UT) techniques have been used for inspections in nuclear and conventional power plants for more than 10 years. Applications involving simple linear arrays, using either
sectional scanning or multiple-angle raster scanning, are now commonly used and accepted. But the power generation industry is continuously looking for inspection solutions to more challenging inspection configurations.
At the end of the 1990s, the first commercial phased array UT systems were used on-site by the early adopters of this new technology, and after 2000, the first formal performance demonstrations for in-service inspection in the nuclear industry were completed, mostly driven by research institutes .
In 2003, portable and battery-operated phased array UT systems were introduced on the non-destructive examination (NDE) market. The availability of such systems strongly contributed to the industry-wide acceptance of phased array technology. Today, in various industries, examination techniques using simple linear arrays are commonly used and accepted, for both manual and automated inspections. In Asia, Europe and North America, NDE inspection vendors have qualified several examination procedures for critical nuclear components. In various other industries, the structural integrity of newly fabricated components is reliably verified by phased array techniques.
But in some cases, for example on rough or uneven surfaces, portable instruments and linear array probes cannot provide adequate inspection results. These challenging applications typically require advanced software features and high-performance phased array hardware.
Rapid weld inspection
Two-dimensional matrix array technology allows for performing beam steering, beam focusing, and electronic scanning in two perpendicular planes. The two-plane steering capability can be used to vary the refracted angle and the skew angle of the ultrasonic beam simultaneously. But in practice, a sufficient number of elements (typically between 8 and 16) are required in each plane to combine adequate steering capability and sufficient active aperture (acoustic energy). Therefore, a phased array system with a large number of simultaneous beam forming channels is required.
As an example, a carbon steel or forged stainless steel butt weld is considered, with a wall thickness of approximately 50mm. The examination technique is based on the use of only two phased array probes, that is a dual 2D matrix array probe, at a nominal frequency of 2.25MHz, on either side of the weld.
For the detection of circumferential flaws, shear wave beams with refracted angles from 40° to 70° and nominal skew angles (i.e. perpendicular to the weld center line) are proposed. The simulated acoustic field for the 50° SW beam is shown in Figure 1. In order to increase the detection probability of flaws in other orientations, and for improved flaw characterization, slightly skewed beams (nominal -20° and nominal +20°) can be used.
For the detection of axial flaws, shear wave beams with refracted angles from 40° to 60° and skew angles with a deviation of 35°, 45° and 55° from the nominal skew are proposed. The simulated acoustic field for the 50° SW beam at a skew of +45° is shown in Figure 2. It has already been shown in formal qualification exercises that such beams can reliably detect axial flaws while scanning from the base material .
Two-dimensional matrix arrays can be used to perform an extremely rapid inspection of a pipe butt weld, while simultaneously looking for circumferential and axial flaws.
This kind of phased array UT examination can be used in lieu of radiographic testing.
Figure 3 shows a schematic representation of the complete examination technique. A total of 159 focal laws (beams) are used for each probe on either side of the weld. In order to fully cover the volume to be examined, three scan lines are typically required. It can be estimated that the Dynaray phased array UT system allows for a full-thickness examination of a 24in NPS pipe weld with a thickness of 50mm in less than 30 minutes. Moreover, as the probes are positioned on the base material, the examination can be performed without removing the weld crown.
Inspecting wavy surfaces
Wavy or tapered surfaces are an important challenge for UT examinations on many occasions. In the nuclear industry, they often lead to incomplete coverage of the volume to be examined during in-service inspection, and as a consequence costly additional measures need to be taken, such as grinding of the weld crown, and/or weld overlay. Flexible phased array probes have been identified as a potential solution to allow for adequate transmission of acoustic energy through wavy or tapered surfaces.
A flexible array, combined with advanced acoustic beam generation tools, can drastically improve the detection capability.
Figure 4 shows how UltraVision 3 software can be used to position the flexible array (2MHz, 24 elements) on a CAD representation of the test specimen. This test specimen has a tapered surface, representative for the dissimilar metal weld region of a PWR pressurizer nozzle, and contains various artificial reflectors.
Obviously, the flexible array improves the transmission of acoustic energy into the material, but this is only one part of the solution. The wavy, tapered surface introduces major distortions in the sound field generated by the phased array probe. The right image of Figure 5 shows the simulated acoustic beam generated in the specimen, when a nominal focal law (i.e. for a flat surface) is used. UltraVision 3 can however generate optimised focal laws that take into account the complex surface. The simulated acoustic beam for the optimised focal law is shown on the left image of Figure 5.
Figure 6 shows actual phased array UT data from the flexible array probe on the considered test specimen.
Signals from optimised and nominal beams were recorded simultaneously (but in different channels) with the Dynaray system and UltraVision 3. The software uses the CAD representation of the test specimen to calculate the individual element delays for the optimised beams. On the left side, the sector-scan view generated with the optimised focal laws shows clear corner reflection and tip signals from one of the 5mm deep notches. On the right side, the sector-scan view generated with the nominal focal laws shows some indication from the corner of the notch, but this indication is not well resolved and incorrectly positioned. The tip signal is not detected with the nominal focal laws. This example clearly illustrates the drastic improvement obtained when using optimised focal laws groups.
In this example, the phased array UT data was recorded during a one-line inspection sequence along the weld centreline. As the surface profile was constant during the scanning sequence the same optimised focal law groups could be used. If a multiple-line sequence is required, or if the flexible array must be moved perpendicular to the weld center line, obviously the optimised focal law groups must be recalculated for each position of the probe.
The UltraVision 3 software includes tools to generate an accurate CAD file of the considered component from either manual profile measurements, optical measurements or UT measurements. From this CAD file, the focal law groups for each probe position are calculated and stored in memory prior to the examination. During the inspection sequence, the processing power of the system allows for rapid retrieval and application of the appropriate optimised focal law group. This feature is called ‘position-dependent focal law groups.’
The experimental results shown in the above case study were obtained with a flexible linear array probe of the first generation. Taking into account the feedback from various trials, our probe manufacturer has now developed improved flexible array probe designs that are ready to be evaluated in industrial conditions.
In addition, the development is not limited to linear array probes, because for some inspection configurations, single and dual flexible 2D matrix arrays are required to provide an appropriate solution.
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Zetec launched a phased array UT technology called Dynaray in 2007. The first units were delivered in the middle of 2008. This phased array UT data acquisition system can be configured with up to 256 simultaneously active channels and up to 4096 focal laws, or beams, to benefit more fully from the versatility of 2D matrix array probes.
D. MacDonald, J. Landrum, M. Dennis and G. Selby, 'Appendix VIII Qualification of Phased Array UT for Piping', 6th EPRI Piping & Bolting Conference, Point Clear (AL), USA, August 2002.
|About the authors|
This is an edited version of a paper presented at the WCNDT conference in October 2008. Guy Maes, [email protected]; Daniel Richard, [email protected] Zetec-Quebec, 875 boul. Charest Ouest, Suite 100, Quebec, Quebec, G1N 2C9, Canada