APPLICATION OF PHASED ARRAY TECHNIQUES ...

IRSN

FRO400099

APPLICATION OF PHASED ARRAY TECHNIQUES TO COARSE GRAIN COMPONENTS INSPECTION

G6rard CATTIAUX, Steve MAHAUT, Jean-Louis GODEFROIT, Olivier ROY

Rapport IRSN/D6partement d6valuation de &W No 624

Octobre 2003

I NSTIT UT DE D t

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RA DI P ROTECTION ET DE E M E N T D t V A L U A T I

SO R ETt N D E

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RAPPORT DES/624

APPLICATION OF PHASED ARRAY TECHNIQUES TO COARSE GRAIN COMPONENTS INSPECTION

G6rard CATTIAUX *, Steve MAHAUT, Jean-Louis GODEFROIT, Olivier Roy

Proceedings Ultrasonic International 2003 Juillet 2003

IRSN/DES/SAMS CEA/LIST/SISC SACLAY Octobre 2003

APPLICATION OF PHASED ARRAY TECHNIQUES TO COARSE COMPONENTS INSPECTION

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Steve NtAHAUT.-, Jean-Louis GODEFROIT, Olivier ROY a, G6rard CA'171AUX b .Commissariat a PEnergie Atornique, LIST/SISC, CEA Saclay, Bdt. 611, 91191 Gif-sur-Yvette cedex, France b nstitut de Radioprotection et de SCiret6 Nucl6aire, DES/SAMS, BP 17, 92262 Fontenay-aux-Roses cedex, France

Abstract Ultrasonic inspection of cast stainless steel components from primary and auxiliary cooling circuits of French Nuclear Power Plant has to face with major difficulties due to the coarse grained structure of these materials. Attenuation losses and structural noise are encountered, which limits the performances of defect detection ability, mostly in terms of degraded signal-to-noise ratio and poor sensitivity. To overcome such problems, theoretical and experimental studies have been achieved at the French Atomic Energy Commission, with support from the French Institute for Radiolo ical Protection and Nuclear Safety. Experimental studies have been performed over stainless steel specimen of known coarse structure (equiaxial grains and/or elongated grains), containing artificial reflectors (cylindrical holes and electroeroded surface breaking notches). Those mock-ups have been inspected using contact probes of different array designs (linear or matrix splitting), and using pulse echo or dual-element techniques. Such arrays allow to control the ultrasonic beam so as to investigate different inspection angles and focusing depths. Experiments were carried out using oblique longitudinal waves, using delay laws computed by a specific model, taking account of acoustical and geometrical properties of the probes and the inspected component. In addition, specific reconstruction techniques have been investigated to enhance the signal-to-noise ratio as well as spatial resolution. These techniques are based on beam-forming summation and multi-angle inspections. Experimental results show that such techniques allow to reduce the speckle noise and to optimise the beam resolution. Those increased performances allow to detect and to size small planar defects located at the inner wall of a thick specimen, using comer and tip diffraction echoes. Keywords phased arrays, stainless steel, beam forming image and signal processing. , Corresponding author. Tel: 331 69 08 47 14; Fax: 331 69 08 75 97; e-mail: steve.mahautgcea.

1. Introduction Cast stainless steel is widely used in pmary and secondary cooling components of pressurized water reactors (PWR), because of its good corrosion resistance, high strength and weldability. However, the performances of ultrasonic inspections of such materials are degraded because of its coarse grain structure, which leads to attenuation losses and scattering noise. Such phenomena have been specifically studied in the nuclear industry, and one may find relevant synthesis about these effects and some recommendation for probes optimization in ternis of frequency and operating mode in [1]. Basically, longitudinal waves generated by low frequency probes (about I MHz) are usually suggested to reduce the backscattered noise and therefore to enhance the signal-to-noise ratio 2 Large aperture focusing

probes are therefore required to preserve a thin lateral resolution (hence a good defects sizing accuracy) in spite of this low frequency, which is difficult to be carried out for contact inspections, because of possible internal reflections in the wedge, as well as unperfect contact between the specimen and the probe. Studies have then be perforined at the French Atomic Energy Commission (CEA), supported by the Institute for Radiological protection and Nuclear Safety (IRSN), to investigate the application of phased arrays techniques for cast stainless steel inspection. 2. Simulation of phased array techniques 2.1 Considerations for the phased array probes design As already mentionned, large aperture focusing probes operating at low frequencies are conventionnaly used to reduce the attenuation losses and backscattered noise These characteristics cannot be readily implemented for contact probes, because large wedges may also give rise to internal echoes inside the wedge which lead to a "blind" detection zone close to the probe (near surface echoes may not be distinguished from the wedge echoes). For such cases, dual-element probes, made of separate transmitting and receiving probes over half-wedges acoustically isolated, are very efficient to remove this dead zone, and the possibility to use large aperture makes them suitable for thick inspection of coarse grain components 3,4]. At last, both transmitting and receiving part of the probe have been splitted into matrix arrrays in order to focus and steer the beam both in the incidence and perpendicular plane. The conception of this dual-element array probe has been made using the modeling tools gathered in the Civa expertise software developed at the CEA [5]. Those tools allow to predict ultrasonic fields radiated by various transducers into complex (geometry and structure) specimen 6], as well as complete inspection result (accounting for beam-defect interaction for various paths between the probe and the defect - direct or comer echoes - and interaction with the boundaries of the specimen). 2.2 Prediction of ultrasonic field in the specimen Figure I shows the ultrasonic fields radiated by the dual-element matrix array, designed to inspect planar or cylindrical specimens (using a shaped wedge to match a pipe of 450 mm external radius) of 70 mm thickness with 45' longitudinal waves. Dimensions and array splitting design of this probe, operating at 800 kHz frequency, are also reported. Fields radiated in the incidence and perpendicular plane are displayed, corresponding to longitudinal (L) waves steered at 45' and focused at 70 mm (at backwall). One can observe that the delay laws applied to the array probe allow to obtain the desired beam characteristics into both planar and cylindrical specimen (focused 45' L-waves, very low array lobes level, almost no shear waves). 2.3 Prediction of the inspection performances Figure 2 displays the simulated inspection of the planar and cylindrical specimen containing vertical planar defects emerging at backwall, from 3 to 15 nun height. As the inspection is performed using 45' L-waves, the echoes received by the dual-element array probe are related to the comer echo path (reflection at backwall and at the defect), and the tip diffraction echoes (with or without reflection at backwall), which allow to size the defect. For both configurations, one can observe that the tip diffraction echo is clearly distinguished from the comer echo for the IOmm height defect. The overall responses of

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defects from 3 to 10 mm height do not exceed large variations (less than 6 dB), which tends to indicate that such defects would be detected within a reasonable signal-to-noise ratio.

3. Application of the phased array inspections over realistic mock-ups 3.1 Description of mock-ups and array probes Cast stainless steel specimen of equiaxial and uniaxial (elongated grains) structures have been used to evaluate the performances of different contact array probes. Artificial defects (side drilled holes and electro-eroded planar notches at backwall) have been implanted in those specimen to evaluate defect detection and sizing abilities. Figure 3 shows typical macrographs of both structures as well as the implantation of the artificial defects (corresponding to the defects dimensions already used for the previous simulated results). The designed dual-element matrix probe has been used to inspect these components, as well as an additional existing contact array probe in pulse echo mode, which was not designed for this application, of 2 MHz central frequency, 128x5Omm' 64 elements). 3.2 Comparison of inspections of a planar stainless steel mock-up with both array probes Figure 4 shows a comparison of performances modification (compared to a erritic steel specimen containing calibration side drilled holes) observed on a cast stainless steel component inspected with both probes previously described. Attenuation losses and signal-to-noise ratio are drastically degraded compared to the ferritic steel inspection using the 2 MHz probe (an amplification of 20 dB is set from the ferritic to the stainless steel inspection, and the SNR in the stainless steel component is about 4 dB), whereas the 800 kHz dual-element array probe allows to obtain a SNR about 14 dB, while the attenuation losses are about 6 dB. It also has to be mentionned that these performances are obtained thanks to the focusing pattern applied to the matrix probe. 3.3 Inspection of a cylindrical mock-up The dual-element matrix array probe has been used to inspect a cylindrical forged stainless steel component. For this configuration, both matrix arrays used at Transmission/Reception were mounted onto an adapted cylindrical wedge. Figure shows the experimental Cscan (213 raster scanning pattern over the cylindrical specimen, as illustrated on the simulation result on figure 2 and Bscan views (cross section images of the inspection) related to the different planar defects from 3 nun to 15 nun height. All the defects are detected, and defect sizing using the tip diffraction echo is performed from the mm height defect. These results confirm the ability of this array design to optimally focus and steer the beam to provide a high resolution and sensitivity (although experiments over a cylindrical cast stainless steel have not been perfon-ned, comparison with the planar stainless steel mock-up shows that for similar materials, it should still be allowed to detect all of these defects with similar SNR as those obtained on the planar mock-up). 3.4 Application of multi-angle reconstruction for the planar cast stainless steel mock-up inspection

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In addition to the inspections carried out with the phased arrays in "conventional" mode (application of one set of delays at transmission and reception for the whole scanning displacement), another technique, making use of multi-angle inspections and summation has been investigated. This technique may be denoted as tomoSAFT", as it relies on summation of echoes received at different scanning positions, but each of these echoes is received following a given arbitrary inspection angle applied both at transmission and reception thanks to a multi-angle delay law settings. This procedure therefore acts as follows as the array probe is continuously scanning the specimen, consecutive shots are made to transmit and receive longitudinal waves focused at the backwall within an arbitrary range of angles. The echo reconstruction is obtained using the echoes from each angle inspection, temporally and spatially shifted according to the defect location at backwall. Figure 6 displays the comparison of the inspection carried out over the planar mock-up using 450 L-waves, and the presented technique with three different angles inspections 40'/45'/50'. The experimental Bscan images from the 45' L-waves inspection and from the "tomoSAFT" technique are displayed, which shows that backscattered echoes have been decreased compared to the defect echoes. Defect echoes are also slightly more focused, as the technique leads to a larger synthetic aperture, both at transmission and reception. Echodynan-iic curves also displayed on this figure also allow to estimate the improvement of the SNR, which is about dB using this technique. It can also be pointed out that some backscattered noise echoes still remain as the technique is only applied for a given focusing depth (at backwall).

4. Conclusion Experimental and theoretical investigations of phased array applications have been carried out to improve the detection and sizing accuracy of defects located in thick cast stainless steel specimen. A specific matrix array probe used in dual-element mode has been designed and experimentally validated over mock-ups with representative material structures, containing electro-eroded planar notches and cylindrical holes for calibration purposes. Experiments have been made with longitudinal waves focused at back-wall, and show that the lateral and temporal resolution allows to detect all defects (from 3 to 5 mm height) and to size them (from 10 mm height) in a 70-mm-thick specimen) with a fairly good sgnalto-noise ratio (higher than 10 dB). Additional tests have also been performed over a cylindrical specimen. At last, experimental studies have been made to investigate multi-angle reconstruction, in order to increase the signal-to-noise ratio and the lateral resolution. Preliminary results obtained with such a technique, which can only be applied with a phased array thanks to its ability to electronically steer and focus the beam at transmission and reception while continuously scanning the specimen, show that one can significantly improved the signal-to-noise ratio.

5. References [I] - G. Maes, B. Hansoul, P. Dombret, Proc. of the 12 and pressure vessel ind., 1994, 197.

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inter. Conf. on NDE in the nuclear

[2 - M. Serre, P. Benoist, D. Villard, N. Demathan, op.cit. , 191. [3 - Y. Kurozwni, in Insight, Vol. 44, N' 7 2002, 437. [4 - M. Delaide, G. Maes, D. Verspeelt 2

nd

Int. Conf. On NDE in relation to Structural

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Integrity for Nuc. And Press. Components, 2000, 347. [5 - S. Chatillon, A. Lh6mery, F. Cartier, P. Calmon, in D. 0. Thompson, D. E. Chimenti (Eds.), Rev. of Prog. in QNDE American Institute of Physics, 2001, 20A, 71 . [61 - S. Mahaut, 0. Roy, C. Beroni, B. Rotter, in Utrasonics,Vol. 40, N' 1-8, 2002, 165.

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Figure 1: Simulation of UT fields radiated by a twin-crystal matrix array probe through a planar (left) and a cylindrical (right) specimen Figure 2 Simulation of inspections perforined by the twin-crystal matrix array probe through a planar and cylindrical mock-ups containing planar notches at backwall Figure 3 Macrographs and artificial defects implantation in the experimental mock-ups Figure 4 Experimental inspection of ferritic steel (left) and cast stainless steel nght) planar calibration mock-ups with array probes at 2 MHz (top) and 00 kHz (bottom) Figure 5 : Experimental inspection of a cylindrical forged stainless steel mock-up with the twin-crystal array probe Figure 6 Comparison of 45' longitudinal waves inspection (left) and multi-angles reconstruction technique (right) carried out with the twin-crystal array probe.

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