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Scanning Modes at the Application of Ultrasonic Phased Array Inspection Systems Authors: H. Wüstenberg, A. Erhard, G. Schenk Contact

1. INTRODUCTION

The first applications of phased array probes in NDT tried to imitate the successful medical ultrasonic diagnostic with phased array scanning systems. But due to the many differences between medical diagnostic and industrial applications the phased array concept had to be modified for NDT concerning the probes, the electronic equipment and the software [5 to 14]. Nowadays there are available some manufacturers of NDT phased array equipments and probes. But the application of phased arrays in NDT is also linked to different modes of there use and the corresponding software. A simple takeover e.g. of sector scans and parallel scans from the medical field satisfies only a small number of NDT problems. There are - instead of this - various other scanning modes currently applied [5, 6, 7, 13, 14]. A successful application of phased arrays in NDT requires the intelligent selection of the different soundfield parameters e.g. beam orientation and position, focusing and coupling curvature correction. This includes also the question of the theoretical modelling for the steering parameters as there are: the delay times and amplifications for each element related to the angle and acoustical focusing, the different amplitude correction factors for the distance (DAC or DGS) and the angle and focusing dependent sensitivity variations. The following chapters are describing some of the essentials concerning the software architecture needed for the operation of a phased array system and in connection with that the different scanning modes which up to now have been applied by BAM during 15 years of experience with phased array applications [1, 2, 3, 4].

2. SOUNDBEAM STEERING POSSIBILITIES WITH LINEAR PHASED ARRAYS

The basic possibilities of phased arrays are summarized with the help of Fig. 1 and 2. A phased array system is based on a probe divided into small single elements, used as transmitters and/or receivers of ultrasonic waves [1, 12]. The division of a radiating surface in single elements which can be exited with different delay times and sensitivities allows an electronic soundfield steering offering the 3 basic possibilities of Fig.1:

Fig 1: Sound Beam Steering by Phased Array Probes

Fig 2: Variation of the Incidence (a) or the Skewing (Y) Angle by Linear Arrays

• One can move a beam along the line of a linear array which is the parallel scan of the medical diagnostic.
• More important for NDT is the sector scanning using shear- or longitudinal waves, depending on the angle of incidence and possibly also on wedge angles as demonstrated in Fig. 2.
• A more or less curved distribution of the delaytimes produces a focusing which can also be used to compensate the influence of curved coupling surfaces.

Fig. 2 demonstrates the basic possibilities of wedge equipped probes with a linear phased array [4].

• The angle can be changed only within the plane of incidence,
• One may produce mainly skewing angle variations within the plane perpendicular to the plane of incidence
• By a combining probe type with a rotated linear phased array system one produces skewing- and incidence angle variations.

The typical possibilities of such arrangements are demonstrated in the diagram below in Fig. 2, where the different type of functions describing the dependency of the angle of incidence from the skewing angle are demonstrated. Those probe arrangements have been the basis for a lot of experiences collected during the last years at the BAM together with other industrial partners like Krautkrämer, the German forgemaster VSG and Siemens.

3. SOFTWARE MODULES FOR PHASED ARRAY APPLICATIONS

There are five different areas of software packages which are needed for the planning, the execution and the evaluation phase of a phased array application. Those are:
• Software to calculate the angles of incidence and skewing, the needed focusing and the distances between probe and defect area and their typical variations along the probe scanning path based on the geometry data of an object, e.g. the coupling surface curvatures, the wallthicknesses etc. This is a job for suitable CAD packages. Application examples are the inner nozzle corner inspection and the inspection of the dome or spherical bottom of nuclear reactor pressure vessels [6]. Fig. 3 is demonstrating the importance of such geometrical calculations. The figure represents the inclined inlet nozzle for an emergency cooling line, emerging into the primary circuit of a pressurized water reactor. The contact technique requires a curvature adapted probe which can only be moved along a scanning line remaining orientated in parallel related to the axis of the main tube. The CAD program has to calculate for each probe position the needed incidence and skewing angles and also the required sound pathes.

Fig 3: Emergency Cooling Inlet Nozzle (inclined)

• Software to generate the exitation distributions for the elements of an array in order to generate the different sound field parameters like beam focusing, beam orientation and index point position. Those exitation patterns are essentially defined by the specific delay time and amplification distributions for each element for the transmitting and for the reception case. The character of those patterns can strongly influence the side or grating lobes of the resulting beam e.g. by an amplitude apodizing or by stochastic fluctuations of the delay times. There are basically two ways for the determination of the exitation patterns: One can use an iterative procedure based on a theoretical soundfield modelling and try to check an assumed exitation pattern with the model. For that purpose we are using a software module which calculates the directivity pattern of pulse exited ultrasonic linear arrays in different distances. The inverse problem, that means to input the required beam parameters and to produce at the output the corresponding exitation patterns, can not be solved generally. However, we have derived a set of approximative analytical formula for the probes of Fig.2 with delay pathes or wedges allowing the direct calculation of the delay time distribution- and amplification values for a given case, e.g. with a desired angle of incidence, a desired focusing distance or curvature compensation for both wave modes. [15 to 21]

  Fig 4: Shear Wave Directivity Pattern in the Plane of Incidence

  
  Those analytical formula are fast enough to be used even during the scanning procedure at pulse repetion rates close to 2 kHz. This may avoid the creation of look up tables for the steering of the phased array equipment during the planning phase of an inspection.
  The sound field calculation program calculates the directivity patterns shown in Fig. 4 where different angles of incidence have been realized by a shear wave probe of 3.7 MHz. The grating lobes can be recognized, they are in a good agreement with the grating lobes observed during the practical applications.
• Software to create look up tables for the steering during an inspection.
• Another important software module is the equipment control software which delivers the delay time and amplification distribution values, time gain control curves etc. to the phased array equipment in order to allow the execution of the desired function during the probe scanning movement. This control software has to cop with the needed fast pulse repetition rate. Typically 1 kHz to 10 kHz pulse repetition rates are possible. Since many phased array applications are limited by late returning echos, e.g. in the case of large turbine shafts, one is forced to use in such cases a pulse repetition rate down to some 40 or 50 Hz. The control software has to carry out the data storage which should be fast enough in order to guarantee that the pulse repetition rate is not limited by the storage procedure. The control software has also to guarantee that all the results are stored in a given structure e.g. characterized by the so called ZEUS data format [22].
• The fifth essential software package is a package for the presentation and evaluation of the results. Here different approaches may be used: from simple time displacement scan (TD-) presentations over C-scan, B-scan to SAFT-reconstructions and echotomography presentations. The different pictures produced by such software packages have to be adapted to the given purpose of specific inspection tasks.

4. SCANNING MODES

The different scanning modes for a phased array equipment can be:
• Free sweeping of the beam e.g. for a sector scan, a parallel scan or even a focus scan for dynamic focusing. Those sweepings can be triggered by the repetition rate frequency with a constant increment between the different shots, that means e.g. a 0.2° step for a sector scan between the different beam angles or also with a variable increment adapting the sector scan or parallel scan to critical areas.
• Angles, focusing, amplification and TGC-curves predetermined by a look up table or calculated during the scanning procedure and triggered by the probe position along the scanning path. This underlines the link between the software modules for geometry and the soundfield modelling.
• The execution of selected and modified inspection functions at specific probe positions according to specific interference conditions, curvatures, expected defects or other specific events, like e.g. a probe approaching to a specific area in a weld, where the internal noise increases and a specific focusing is needed.

The storage of the inspection results (e.g. containing the digitized A-scans, the probe parameters and the probe positions) has for all different scanning modes to guarantee that during the evaluation of the results images can be produced for specific inspection functions, angles or scanning pathes. That means that the data format (e.g. ZEUS format ) must allow the access to A-scans according to different selection criteria.
The execution of the selected inspection functions needs special preparations at the equipment hardware in order to allow a versatile adaptation of the software to the specific sequence of the required inspection functions. It should especially be possible for a fast repetition rate that a position dependent triggering of specific function sets can be supported by the intermediate storage of the steering parameter data ( delay time distributions etc. ).

The following examples are describing application cases for the different scanning modes.

5. APPLICATION OF A SCANNING MODE WITH POSITION INDEPENDENT BEAM SWEEPING

For the inservice inspection of steam turbines and the manufacturing inspection of turbine shafts and generators we have developed a special software package to present the results as a socalled echotomography. Fig. 5 shows a turbine and some typical probe positions for an inservice inspection situation. The scanning mode is a free beam sweeping approach, which is only triggered by a pulse repetition rate frequency low enough to suppress interferences by late returning echos (e.g. 45 Hz), which are a typical phenomenon for the low attenuation steel of large turbine shafts. The scanning speed must be as low as to allow a sufficient coverage with angles and sector scans. Typically an angular increment of 2° and a rotating speed of 0.5 to 1 turns per minute must be used. By the storage procedure the A-scan data are linked to the actual probe position.

Fig 5: Inservice Inspection at a Steam Turbine with Phased Array Probes


Fig. 6 shows the results from an A-scan, from a sector-scan formed by A-scans and the echotomogram created by an arithmetic superposition of all sector scans. 2 single inclusions can clearly be separated.

Fig 6: Generation of an Echotomogram

6. APPLICATION OF INTELLIGENT SCANNING MODES WITH POSITION AND FUNCTION DEPENDENT STEERING OF THE PHASED ARRAY EQUIPMENT

As shown in Fig. 3 for the inspection of the nozzle inner corner area at an emergency cooling inlet nozzle, fairly large probes must be moved along the scanning path. Since the probe can not be rotated during that movement, it is essential to vary the angles of incidence and skewing electronically. This has been realized by a multi-array probe containing 4 different modules in one probe housing as demonstrated in Fig. 7 top right-hand side. The module 4 of this probe had been used to detect 4 notches within the nozzle inner corner area as demonstrated in Fig. 7 right-hand side bottom. The resulting TD-scan is shown in the same figure left-hand side. One can clearly recognize the indications of the 4 different notches. It is also possible to estimate the different notch depth represented by the echo-amplitude and dynamic behaviour. This application required a careful calculation of look up tables, which had been used during the scanning movement to feed to the phased array equipment at each position an adapted steering data set for the probe soundfield and sensitivity.

Fig 7: Inspection of the Inner Nozzle Corner with a Phased Array Probe

Fig 9: Detection of Test Notches by a Phased Array Probe

Fig 8: Ultrasonic Inspection of Railway Axles with Phased Arrays

Another example for an event controlled scanning mode is the inspection of axles from railroad wheel sets. Axles without an inner hole are difficult to inspect without dismantling the whole set. The application of phased array probes at different coupling positions as demonstrated in Fig. 8 allows the surveillance of very large areas at the surface of such axles without dismantling them. Fig. 9 shows the results of the phased array scanning where TD-scans have been produced at different angles, thus demonstrating the possibility to detect the different reference defects at the indicated positions.
The last example is representing the inspection of a girth weld at a pipeline. A typical probe arrangement according to RTD Rotterdam is shown in Fig. 10. In order to reduce the need for a specialized multiple probe arrangement requiring a careful individual adaptation to each wallthickness and tube diameter, a phased array realtime scanner as demonstrated in Fig. 11, can be used. Such a phased array probe, in case of Fig 11 a 64 element probe for 3 MHz, allows the movement of a beam and the simultaneous angle sweep. Another important feature of such a probe and the connected electronic equipment is the possibility of specific tandem inspection as proposed by Fig. 10.

Fig 10: Probe Arrangement for Girth Weld Inspection on Pipelines

Fig 11: Weld Inspection Using a Real-Time-Scanner

7. CONCLUSIONS

The scanning modes of a phased array inspection are linked to the equipment possibilities and the software features.

Five software modules are needed for the planning, execution and evaluation phase:

• Calculation of the geometry data by CAD or similar packages
• Soundfield modelling and/or analytical formula for the beam forming
• Creation of look up tables for the steering during an inspection
• Equipment control software during the execution phase
• Presentation and evaluation of the results

Essentially three scanning modes can be discriminated:
• Free sweeping of a beam
• Execution of selected inspection function sets linked to the probe position
• Execution of the selected inspection function triggered by special events

The last 2 modes require special precautions at the equipment hardware in order to allow a versatile adaptation of the software to the specific problem.

8. LITERATURE

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