The German version of this paper was presented at the DGZfP Symposium Wissenschaftliches Kolloquium Grundlagen der Zerstörungsfreien Prüfung(Research in NDT) during the MTQ Exhibition Report 12.-15.11.1996 in Dortmund D.

Abstract

Table of contents
1 Introduction
2 Phased Array Principle
3 Broad-band Probes with Composite Crystal Technology
4 Universal Phased Array Probe
5 Potential NDE Applications for the Phased Array Ultrasonic Probe
6 Summary and Outlook
7 References

1. Introduction

The codes and standards stipulate performance of ultrasonic examination within the scope of mechanized inservice inspections (ISI) of welds in nuclear power plants. Inspection systems with various scanning directions and angles of incidence including tandem functions are used for this purpose. Because space is limited, it is frequently impossible to mount all of the essential conventional probes on a single inspection system at any time. Time-consuming conversion work involving a high dose rate exposure and multiple scanning passes are the inevitable consequences of this. Development activities have therefore been concentrated on replacing conventional UT probes with phased array probes. One UT phased array probe replaces several conventional probes.

The phased array probe is a UT probe in which the angle of incidence is controlled electronically by means of a suitable ultrasonic instrument. Digital beam angle adjustments are made in the horizontal and vertical directions as appropriate to the geometry of the item to be inspected (orientation of welds to be examined relative to probe). The phased array probes used up to now are large with a partially restricted useful angle of incidence range - in effect, they were all special-purpose probes.

The aim of development work was to design a probe which would meet the following requirements:

1. Compact design
2. Angle of incidence from 0° to more than 70° in steel (Fig. 1).
3. Broad band
4. UT examination with wave mode conversion and tandem functions
5. Good sensitivity
6. Good signal-to-noise ratio
7. Rugged design

2. Phased Array Principle

The phased array probe combines the functions of several conventional probes in a single casing. It is no longer necessary to use different wedges to set each of the various angles of incidence, as was the case with conventional probes. The probe can now be adjusted and the angle of incidence can be set digitally (Fig.1).

A phased array probe consists of a linear array with 16 crystal elements. The crystal is acoustically dampened in order to obtain a maximum sound pressure for the material to be examined. Vertically or horizontally adjustable beam angle array probes can therefore be designed and built for the contact examination technique (Fig. 2 and 3). The more crystal elements used in the probe, the better the scanning range of the sound beam will be. Studies which are not the subject of this report have shown that the 16 element array is a good compromise with respect to conductor number, beam angle adjustability and probe size.

Figure 2: Linear Array

Figure 3: Beam angle ranges - vertical and horizontal adjustment

If the crystal is set up direct on a examination surface, the phased array probe can be controlled in an range from -90° to +90° (Fig. 2) . If the sound beam is set to 90° using this 0° array, it is evident that the effective crystal size reduces with increasing angle. The phased array probe has therefore been designed with a wedge and a fixed natural angle of incidence. Angles of incidence from 0° to 70° are possible with this array. The phased array principle is based on the time-delayed activation of each element. The array elements are shown with signaling leads in Fig. 4. The crystal elements are excited with a burst of energy and emit a wave front. The echo is input as an RF pulse. The angle of reception does not have to be the same as the angle of transmission. This is a particularily useful feature when working with the wave mode conversion technique. In this case, a signal is transmitted as a longitudinal wave and, following wave mode conversion at a reflector, received as a transverse wave (Fig. 1) . It is also possible to operate two element groups in TR (transmitter/receiver) mode. A focal point can also be set electronically.

TR mode (transmitter / receiver) LLS technique (receiver situation)
Figure 4: Control Options for the Linear Array

The codes and standards stipulate the following angles of incidence for the nondestructive examination of welds in thick-walled components: 45° and 60° and additionaly the tandem function for volume zones, 70° for the zone near the scanning surface, and 0° for checking diluminations, probe-to-specimen coupling and measuring wall thickness. The use of phased array probes reduces the number of probes required from 14 to 4. The probe system is therefore much smaller and more compact.

3. Broad-band Probes with Composite Crystal Technology

The array crystal elements measuring 1.5x8 mm from standard PZT crystals were compared with composite crystals in a 0° probe. The back wall echo obtained from a steel calibration block was evaluated for this purpose. The highly-dampened probe with composite crystal technology features a short pulse response and a broad-band spectrum (Fig. 5). The probe with PZT crystal is less dampened, and the pulse response continues to resonate. Its frequency spectrum has a narrow band.

Figure 5: Comparison of PZT and Composite Technology
Measuring technique	PZT	Composite
Pulse of back wall echo
Pulse width of transmitter signal with back wall echo
Frequency spectrum of back wall echo

4. Universal Phased Array Probe (abbreviated: VU probe)

Figure 6: Universal Phased Array Probe (26Kb)

Development of the universal phased array probe was based on composite crystal material and a new probe design (Fig. 6). It offers the following features:
All aspects of the target specification have been met:

1. Vertical beam adjustment linear array probe
2. Composite technology broad-band ultrasonic probe
3. 16-element 2MHz monocrystal
4. Wide angle of incidence range from 0° to 70° in steel (Fig. 8) ==>
   • ideal for LLS wave mode conversion function and tandem function
5. Broad-band spectrum ==>
   • the large bandwidth supplies more broadly based information from the component
6. Short sonic pulse ==>
   • has a shorter dead zone after initial pulse
7. Good sensitivity in steel ==>
   • improves detection of discontinuities
8. Good signal-to-noise ratio ==>
   • an improvement on its predecessors
9. Good detectability of discontinuities in the near-surface examination zone
10. Better resolution ==>
    • provides better information on discontinuities
11. Compact probe design ==>
    • the size of the probe housing no longer obstructs inspections; good probe-to-surface contact even on items with "difficult" geometry
12. Compact, rugged probe design ==>
    • stainless steel housing for use in nuclear power plants, including a special, watertight coaxial plug connector (Fig. 7)

Figure 7: Schematic Diagram

Figure 8: Structure of Probe

Half cylinder: R=100 mm	New phased array design: Universal phased array probe	Conventional phased array design: LLT1.5 phased array probe
Figure 9: Comparison of Universal and LLT1.5 Phased Array Probe

In order to appreciate the performance of the universal phased array probe, it has been compared with the previous probe, the LLT1.5 (Fig. 9).The universal phased array probe possesses a considerably shorter dead zone after the initial pulse than the LLT1.5 phased array probe while retaining a good signal-to-noise ratio. The measurements were performed on a half cylinder using a transverse wave with a 45° angle of incidence. The back reflection signal is shown.

Using a longitudinal wave form with an angle of incidence of 70° or 79°, it is possible to detect notches made in the surface of the test block or transverse side-drilled holes just beneath the test surface (Fig.10).

Figure 10: Examination of Near-surface Reflectors
Measurement performed on calibration block K1, 4mm notches K1 side view:	Measurement performed on calibration block, 2mm side-drilled hole at a depth of 6mm Calibration block:
Angle of incidence = 79° longitudinal	Angle of incidence = 70° longitudinal

5. Potential NDE Applications for the Phased Array Ultrasonic Probe

If we consider the various conditions which limit nondestructive examination techniques, use of phased array probes can be seen to offer the following advantages:
Table 1

Boundary conditions	=>	Examination technique advantages
Difficult geometry	=>	Angle of incidence can be adjusted and scanned for the particular component
		Possibility of setting different wave modes
		A single probe for angle beam scanning, contact and wall thickness measurement
Impaired access to components	=>	Compact probe system
Inspection requirements demand many test functions	=>	Considerably fewer probes required thanks to phased array (one probe per scanning direction)
High dose rates in the nuclear or chemical industry	=>	Fewer or no conversions of probe system required
Small traversing ranges	=>	Compact probe system, can be advanced right up to obstruction
Short inspection times	=>	Only one scan needed
Underwater application	=>	Watertight casing and plug connector
Reproducibility of inspection data	=>	One exact probe index for all test functions
Evaluation possibilities	=>	Possibility of combining data
		Analytical features using contour tracing or tomography

6. Summary and Outlook

The universal phased array probe with composite crystal technology is able to replace a number of different conventional ultrasonic probes. It covers angles of incidence ranging from 0° to more than 70°. Even with an angle of incidence of 70° it is still possible to detect reflectors just under the surface with an adequate signal-to-noise ratio. Due to the element structure of the phased array probe, it is possible to program a focus, side lobe suppression, transmitter/receiver mode and wave mode conversion mode using a single electronic phased array system. This system can even perform complex examination tasks. Practical experience has already been gained with the universal probe. The hard requirements, the very tough time schedule and the minimized inspection system for the inspection with limitations were 100% fulfilled.

References

1. H. Wüstenberg, BAM, Berlin; W. Rathgeb, M. Agboghoroma, E. Fischer, Siemens KWU, Erlangen "Recent Applications of UT Phased Array Techniques for Inservice Inspection of Primary Components" Proceedings of the 11th International Conference on NDE in the Nuclear Pressure Vessel Industries, Albuquerque, New Mexico, USA, 30 April 1992
2. E. Fischer, M. Dalichow, Siemens, Erlangen; M. Tagliamonte, Siemens SPC, USA; H. Wüstenberg, BAM, Berlin "Advanced Ultrasonic Inspection System for the ID-Inspection of Reactor Pressure Vessels of BWRs" Presentation during 12th International Conference on NDE in the Nuclear Pressure Vessel Industries, Philadelphia, Pennsylvania, USA, 11 October 1993

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