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.
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:
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.
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.
|Figure 5: Comparison of PZT and Composite Technology|
|Pulse of back wall echo|
|Pulse width of transmitter signal with back wall echo|
|Frequency spectrum of back wall echo|
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:
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|
|Angle of incidence = 79° longitudinal||Angle of incidence = 70° longitudinal|
|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|