|NDT.net - October 2002, Vol. 7 No.10|
Special single element as well as array immersion technique probes have been developed for both flaw detection and dimension measurement application, like bar and pipe testing, by means of automatic rotating ultrasonic inspection units. The probes have been built by the use of standard piezoelectric composite material, which was produced by dice and fill method for the standard probe manufacturing.
Probes with centre frequencies between 1 MHz and 10 MHz have been manufactured and checked in automatic testing machines of different types. A more than one-year-old experience for the use of such transducers in standard production units exists.
Due to the higher sensitivity to wear by the lower lens thickness and due to the lighter backing, composite transducers regularly show a reduced lifetime in rotating units with high rotational speed caused by the high centrifugal forces up to about 3000 g. A special manufacturing procedure was developed to increase lifetime comparable to the standard ceramic transducers.
Normally, the sensitivity and the signal-to-noise-ratio of composite transducers are only increased by a value of about 4 dB compared with standard ceramic transducers. This increase is caused by the higher coupling factor of the composite material itself and by the better acoustical impedance matching to the polymer material of the lens in contrast to the high impedance difference between ceramic and polymer material. For the standard immersion technique applications, however, cylindrical and spherical focused probes are used. The heights of polymer lens material between centre and edge are different by focusing with a standard lens for ceramic crystals. The sensitivity can be highly increased due to the loss of lens attenuation by curving the composite material itself and using only a matching layer of constant thickness. A better sound beam forming is possible, too. Additionally, the near surface resolution and, for materials with higher ultrasound attenuation like stainless steel for example, the defect sensitivity are increased caused by higher bandwidth of the material itself. As a result, an increase of sensitivity and sometimes also signal-to-noise-ratio between about 6 dB and more than 12 dB can be achieved.
The combination of high sensitivity and extreme resolution is often required for automatic testing machines, which are using mainly immersion technique probes. According to G. Splitt (1) however the needs of high resolution and high sensitivity are antipodes for traditional probes.
The development of piezocomposite materials for ultrasonic transducers in the middle of the nineties first for the medical probes and then for the standard contact probes gave us the change also to optimise the probes of automatic ultrasonic testing machines to improve the defect sensitivity and resolution.
As already pointed out by Paul Meyer (2) for the standard contact probes, also the improved immersion technique transducers should provide the following features to reach this goal:
This paper presents first results of probes especially adapted to rotating ultrasonic inspection units sold worldwide. The use of the new piezocomposite material and special production procedures for the manufacturing of the immersion technique transducers increases probe sensitivity, signal-to-noise-ratio and resolution.
The increase is achieved by:
A special manufacturing procedure additionally ensures a long lifetime even under high centrifugal forces in the rotating units.
Discussion of the advantages of the piezocomposite material
As described above the use of the new material for the manufacturing of ultrasonic probes gives the following advantages:
Coupling Factor and Acoustic Impedance Matching
Generally the piezocomposite material is produced by means of a dice and fill method. That means the ceramic plate is diced into rods, filled up with polymer and the thickness is grinded to reach the wished frequency whereby the pitch of the rod lattice should correspond to the wavelength to avoid radial modes and grating lobes. Figure one shows this principle for the 1-3 type piezocomposite normally used.
|Fig 1: Ceramic rods in a polymer matrix 1).|
For such a material the thickness coupling factor kt and consequently the electro acoustic efficiency is higher as for a standard PZT ceramic. Additionally, the acoustical impedance is reduced due to the percentage of polymer material because the value for the polymer is lower than for the ceramic. With a value of about 7 Mrayl for the composite material compared with the 29 Mrayl of the standard PZT ceramic the transmission losses via the polymer lens material (about 3 Mrayl) to the coupling water are dramatically lower.
Due to the higher coupling factor and the lower impedance mismatch an increase of the absolute signal amplitude is achieved by using the composite material for manufacturing immersion technique transducers. This means that the absolute output voltage of the defect signal received by the probe is higher but not consequently also the signal-to-noise-ratio because any acoustical noise is increased, too. For the automatic ultrasonic inspection units, however, already this increase is very helpful because now lower amplification are needed and the influence of the electrical noise by the surrounding equipment is reduced.
The desired increase of defect resolution can be achieved by an increase of the transducer bandwidth. Normally, a different type of higher damped ceramic material (lead meta- niobate instead of lead zirconate-titanate) and a heavy backing are used to build standard high-resolution probes. But the bandwidth improvements - reached by this way - yield to a dramatic loss in sensitivity. A big advantage of the composite material is now that this improvement of bandwidth is achieved by the structure of the material itself. The bandwidth is increased by the lattice structure itself. Filling with epoxy damps the ceramic in addition. This increase of bandwidth without loss of sensitivity shows not only an advantage in materials with higher attenuation. Figure 2 presents a comparison of transmitted and attenuated spectra of a material with moderate attenuation, calculated by Paul A. Meyer (2).
|Fig 2: Comparison of transmitted and attenuated spectra of a material with moderate attenuation 2).|
One can clearly recognize from this figure that already for a moderate attenuating material a high amplitude decrease occurs for the monolithic material. In spite of the amplitude shift (due to the normalization) there is no decrease of amplitude for the composite material caused by the high bandwidth. Consequently the composite transducer is able to receive the lower frequency parts of a defect signal with higher amplitude than a transducer made of a monolithic ceramic. Figure 3 shows the calculated corresponding received RF waveforms for such spectra.
|Fig 3: Received RF waveforms with moderate attenuation 2).|
Thus the piezocomposite material yields for material with moderate attenuation not only to an increase of signal amplitude but also to a higher signal-to-noise-ratio for defect detection.
As most of the immersion technique transducers, depending on the application, are focalised by using an additional polymer lens in front of the crystal to concentrate the energy into the material, the energy of the sound field has to be transferred via different heights of the lens due to the focalising radius. Especially for highly focussed large crystal probes the difference of the lens thickness from centre to edge of the crystal is very high so that the optimal thickness for an impedance matching layer of a quarter of the wavelength can only be realized for a very small centre point or line.
|Fig 4: Influence of lens refraction on the sound field of line focussed probes.|
When the piezocomposite material, however, is produced by means of a flexible polymer between the ceramic rods then the crystal itself can be curved to realize the desired focalisation without using any refraction lens. For such probes not only the central beam but also the side beams are emitted directly to the focal spot without any refraction at a lens interface.
Please refer to picture 4 for a presentation of the principal of the different propagation of central and side beams for standard lens and curved piezocomposite.
For the piezocomposite design not only the losses in the lens itself are reduced but also the side loop behaviour of the sound field is improved by the more homogeneous wave emission. So the complete sound field of such a probe is optimised. In addition, the sensitivity against misfits of the incident angle set-up in rotating testing machines is reduced.
Nevertheless, an impedance matching layer between crystal and water is necessary to optimise the layout for piezocomposite probes, too. But now this layer can be realized in a thickness of a quarter of wavelength over the complete crystal area to improve the energy transfer and to reduce the interface echoes.
In addition such a layer is absolutely necessary to protect the electrode against the heavy load in the rotating testing machines. Without such a protective layer the transducer would be destroyed in a short time by an erosion procedure, caused by small dust and scale parts in the coupling water.
As described above, the thickness of the lens cant be optimised on low reflection over the complete crystal area for standard ceramic crystal probes, which are focalised by means of a polymer lens. Thus, a series of more or less high lens interface reflection echoes occur after the main pulse for each transferred ultrasonic signal. This echo series interferes with the ringing of the main pulse for resonant transducer. For focalised high-resolution probes, consequently, such signals are a big problem for the transducer design and decrease the possible bandwidth in addition. This disadvantage is avoided when the transducer is focalised by curving the piezocomposite, too.
A heavy backing is not necessary to increase the bandwidth of the composite transducers due to the structure of the piezocomposite material itself. Consequently, the composite probes for static applications are produced mostly with no or only a light backing. But the load of the water column on the probe surface in rotating testing machines would destroy such probes without a stabilizing backing within a short time. So, it was necessary to select a special type of backing material and a special manufacturing procedure which dont lower the sensitivity but hold the high centrifugal forces of about 3000 g which are loaded on the probe during rotation in a high speed testing machine. So now the lifetime of the composite probes is comparable to the standard probes by means of this solution.
Caused by the rising request for defect resolution of automatic testing machines in the past it was a demand to prove the above described theoretical approach for some actual application. The following first results are presented where the absolute amplitude output and the signal-to-noise ratio are compared for standard ceramic and piezocomposite probes, both with optimised layout on the same artificial test defect.
For the first example the transducer certifications, which are provided for each transducer individually, are compared in addition. For this application the demand was to increase the longitudinal defect detection capability in a rotating testing machine for the inspection of tubes and bars up to an outer diameter of 90 mm, a so-called ROTA 90Z. In such units often array probes are used where the crystal of the probe is divided in several individual elements, which are connected to individual electronic evaluation channels, to increase testing speed.
|Fig 5: RF-signal and spectrum of 5 MHz line focussed array probe left standard PZT ceramic / right piezocomposite.|
Fig 6: Focal point and area (length and width) of 5 MHz line focussed array
probes left standard PZT ceramic / right piezocomposite.
Actually, there was used a two element array probe with a centre frequency of 5 MHz and a focal point of about 30 mm in water for the detection of longitudinal flaws. Such probes are working with an incident angle of about 18 degrees in water to create a 45 degrees angled beam shear wave in steel to detect external and internal defects in the tested material. In the transducer certification sheets, partly shown above, not only the RF signal and the corresponding spectrum of the interface echo at the focal point but also the focal distance curve and the focal area at the focal point with information about length and width are checked. Figure 5 shows the comparison of the RF-signal of the interface echo water / steel and the corresponding spectrum between the two probes for one array element. Figure 6 presents the two results of the sound field measurements carried out in a water tank scanner by means of a 3 mm half ball reflector. The sound field behaviour of both probes shows only a few differences. The characteristic of the piezocomposite seems to be a little bit more homogeneous but the discussed difference in the side loop behaviour is not obvious because both measurements have been performed at the focal point. From figure 5, however, the high increase in bandwidth for the piezocomposite transducer can clearly be recognized in the shape of the RF-signal and especially in the spectrum. Additionally, the difference in the absolute received signal voltage output of the probe for a 300 V transmitter pulse is 14 dB.
Fig 7: A-scan of 5% longitudinal external defect - 5 MHz line focussed
array probes left standard PZT ceramic / right piezocomposite.
The question is now, whether this is also true for the defect detection. So the resolution of both probes has been compared on an artificial test defect. A test tube with an outer diameter of 21.4 mm and a wall thickness of 3.7 mm was used for the comparison where a longitudinal external defect with 5% of the wall thickness depth was machined in. Figure 7 shows the resulting A-scans of the comparison.
As it is already obvious in the certification sheets, also for the defect detection the absolute output of the piezocomposite probe is 12 dB higher. The effect of the side loops, which can be detected on the small echoes left and right of the main defect echo, is reduced and the form of the echo is sharper for the piezocomposite probe. But due to the acoustical noise from the tube caused by the scattering during the refraction at the inside surface there is only a small increase in signal-to-noise ratio for this application. Also the bandwidth effect on the attenuation doesnt give any improvement for this application because the material is only low absorbing and the wall thickness is very small. Nevertheless, the yield of gain enhances this application, too, because not the sensitivity against electrical noise from the surrounding is reduced. In addition the sensitivity against angle misfits during adjustment or out-of-centre-line problems are reduced.
The next example, however, presents a complete different result. It was demanded for this application to improve the sensitivity of the transversal defect test for higher wall thickness. Here, the sensitivity is poor for standard probes due to the small pulse response caused by the reason that the focal line is perpendicular to the defect orientation to fulfil 100 % coverage. The probes to compare are installed in a ROTA 180S, which is especially designed for testing black tubes and bars. The reference defect is a 5% external transversal notch inserted into a tube with 139.7 mm diameter and 12.0 mm wall thickness.
|Fig 8: A-scan of 5% transversal external defect - 5 MHz line focussed transducers left standard PZT ceramic / right piezocomposite.|
Figure 8 shows both A-scans for the standard PZT and the piezocomposite probe. In contrast to the first example, now not only the absolute amplitude has increased by 23 dB but also the signal to noise ratio has improved by 7 dB. This is caused by the improvement of bandwidth and additionally by the influence of the optimised sound field of the piezocomposite probe by curving of the material itself.
The following table 1 gives a short scheme of the piezocomposite probes already in use. There is only a small amount of applications where we had the possibility to compare withstandard probes because now directly such probes are manufactured for most of the critical specifications. So it is not possible to provide more comparative examinations for the two different probe layouts.
China, Rota 180S, Tube 139.7x12.0, 5% trans. ext., 5 MHz ||52 ||20 ||29 ||27
China, Rota 180S, Tube 38.0x3.0, 5% trans. int., 5 MHz ||43 ||21 ||22 ||27
Spain, Rota 90Z, Tube 21.4x3.7, 5% long. ext., 5 MHz ||38 ||20 ||26 ||21
China, Rota 130S, Tube 73.0x6.9, 5% long. ext., 5 MHz ||31 ||26 ||22 ||28
||Germ., Rota 45Z, Bar 8.0, 10x0.7 dia. core, 10 MHz ||37 ||20 ||34 ||20*)
||Table 1: Comparison between standard and composite probes.|
Nevertheless, what one can clearly recognise from this table is the fact that for each piezocomposite probe always the absolute signal output is increased. The value is between 3 and 23 dB. Already this is a big advantage for the automatic testing machines because they are mainly working in industrial surroundings where a lot of electrical noise disturbance occurs. When a lower gain for the receiver is necessary to detect the defect signals the influence of the electrical noise on the testing unit is reduced. In addition the time-of-flight resolution that means the axial resolution is improved by the higher bandwidth.
Due to the bandwidth effect on the attenuation and by the optimisation of the sound field for some application also the signal-to-noise ratio is improved up to 7 dB.
Not only absolute signal output but also sensitivity and signal-to-noise-ratio are increased by using composite material for the manufacturing of immersion technique transducers, which are used in automatic rotating ultrasonic inspection units. The increase is achieved by the better impedance matching of the piezoelectric material to the lens material, by optimisation of the sound beam when curving the composite material itself and by the higher bandwidth of the ceramic / polymer composite. A special manufacturing procedure ensures the necessary lifetime against the high centrifugal forces in the rotating units.
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