New transducer materials are used when designing probes thus leading to new and economical solutions to critical test tasks. Here we should mention polymer and piezocomposite transducer materials which have considerable advantages with regard to resolution, scan width and amplitude of the ultrasonic pulse.
With automatic plate testing it is important to employ economical methods. Therefore, to keep test time short probes are needed which cover a wide test track at a specified flaw detection level. Polymer probes with optimum element diameters have proven to be successful in solving these tasks.
For example, it is required to detect a 5 mm diameter flat bottom hole with a scan width of at least 25 mm at a flaw depth of between 1.1 and 34 mm on plates applied as raw material for production of electric resistance welded tubes. A 10 MHz polymer probe with element dimensions of 6 x 33 mm² can be optimally used. As shown in Fig. 1, this probe has a scan width of 28 mm at -3 dB and a very low amplitude modulation of less than 1 dB over the complete sound beam width.
Fig.1 : Sound beam profile of a 10 MHz polymer probe with a 6x33 mm² element parallel to the lenght axis at 3 different depth z.
Fig. 2: Near resolution of a 10 MHz paint brush probe with|
a polymer (top) or a ceramic (bottom) 6 x 33 mm² element
for a 5 mm flat bottom hole at a depth of 1.1 mm (left)
and an interface echo without test reflector (right)
The method is based on the different attenuation of the ultrasonic beam in the welding spot and on the more or less good reflection at the interfaces between the welded plates. Relatively higher frequencies are required in order to obtain a reliable indication about the quality of the spot weld by observing the echo sequence. In practice, the highly damped 20 MHz polymer probes have been proven and enable the required resolution of the intermediate echoes from the individual weld layers. The probes are designed with an integrated water delay line which is enclosed by a thin rubber membrane. This special construction enables flexible probe coupling which is necessary due to the uneven weld spot.
Probes with different element diameters are required to determine the diameter of the weld spot. Typical values are in the range between 3 and 6 mm. In order to achieve perfect evaluation of the echo display it is necessary that the probe does not generate any interfering noise echoes which would otherwise disturb the echo sequence coming from the weld spot. This is also a reason why polymer probes are preferably applied in practice because they are well damped and do not produce any radial modes which, with ceramic probes, would cause interference indications.
Extreme ambient conditions such as high pressure, high temperature and aggressive mediums require a special probe design when testing pipelines. They must be especially protected, in mechanical construction, against these influences. At the same time they must be small because many of them must be contained in a pig, having a size defined by the inner diameter of the pipeline, in order to carry out a complete test fully covering the pipeline's circumference. Fig. 4 shows various sizes which are designed for a maximum temperature of 125 °C and a pressure of up to 200 bar.
Working together with the Federal Institute for Material Research and Testing (BAM), angle beam probes and TR probes were developed which could withstand continuous heating up to 250 °C, above this value the probe must be cooled off after measurement. Maximum permissible contact temperature for angle beam probes is about 350 °C; the delay line material can disintegrate at higher temperatures. The delay line material of TR probes is a special ceramic and can be used up to 800 °C.
Instead of using the normal "Vespel" as wedge material for angle beam probes, a new polymer is applied which has a very low sound attenuation even at high temperatures comparable to polystyrene. This new polymer is also used for dual probes at medium temperatures which can be applied up to 350 °C. In both cases a higher probe sensitivity results giving an extended measurement range.
Magnetic probes also offer advantages for other NDT tasks, e.g. instrument calibration or reporting with a stable coupled probe. for example, this is the case with thickness measurement when measured values must be stored, or when measurement positions are difficult to access and the operator has problems keeping the probe coupling steady.
|PROBE TYPE||SWB|| WB
||Element size [mm²]
|| 14 x 14
|| 20 x 22
|| 2 or 5
|| 2 or 4
||Contact face [mm²]
|| 21 x 36
|| 29 x 53
||exit point [mm] ||12 - 18 (depend. on angle)
||Bandwidth [%] ||50
Several types of piezocomposites have been investigated but today the 1-3 type piezocomposite is mostly used to build ultrasonic probes. Parallel orientated piezoceramic rods are embedded into a polymer matrix as shown in Fig. 7. This has the following advantages as opposed to the pure piezoelectric ceramic (refer to table 2):
The early recognition of possible weak points, e.g. on pipelines or on flange joints in the chemical industry, by using special ultrasonic probes generates considerable economical and ecological advantages. The better the probe is matched to the test task, the more reliable the early detection of possible damages becomes before it is a threat to people and environment.