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Ultrasonic Testing in pipe extrusion
Transducer and Sound Field

Transducers in the range of 1 - 30 MHz are applied, the frequencies 1, 2, 5 and 10 MHz are the most common. Transducers for automatic testing are applied with water couplings and so-called immersion transducers. Generally, other transducer types are applied in ultrasonics, but not in the field of plastic pipe extrusions. The transducer is available in a cylindrical housing and can therefore be easily assembled into the holding devices. Diameters are approx. 10 - 25 mm with a transducer length of approx. 20 - 60 mm. Mostly they are equipped with a mould-in coaxial cable and a high frequency plug connection at the other end. The transducer itself is waterproof, the plug connection is protected against splash water only and should therefore be placed in a dry space and connected with the equipment via an extension. Some manufacturers supply transducers with plug connections on the housing, however, they cannot guarantee reliable operation over a long period.

Construction Ultrasonic transducer

The piezo element is loaded with a damping body at the rear, this generates a short pulse. The front consis of a thin protection layer. If the transducer is applied in the direct coupling method, a wear-resistant ceramic surfaces isadded.

The essential characteristics of a transducer are as follows:

Sound field
The sound field of a transducer can be depicted as a standard, thus independent of piezo diameter and frequency
Frequency and Sound attenuation
Measurement of smaller wall thickness requires a high frequency in order to permit pulse separation between the interface echo and the backwall echo, to enable evaluation. For a wall thickness of 0,5 mm, 10 MHz transducers are used. For a large wall thickness a lower frequency (1 MHz) is required as the sound attenuation in plastic material is very high when high frequencies are applied. The pulse separation for large thickness is not the most important factor as the distance between the interface and the backwall echo is very large.
StoƟwelleShock wave transducer low damped transducer
Shock wave transducers should always be used for wall thickness measurement. For smaller wall thicknesses this is as important for the pulse separation as is the frequency itself. For large wall thickness the shock wave is required also for a perfect start and stop trigger of the time measurement. Low damped transducers are not recommended.
The diameter of the transducer determines the sound energy transmitted. As the received echoes, and their amplitude, are important, the transducer diameter should relate to the pipe diameter. This means the sound field has to be directed to the pipe and enter into it. Further, not only the sound field in the coupling medium has to be considered, this applies also to the sound field in the wall. Due to the pipe bending there is a slight defocusing effect.
Transducer shape
Mostly plane transducers are used. Transducers focused lengthwise are used for pipe diameters of less then 6 mm in order to concentrate the sound energy to the surface line of the pipe. Transducers with point focusing are only of importance with flaw detection when flaws smaller than 1 mm size have to be detected.

Essential for the measurement is a precise alignment of the transducer to the pipe axis. Thus the pipe is guided to remain rectangular, or the transducer is mounted in a cardan joint fixture and guided via a prism on the pipe surface.

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Rolf Diederichs 18.Dec.1995, info@ndt.net

/DB:Article /AU:Diederichs_R /IN:NDTnet /CN:DE /CT:UT /CT:transducer /CT:process /CT:plastic /ED:1996-01