Abstract

Table of content
1. Introduction
   
2. Desirable Characteristics of Ultrasonic Transducers
   
3. Historical Design Perspective
   
4. Probes for Spot Weld Testing
   
5. Testing of Pipelines
   
6. High Temperature Measurement
   
7. Magnetic Probes
   
8. New Angle Beam Probes
   
9. Piezocomposite Material
   
10. Conclusions
    
11. Literature
    

1. Introduction

Today, within the field of non-destructive material testing with ultrasound many standard tasks have been solved and do not require further development. However, there are ever increasing test problems which can only be solved by specially matched probes. For example, we can think of the testing of pipelines for cracks and corrosion using fully automated pigs, as well as spot weld testing, component testing at high temperatures, the measurement of bolt tension with magnetic probes or the automatic testing of brake linings with special roller probes.

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.

2. Desirable Characteristics of Ultrasonic Transducers

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.

3. Historical Design Perspective

Another advantage, in addition to the almost rectangular sound beam profile, is the good resolution power of this polymer probe compared to the conventional probe with a ceramic element. The polymer probe shows a distinctly narrower interface echo so that the flaw echo at a depth of 1.1 mm is clearly resolved. Its signal-to-noise ratio is more than 20 dB compared to only less than 6 dB for the ceramic probe (Fig. 2). Therefore, for an automatic test only the polymer probe has a sufficient signal-to-noise ratio.

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)

4. Probes for Spot Weld Testing

Destructive test methods were used in the past for testing spot welds in the car industry because these met the required demands for vehicle safety. Today, non-destructive testing with ultrasonics is becoming more and more accepted. It is considerably more economical because the tested vehicles can be used further and it has in the mean time achieved a high degree of reliability.

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.

13KB Fig. 3: Probe for spot weld testing with flexible coupling.

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.

5. Testing of Pipelines

With pipeline testing, the probes are used for thickness measurement in order to detect possible corrosion points and also for crack detection in order to detect cracks which occur due to mechanical stress. The paramount factor for both applications is the prevention of incalculable damage to people and environment. Today therefore, all large pipelines are tested by pigs which are driven through at regular intervals and which measure possible degradation using ultrasonics whereby the results are recorded on a data carrier which, after the test run, are evaluated outside of the pipeline.

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.

13KB Fig. 4: Various probes for pipeline testing

6. High Temperature Measurement

The capability to test hot parts with ultrasonics can greatly improve the profitability of the test. It is often very costly to shut down an installation just to carry out an ultrasonic test at a low temperature. In the same way, it is better to directly test the welds on a prewarmed part in the hot condition instead of waiting until the part has cooled down. In order to achieve this, delay lines made of heat resistant material are used since a long time ago in front of conventional straight beam probes thus enabling tests to be carried out on hot parts. The delay line can only be coupled to the surface for short periods when the temperature is above 100 °C, it must be cooled off between these times because the probe itself can only withstand temperatures up to about 60 °C. This is perhaps more tolerable with thickness testing but considerably limits flaw testing. The short coupling period is not sufficient for continuous flaw detection on a hot part.

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.

7. Magnetic Probes

When measuring the mechanical stress on bolts using ultrasonics it is important to achieve reproducible probe coupling because this influences the measurement accuracy. In this case, probes with integrated magnets are successful. Fig. 5 shows a magnetic probe (5 MHz, 10 mm element) fixed on a bolt head together with the Boltmike instrument for bolt stress measurement.

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.

32KB Fig. 5: Bolt stress measurement with a magnetic probe

8. New Angle Beam Probes

For weld testing there are also new probes designed for tasks which were difficult up until now. Well established angle beam probes having element sizes of 8x9 or 20x22 mm² have their application limitations. Angle beam probes with large elements are, due to their sharply focused sound beam, suitable for testing at greater depths but owing to their size they cannot always be satisfactorily coupled and positioned sufficiently close to the weld. These problems do not occur with miniature angle beam probes suitable for testing thinner welds. However, owing to the small probe element, the test sensitivity is strongly reduced at larger depths.

20KB Fig. 6: New angle beam probes SWB with intermediate size between MWB (small) and WB (large)

9. Piezocomposite Material

Piezocomposite material is an important update of existing piezoceramic. R. Newnham (1) at the Materials Research Laboratory of Pennsylvania State University, USA, has pioneered much of the basic development work. A mixture of polymer material and piezoceramic is used to modify specific features of the ceramic.

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):

1. Improved electroacoustic efficiency
2. Reduced planar coupling
3. Low acoustic impedance
4. Mechanical flexibility
For more information access to an article in detailes about Composite Transducers.

10. Conclusions

The above examples are only a small selection from the many special probes which are matched to the specific task. They show that NDT with ultrasonics is economical when optimum probes are used. The application of special transducer element materials enables existing barriers to be overcome with regard to resolution and sensitivity. These transducer materials are, thanks to modern technology such as metallizing by high vacuum evaporation and micro-mechanics, produced in reliable quality and at acceptable costs. Therefore, there is no restriction using them for critical applications when necessary.

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.

7. Literature

1. R.E. Newnham et al., Composite piezoelectric transducers, Mater. Eng., vol. 2, p. 93 - 106 (1980)
2. Ultraschall-Prüfköpfe für 650 °C Dauertemperatur, H. Mrasek, D. Gohlke, K. Matthies, E. Neumann, BAM, Berlin , NDT annual Conference 1996, 13. - 15. May in Lindau. Conference Report

Author`s Profile

32KB Dr. Gerhard Splitt , born 1943, studied physics at the University of Hamburg and received there his PhD on solid state physics in 1975. From 1969 until 1975 he worked as a scientist at the Institute of Applied Physics of the University of Hamburg. Since 1976 he is in charge of the department for probe development at Krautkraemer GmbH in Huerth, Germany.
E-mail: Gsplitt@krautkramer.de

| UT-Frontpage | |Top to this page|