Abstract

Technical ceramics are an important part of the various construction materials in modern machine engineering. It would be advantagous to detect defects or inhomogeniousities in the early process of production to reduce the production costs. The non-destructive characterisation methods, especially the ultrasonic testing, to measure fast and with low costs thee inhomogeniousities and different material properties, are very interesting.

The main point of this work is the optimisation of different ultrasonic testing methods for the testing of green ceramic bodies. The problem is to couple the ultrasonic impulse into the testing material. Because of the destruction of the green ceramic the conventional coupling possibilities with water or other liquids are not applicable. An alternative way is presented only by the dry coupling method. In this work, different kinds of ultrasonic dry coupling will be tested for their sensitivity and lateral resolution to detect material mismatches. With dry coupling methods it is very difficult to analyse the lateral resolution of a real defect in green ceramic bodies. Appropriate devices that allow an water immersion technique for the green ceramic testing will be demonstrated. Another possibility of ultrasonic dry coupling with air-coupled ultrasonic transducers will be supposed.
The aim of this research is an ultrasonic areascan of green ceramic bodies. First results of combinations of different coupling techniques will be discussed.

Table of contents

1. Ultrasonic contact coupling
2. Ultrasonic coupling in immersion technique
3. Air coupled ultrasonic testing
4. References

1. Ultrasonic contact coupling

The shape and amplitude of the received signal, the selection of suitable transducers for transmitting and receiving and optimised trigger pulse is relevant for the measurement of the ultrasonic propagation. The optimised configuration and adjustment of the apparatusses depend on the transfer reaction of the material. In the reverse case, we could get information about these configurations of the apparatuses which would be difficult to find only with empirical tests. Generally, the obtainable precision of the measured propagation time is increasing with higher frequencies. With increasing frequencies the material is impairing the ultrasonic waves on the other hand. The criterions for the selection of the transmitting pulse and the transducers could be gathered from the complex transfer function of the transfer way transmitter-specimen-receiver. The amount of this complex function is the amplitude response:


(1) The frequencies with the lowest damping were detectable for 50, 130 and 600 kHz. For these frequencies the transmitting pulse should contain adequate harmonic parts with regard to a good transmissibility of the material (fig. 1).

Fig. 1: amplitude response and frequency content of transmitting pulses with different combinations of transducers - For FFT: Blackman-sampling in the time domain:

With digital recording and a transmitting voltage of about 800 V a measuring uncertainty of the ultrasonic transfer time of 0,42 µs was obtainable for a green state SSiC-specimen with a minimal thickness of 40 mm and a test frequency of 600 kHz. This measuring uncertainty was destined for the reproducibility of the coupling conditions between transducer and specimen.
The aimed measuring uncertainty for a cylindrical green ceramic body with a length of 372 mm amounted to 0,27% for a test frequency of 50 kHz.
In a second test series, the possibilities of the contact coupling with conventional transducers for the testing of green ceramics were examined. Measurements were made at different specimens with contact coupling transducers in pulse-echo testing method. The nominal frequencies of the transducers were 1 MHz and 2 MHz . The ultrasonic system HILLSCAN 3000 of the Fa. Dr.W.Hillger and a device USIP 12 of the Fa. Krautkrämer were used.

Fig 2 and Fig 3 Fig. 2: A-scan of a blind hole with a diameter of 2 mm in a depth of 28 mm in a 33 mm SiC green ceramic pulse echo testing method signal amplification 81dB, broad band transducer 1 MHz Fig. 3: A-scan of a 33 mm SiC green ceramic without model defects pulse echo testing method signal amplification 81dB, broad band transducer 1 MHz Amplitude response of a 5 mm SiC bending bulk produced with two air coupled capacitance transducers in ultrasonic through-transmission testing

Figure 2 and 3 show A-scans of model defects in SiC green ceramic bodies. A green ceramic step bar and a cylindrical ring with different blind holes were test specimens. The blind holes were produced in various depths and with various diameters. The hole with a diameter of 2 mm in a depth of 28 mm SiC-ceramic was the smallest hole repeatedly detectable.

The use of transducers without protective layer may cause the destruction of the green ceramics surface. The surface of the ceramic adjusts to the transducer surface. With a corresponding higher contact pressure the ultrasonic coupling and the reflected signal amplitude could be improved.
The velocity vl of the longitudinal waves in green ceramic was measured in the impulse-echo testing method to describe the smallest differences D in the density which can measured in ultrasonic testing. A 1 MHz broadband transducer from Panametrics was used. The smallest differences in density tested amounted to 0,017 g/cm^³ for the SiC-ceramics (fig. 4) and to 0,023 g/cm^³ for Al₂O₃-ceramics.

fig. 4: determination of density differences in SiC- bending bars

Differences of the transfer time in the same sample caused by other materials or inhomogeneities in the ceramic could be better detected with an contactless ultrasonic areascan. Then the coupling conditions would be equal over the scanned sample. Therefore, the determination of synthesised model defects, like metal- or wood splits, plastic materials or prepressed ceramic material, was centred to the immersion technique in water.

2. Ultrasonic coupling in immersion technique

In ultrasonic contact technique it is difficult to realise an area scan because of the different coupling conditions on every point of the scanned area. It is only possible to take a test at certain points. For this reason, two devices were constructed for testing green ceramics in immersion technique with a waterbag. (fig.5)

fig. 5: immersions technique device for the ultrasonic testing of green ceramic bodies

The first device was constructed with a coupling pad of the firm Sonotec GmbH Halle. This coupling pad is a solid silicon coupling gel with acoustical properties of water. The advantage of the material is to osculate to the surface of the sample.
The coupling pad forms with two clamps a bag which can filled with water. The bag of the second device was produced from PE-foil. For a better coupling, an adhesive tape was placed between bag and sample.
The devices filled with water make a testing of the green ceramic bodies in immersion technique possible without the direct contact between ceramic and liquid. So the green ceramic could not destroyed be by the liquid and the coupling conditions are equal over the scanned area.

The best coupling for the system sample-device-transducer was produced by the PE-foil device. In the case of the silicon pad waterbag it would be supposed that the air gap between waterbag and surface of the sample and the damping of the silicon pad are too high to get an usable ultrasonic signal from the back surface of the sample. This device and the testing equipment will be optimised in the future work.

fig. 6: C-Scan of bending bulks with different densities fig. 7: recording of the back surface echo in a SiC-probe with prepressed SiC-material fig. 8: embedded Teflon disks simulate an area of cracks in a SiC-sample. The amplitude of the flaw echo was rated.

Figures 6 to 8 show scanned pictures of green ceramics with different densities and model defects. These samples where scanned in immersion technique with the PE-waterbag. The transducer was a focused transducer for immersion technique with a nominal frequency of 2.25 MHz. The samples were green ceramic bodies with different synthesised defects which were produced during the pressing process. Defects with an area of more than 10 mm in diameter, like Teflon disks, small Styrofoam sticks or plastic tubes with a diameter of 1mm could be detected with high resolution. The amplitude of the flaw echo was evaluated. (fig. 8) Areas (( 10mm) with small defects like 2 mm metal or wood splits and prepressed material could be detected with the evaluation of the back surface echo. (fig. 7) The SiC-samples had a thickness of 15 mm. For this thickness, single defects smaller than 5 mm were not detectable in green ceramic bodies. Other possibilities to minimise the air gap between the waterbag and the green ceramic sample are examined. In this way the insertion of ultrasonic energy into the green ceramic material could be increased.

3. Air coupled ultrasonic testing

Fig 9 and Fig 10 Fig. 9: Amplitude response of a 10mm Plexiglas-probe produced with two air coupled capacitance transducers in ultrasonic through-transmission testing Fig. 10: Amplitude response of a 5 mm SiC bending bulk produced with two air coupled capacitance transducers in ultrasonic through-transmission testing

Air coupled ultrasonic waves produced with capacitance transducers were used for studies in the non-destructive green ceramic testing.[2-6] The capacitance transducer works with the technical principle of a condenser transmitter and microphone. The first results of the tests with the capacitance transducers are shown in fig. 9 and fig. 10. The materials of the samples were acrylic glass and SiC green ceramic. The transducers were used in a distance of 3 mm to the front and back surface of the sample and in the through-transmission testing method. The frequency response of the received ultrasonic signals showed the maximum of the amplitudes between 300 to 800 kHz. For this reason, way the aircoupled ultrasonic testing is less suitable for the detection of defects at present, but it presents the possibility of an areascan to determine the allocation of the density in materials with lower acoustic impedance. For the time being, the material thickness that can be tested with aircoupled ultrasonic waves amount 10 mm for Plexiglas and silicon nitride, and 5 mm for silicon carbide. The received amplitude in SiC had a signal-noise-ratio of only 6 dB.
Future studies will research the possibilities of the aircoupled ultrasonic technique. The aim is to scan the full volume of the green ceramic bodies in through-transmission technique.

References

1. W. Kuhl, G.R. Schodder, F.-K. Schröder Condenser transmitter and microphones with solid dielectric for airborne ultrasonics Acustica, 4 (1954) 5, S. 519 - 532
   
2. D.A. Hutchins and D.W. Schindel Advances in non-contact and air-coupled transducers IEEE International Ultrasonics Symposium Cannes (1994) S. 1245 - 1260
   
3. H. Carr, W.S.H. Munro, M. Rafiq, C. Wykes Developments in capacitive transducers Nondestr. Test. Eval., 10 (1991) S. 3-13
   
4. D.W. Schindel, D.A. Hutchins, L. Zou, M. Sayer Capacitance devices for the generation of air-borne ultrasonic fields IEEE Ultrasonic Symposium (1992) S. 843-846
   
5. H. Carr, C. Wykes Diagnostic measurements in capacitive transducers, Ultrasonics 31 (1993) 1, S. 13-20
   
6. D.W. Schindel, D.A. Hutchins Capacitance devices for the controlled generation of ultrasonic fields in liquids IEEE Ultrasonic Symposium (1991) S. 301-304

Authors

Prof. Dr.-Ing. habil. Horst-Dieter Tietz
Rektor der Westsächsischen Hochschule Zwickau (FH)
ordentlicher Professor des Fachgebietes Werkstoffe/ Qualitätsmanagement
Prof. Dr.-Ing. habil. Manfred Dietz
Fachgebietssprecher des FG Werkstoffe/ Qualitätsmanagement
ordentlicher Professor des Fachgebietes Werkstoffe/ Qualitätsmanagement
Dipl.-Ing. Beate May
wissenschaftlicher Mitarbeiter der WSH Zwickau (FH)
FG WS/QM
Email: Beate.May@fh-zwickau.de
Dipl.-Ing. Lutz Bühling
wissenschaftlicher Mitarbeiter der WSH Zwickau (FH)
FG WS/QM

The paper was presented on the DGZfP annual NDT confernce in Dresden in May '97

The Paper was presented on: Focus on Ceramic and Thickness Measurement in UTonline 10/97