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.
(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/cm³ for the SiC-ceramics (fig. 4) and to 0,023 g/cm³ for Al2O3-ceramics.
fig. 4: determination of density differences in SiC- bending bars
fig. 5: immersions technique device for the ultrasonic testing of green ceramic bodies
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.
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
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
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