FIFTH CANADIAN CONFERENCE ON NONDESTRUCTIVE ... - IAEA

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The system described in this paper (see Fig. 1) can be operated at frequencies up
to 100 MHz. The puiser of the system consists of a 1 Hz to 100 MHz pulse
generator (Philips PM5771) and a broadband (0.15 to 300 MHz) power amplifier
(EIN403LA). The pulse generator is capable of providing pulses as narrow as 5 ns
which are required for 100 MHz transducers. The high frequency transducers*
employ a quartz buffer rod with a piezoelectric lithium niobate crystal mounted
on one end and a lens machined on the opposite end. Focused transducers (12 mm
focal length in water) with a narrow beam diameter (500 ym at focal point) were
used.
The detecting system uses a broadband (0.1-1300 MHz) amplifier with 48 dB gain
(HP8447F) and a digital scope (HP1980). The latter has a bandwidth of 5 Hz to
100 MHz and consists of two 100 MHz analogue measurement channels and an analogue
to digital converter. When used together with a digital waveform storage unit
(HP19860), repetitive ultrasonic signals of up to 100 MHz can be averaged (up to
64 times), digitized (up to 501 points per waveform in main or delay modes), and
stored in the memory. The stored waveforms are transferred to a microcomputer
(HP9845) for analysis in the time and/or frequency domains.
2.3 Detection Technique
Surface or Rayleigh waves were used to detect surface cracks in silicon nitride.
These waves are generated by the mode conversion of compressional waves incident
at the Rayleigh angle, 9R, onto the surface of the test material. The
conventional method of generating surface waves is to use a wedge transducer with
a wedge angle of 9 g in contact with the test specimen. Preliminary work*- ' '
however, indicated that the contact method is not suitable for testing ceramics
at high frequencies due to coupling problems. Consequently, the technique of
non-contact mode conversion in an immersion tank was used in the present work.
Figure 2 shows the basic configuration for the generation and detection of the
Rayleigh waves. In this configuration the Rayleigh angle is given by:
6R = arc sin -y— (1 )
R
where V is the acoustic velocity in water and VR is the Rayleigh wave velocity in
the test material. For silicon nitride 9R=15.7 . The Rayleigh waves reflected
by sharp surface discontinuities radiate energy back into water at an angle of
9Rand this energy is received by the transducer. These are referred to as "leaky
Rayleigh waves". Thus, as the transducer is scanned parallel to the surface of
the test material, strong signals from surface flaws are detected. Figure 3
shows a typical signal from a 100 pm Knoop indentation detected by leaky Rayleigh
waves at a frequency of 50 MHz.
The detectability of surface flaws by leaky Raleigh waves is dependent on the
transducer frequency. This is demonstrated in the experimental results of Fig. 4
which show the smallest Knoop indentation detectable at different frequencies.
This figure indicates that for the experimental conditions of the present work, a
* Precision Acoustic Devices, Inc.