Re: To all: Is there any disadvantage for composite transducers?

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Re: To all: Is there any disadvantage for composite transducers?
Posted by: Gerhard Splitt , E-mail: Sp@kk-a-col.mhs.compuserve.com, on October 01, 1996 at 16:53:50:
In Reply to: Re: To all: Is there any disadvantage for composite transducers? posted by : Bill Grandia on September 24, 1996 at 08:51:11:

Sorry for the delayed answer which is due to too much trouble last week.
The comments of Bill Grandia have shown that piezocomposite transducers are
suitable for nearly all probes except for hardfaced probes, which will be coupled
directly to steel. But even there could be some advantage depending on crystal
diameter, frequency and wear plate material. A normal PZT transducer will produce
radial ringing which can result in spurious echos in the hardface probe. With a
piezocomposite transducer the radial coupling is reduced to a small amount making
the hardface probe more quiet.
Of course there are some other restrictions for using piezocomposite transducers for
all types of probes.
1. The material is more costly, therefore it will not be used in cheap probes.
2. It is difficult and extremely costly to fabricate high frequency transducers. To avoid
additional ringing the periodicity of the 1-3 piezocomposite matrix should be smaller
than halve the wavelength of shear waves in the epoxy matrix for the highest
frequency within the pass band of the probe. For 15 MHz probe e.g. this results in
very small ceramic posts of 0.04 mm edge length and very small separating epoxy
lines of 0.015 mm (for 20 MHz upper pass band and shear wave velocity of 1100
m/s)!
3. Piezocomposite material is not suitable for temperatures above 130 degree C
approx.
Gerhard Splitt

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Comments:
: : Sorry for the delayed answer which is due to too much trouble last week. : The comments of Bill Grandia have shown that piezocomposite transducers are : suitable for nearly all probes except for hardfaced probes, which will be coupled : directly to steel. But even there could be some advantage depending on crystal : diameter, frequency and wear plate material. A normal PZT transducer will produce : radial ringing which can result in spurious echos in the hardface probe. With a : piezocomposite transducer the radial coupling is reduced to a small amount making : the hardface probe more quiet. : Of course there are some other restrictions for using piezocomposite transducers for : all types of probes. : 1. The material is more costly, therefore it will not be used in cheap probes. : 2. It is difficult and extremely costly to fabricate high frequency transducers. To avoid : additional ringing the periodicity of the 1-3 piezocomposite matrix should be smaller : than halve the wavelength of shear waves in the epoxy matrix for the highest : frequency within the pass band of the probe. For 15 MHz probe e.g. this results in : very small ceramic posts of 0.04 mm edge length and very small separating epoxy : lines of 0.015 mm (for 20 MHz upper pass band and shear wave velocity of 1100 : m/s)! : 3. Piezocomposite material is not suitable for temperatures above 130 degree C : approx. : Gerhard Splitt :

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