PRODUCTION OF ULTRASONIC WAVES: THE PIEZO-ELECTRIC EFFECT, THE PULSE-ECHO SYSTEM

PRODUCTION OF ULTRASONIC WAVES

In ultrasonic testing sound waves are used with a frequency of 20 KHz upwards. In testing metals a range of 1 MHz to 6 MHz is generally
used. To produce these high frequencies use is made ofthePiezo-Electric Effect.

THE PIEZO-ELECTRIC EFFECT

 

Certain crystalline substances change their shape slightly when an electrical potential is applied across opposite surfaces of the crystal, and conversely develop an electrical potential when they are subjected to mechanical pressure or shock. This is known as the Piezo-Electric Effect.

If an alternating voltage is applied to the crystal, then it will expand and contract as the voltage changes. Although the expansions and contractions will be of the same frequency as the alternating voltage, each crystal has a natural or resonant frequency at which it tends to vibrate most readily.

The resonant frequency of the crystal is directly related to its thickness. If the crystal in Figure  was given a very short pulse of voltage by closing and immediately re-opening the switch then it would vibrate for a very short period at its resonant frequency before the expansions and contractions died away.

If the crystal was to be given a sharp knock by mechanical means, then it would also vibrate for a short period at its resonant frequency producing an alternating potential across its surfaces at the same frequency.

Such devices which convert electrical energy to mechanical energy or vice-versa are termed transducers. In ultrasonic testing equipment the transducer is incorporated in a device termed an ultrasonic probe. A loudspeaker is another common transducer.

Piezo electric transducers can be manufactured from a number of materials including quartz and ceramics. Common ceramics used in the ultrasonic transducer are barium titanate and lead zirconate titanate.
 
The vibrating crystal is used to produce ultrasonic compression waves within the probe.

 

 

PRODUCTION OF ULTRASONIC WAVES In ultrasonic testing sound waves are used with a frequency of 20 KHz upwards. In testing metals a range of 1 MHz to 6 MHz is generally used. To produce these high frequencies use is made of the Piezo-Electric Effect.    THE PIEZO-ELECTRIC EFFECT   Certain crystalline substances change their shape slightly when an electrical potential is applied across opposite surfaces of the crystal, and conversely develop an electrical potential when they are subjected to mechanical pressure or shock. This is known as the Piezo-Electric Effect. If an alternating voltage is applied to the crystal, then it will expand and contract as the voltage changes. Although the expansions and contractions will be of the same frequency as the alternating voltage, each crystal has a natural or resonant frequency at which it tends to vibrate most readily. The resonant frequency of the crystal is directly related to its thickness. If the crystal in Figure 8 was given a very short pulse of voltage by closing and immediately re-opening the switch then it would vibrate for a very short period at its resonant frequency before the expansions and contractions died away. If the crystal was to be given a sharp knock by mechanical means, then it would also vibrate for a short period at its resonant frequency producing an alternating potential across its surfaces at the same frequency. Such devices which convert electrical energy to mechanical energy or vice-versa are termed transducers. In ultrasonic testing equipment the transducer is incorporated in a device termed an ultrasonic probe. A loudspeaker is another common transducer. Piezo electric transducers can be manufactured from a number of materials including quartz and ceramics. Common ceramics used in the ultrasonic transducer are barium titanate and lead zirconate titanate. The vibrating crystal is used to produce ultrasonic compression waves within the probe.
THE PIEZO-ELECTRIC EFFECT

 

 

THE PULSE-ECHO SYSTEM

 

The most common system used in ultrasonic thickness measurement and ultrasonic flaw detection is the pulse-echo system. Here the piezo-electric transducer is repeatedly excited for a short duration to produce sound wave pulses. There is a delay of micro-seconds between each pulse, (see Fig).

These sound wave pulses travel through the material under test until they meet an interface or boundary, where they are reflected back. If the sound hits the interface at right angles then the reflected sound travels back to the probe as an echo. Echoes coming back to the probe are reconverted into electrical signals and the time between transmitting the pulse and receiving the echo is electronically measured.

By calibrating the ultrasonic equipment for the speed of sound in the test material the equipment is able to display the time taken for the pulse-echo to travel through the material as a distance.

 

 

 

THE PULSE-ECHO SYSTEM The most common system used in ultrasonic thickness measurement and ultrasonic flaw detection is the pulse-echo system. Here the piezo-electric transducer is repeatedly excited for a short duration to produce sound wave pulses. There is a delay of micro-seconds between each pulse, (see Fig).  These sound wave pulses travel through the material under test until they meet an interface or boundary, where they are reflected back. If the sound hits the interface at right angles then the reflected sound travels back to the probe as an echo. Echoes coming back to the probe are reconverted into electrical signals and the time between transmitting the pulse and receiving the echo is electronically measured.  By calibrating the ultrasonic equipment for the speed of sound in the test material the equipment is able to display the time taken for the pulse-echo to travel through the material as a distance.
THE PULSE-ECHO SYSTEM

 

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