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The school of Applied Non-Destructive Examination. cc. Ultrasonic Inspection Level I 2010.01 PRODUCTION OF ULTRASONIC WAVES. The Piezo Electric Effect. The mechanical vibrations (sound energy) required for the ultrasonic method, can be generated by "electromechanical crystals / transducers". These are the devices used to transform electrical energy into mechanical energy and vice versa, known as piezoelectric transducers. These units are capable of operating at various frequencies, ranging from the low kHz into the MHz range. These electromechanical transducers used for ultrasonic inspections are more commonly referred to as transducers, transmitters, receivers or crystals. The transducers / crystals in turn are mounted in housing and together, the assembly of housing and transducer etc., are commonly referred to as probes, transducers or search units. Certain crystalline materials exhibit this piezo-electric effect. Naturally occurring crystals include Quartz, Lithium Sulphate and Rochelle salt . The commonly used polarized (man made) ceramic transducers are Barium Titanate, Lead Zirconate - Titanate , and Lead Metaniobate. In order to use the piezo-electric properties of a natural crystal for a particular purpose it is necessary to cut the crystal in a specific way. The magnitude and character of the piezo-electric effect in any piezo material depends to a great extent on the relationship of the applied force or electrical field, to the axis of the crystal material. The X - cut crystals produce a longitudinal ultrasonic mode of sound waves. Whereas a crystal cut with its major face at a right angle to the Y axis will be known as a Y - cut crystal. If this crystal is now excited by an electrical pulse it will produce shear waves when in direct contact with the material - remember we are referring to natural crystals. An interesting bit of information: Recently a university in HAIFA built an “electrical road’ using Quarts crystals. A rubber compound with strategically placed crystals was placed over 30 meters of an existing roadway. When a truck passed over this electrical road, using the Piezo-electrric effect, it generated 2000 Volts of electricity! Enough to light up a number of street lamps as the truck went past. It is said that 1km of roadway can generate enough electricity for 40 homes. Back to Ultrasonics. All hand held probes used for flaw detection purposes have "X - cut" crystals fitted and as such produce longitudinal waves. Reciprocal* receiving and giving in the same proportion. 1
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The school of Applied Non-Destructive Examination. cc. Ultrasonic Inspection Level I 2010.01 PRODUCTION OF ULTRASONIC WAVES. Transducer materials and properties. The ceramic materials for example Barium Titanate , when produced do not exhibit the piezo-electric effect. In order for them to produce the piezo effect they must first be polarized.
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  • Fall '17
  • ZAHEER AHMED
  • Wave mechanics, S-wave, Rayleigh wave, oo oo oo

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