Effect of curie temperature typically there is a

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Effect of Curie Temperature Typically, there is a temperature for piezoelectric ceramics at which the properties of the ceramic change permanently and the ceramic element no longer exhibits piezoelectricity. This temperature is known as the curie temperature and is the point at which a material moves from ferroelectric to paraelectric phase. Piezoelectric ceramic elements have been used successfully within 50 °C (122 °F) of their curie temperature. 45 The curie temperature of lead zirconate titanate ceramics is 300 to 400 °C (572 to 752 °F) depending on the type of lead zirconate titanate. Other piezoelectric materials have lower curie temperatures (barium titanate at 120 °C [258 °F]) and higher (lithium niobate at 1210 °C [2210 °F]). 49 Testing limitations are therefore encountered in environments where static elevated temperatures cause the loss of piezoelectricity in the transducer’s active elements. In addition, failure may occur in other transducer components not designed for high temperature applications. Effect of Fluctuating Temperature Special problems are encountered when transducers are placed in environments with widely changing temperatures. Piezoelectric ceramic active elements have small domains in which the electrical polarization is in one direction. Temperature changes can cause some of these domains to flip, resulting in a spurious electrical signal that is not easily distinguished from the signal produced by an acoustic emission hit in the test object. In a lead zirconate titanate element, a temperature change of 100 °C (212 °F) can cause an appreciable number of these domain flips. 45 Ceramic elements should be allowed to reach thermal equilibrium before data are taken at differing temperatures. If acoustic emission testing must be done during large temperature changes, then single-crystal piezoelectric materials such as quartz are recommended. 45 Acoustic waveguides may also be used to buffer the transducer from large temperature changes. Transducer Calibration Terminology of Transducer Calibration Calibration. The calibration of a transducer is the measurement of its voltage output into an established electrical load for a given mechanical input. The subject of what should be the mechanical input is discussed below. Calibration results may be expressed either as a frequency response or as an impulse response. Test Block. A transducer is attached to the surface of a solid object either for measuring hits in the object or for calibration of the transducer. In this discussion, that solid object is called the test block. Displacement. Displacement is the dynamic particle motion of a point in or on the test block. Displacement is a function of time and three position variables. Here, the word velocity or acceleration could replace displacement .
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  • Fall '19
  • Nondestructive testing, Acoustic Emission, Acoustic Emission Testing

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