Characteristics of Piezoelectric Transducers

7/30/2019 Characteristics of Piezoelectric Transducers

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Characteristics of Piezoelectric

Transducers

The transducer incorporates apiezoelectric element, whichconverts electrical signals intomechanical vibrations(transmit mode) andmechanical vibrations intoelectrical signals (receivemode).


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Composition

Zirconato titanato de plomo Titanato de bario Metaniobato de plomo

Sulfato de litio Quarzo Niobato de litio


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Band width and frequency

Bandwidth, or range of frequencies. The frequencynoted on a transducer is the central or center frequencyand depends primarily on the backing material.

Highly damped transducers will respond to frequenciesabove and below the central frequency.

The broad frequency range provides a transducer with

high resolving power.Less damped transducers will exhibit a narrowerfrequency range and poorer resolving power, but greaterpenetration


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US BEAMThe intensity of the sound is indicated by color, with

lighter colors indicating higher intensity.


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Acoustic pressure and far field

xxD

senPP

21

22

042

Acoustic pressure at the beam axis

)12(4

)12( 222

m

mDX m

Far field: m= 0,1,2,3 Far field pressure: X > 3.7N


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Far field


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Near field and far field


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Ultrasonic field for a straight probe

Campo en eje de palpador (excitacin con pulsos


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Transducer Beam Spread

Beam spread is largely determined by the frequency and diameter of thetransducer. Beam spread is greater when using a low frequency transducer thanwhen using a high frequency transducer. As the diameter of the transducerincreases, the beam spread will be reduced.


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Near Field- Far Field- Divergence

The nearfield

BeamDivergence


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Angulo medio de divergencia del

hazTheoretical value of the half angle d in relation to thediameter and the frequency of the transducer for asound velocity of 1500 m/s (water).


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Beam characteristics


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The US beam

Beam Diameter Focal Zone


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Transducer Types

Contact transducers are generally hand manipulated. They have elementsprotected in a rugged casing to withstand sliding contact with a variety ofmaterials. These transducers have an ergonomic design so that they are easy to gripand move along a surface. They often have replaceable wear plates to lengthen theiruseful life.


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Immersion transducers

Do not contact the component.These transducers are designed

to operate in a liquid

environment and all connections

are watertight.


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Dual element transducers

Contain two independently

operated elements in a singlehousing. One of the elementstransmits and the otherreceives the ultrasonic signal


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Delay line transducers

Provide versatility with a varietyof replaceable options.Removable delay line, surfaceconforming membrane, and

protective wear cap options canmake a single transducereffective for a wide range ofapplications. As the nameimplies, the primary function of

a delay line transducer is tointroduce a time delay betweenthe generation of the sound

wave and the arrival of anyreflected waves.


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Angle beam transducers

Angle beam and wedges aretypically used to introduce arefracted shear wave into thetest material. Transducerscan be purchased in a varietyof fixed angles or inadjustable versions where the

user determines the angles ofincidence and refraction.


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Couplant

A couplant is a material(usually liquid) that facilitatesthe transmission of ultrasonic

energy from the transducerinto the test specimen.In contact ultrasonic testing athin film of oil, glycerin or

water is generally usedbetween the transducer andthe test surface.


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The ultrasonic flaw detector


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Data Presentation

The three most common formats are knowin the NDT world asA-scan, B-scan and C-scan presentations.

A-Scan Presentation

The A-scan presentation displays the amount of

received ultrasonic energy as a function of time.Most instruments with an A-scan display allow thesignal to be displayed in its natural radio frequencyform (RF), as a fully rectified RF signal, or as eitherthe positive or negative half of the RF signal. In the

A-scan presentation, relative discontinuity size canbe estimated by comparing the signal amplitudeobtained from an unknown reflector to that from aknown reflector. Reflector depth can be determinedby the position of the signal on the horizontalsweep.


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Data Presentation

In the B-scan, the time-of-flight (traveltime) of the sound energy is displayed along

the vertical axis and the linear position ofthe transducer is displayed along thehorizontal axis. From the B-scan, the depthof the reflector and its approximate lineardimensions in the scan direction can bedetermined


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Data PresentationC-Scan PresentationThe C-scan presentation provides a plan-type view of thelocation and size of test specimen features. The plane ofthe image is parallel to the scan pattern of the transducer.C-scan presentations are produced with an automateddata acquisition system, such as a computer controlledimmersion scanning system. Typically, a data collectiongate is established on the A-scan and the amplitude orthe time-of-flight of the signal is recorded at regularintervals as the transducer is scanned over the test piece.The relative signal amplitude or the time-of-flight is

displayed as a shade of gray or a color for each of thepositions where data was recorded. The C-scanpresentation provides an image of the features thatreflect and scatter the sound within and on the surfacesof the test piece.


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Phase array


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Normal Beam Inspection

Pulse-echo ultrasonic measurements can determine the locationof a discontinuity in a part or structure by accurately measuringthe time required for a short ultrasonic pulse generated by atransducer to travel through a thickness of material, reflect fromthe back or the surface of a discontinuity, and be returned to the

transducer. In thickness gauging, ultrasonic techniques permit quick and

reliable measurement of thickness without requiring access toboth sides of a part. Accuracy's as high as 1 micron.

It is possible to measure most engineering materials

ultrasonically, including metals, plastic, ceramics, composites,epoxies, and glass as well as liquid levels and the thickness ofcertain biological specimens.


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Angle Beams

Angle Beam Transducers and wedges are typically used to introduce ashear wave into the test material.


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Calibration

Calibration refers to the act of evaluating and adjustingthe precision and accuracy of measurement equipment


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AWS V1 or IIW


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Bloque V2


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Multiple reflections at the edge of

a plate


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Test on pipes

D

d21sin

))tan(2( dfs s

)cos

2

(

d

fw ws


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Weld testing


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Weld testing

Ecodynamic curves


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Ecodynamic curves

Envolvente?


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Flaw location


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Flaw location and sizing


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Crack Tip Diffraction

When the geometry ofthe part is relativelyuncomplicated and the

orientation of a flaw iswell known, the length(a) of a crack can bedetermined by a

technique known as tipdiffraction


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Tips for U.S. scanning


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Reflection from small reflectors


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AVG Diagram


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Sound attenuation


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Calibracin de sensibilidad


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Curvas DAC

Distance Amplitude Correction (DAC)Acoustic signals from the same reflecting surface will havedifferent amplitudes at different distances from the transducer.Distance amplitude correction (DAC) provides a means ofestablishing a graphic reference level sensitivity as a function of

sweep distance on the A-scan display

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Characteristics of Piezoelectric Transducers