understanding acoustic emission testing reading 2 ndthb vol5 part 2

Charlie Chong/ Fion Zhang

Understanding Acoustic Emission Testing, AET-Reading II, Part 2 NDTHB-Ed3 Vol.5My Pre-exam ASNT Self Study Notes10th September 2015


Aerospace Applications

The Magical Book of Neutron Radiography


ASNT Certification GuideNDT Level III / PdM Level IIIAE - Acoustic Emission TestingLength: 4 hours Questions: 135

1 Principles and Theory• Characteristics of acoustic emission testing• Materials and deformation• Sources of acoustic emission• Wave propagation• Attenuation• Kaiser and Felicity effects, and Felicity ratio• Terminology (refer to acoustic emission glossary, ASTM 1316)


• Signal conditioning• Signal detection• Signal processing• Source location• Advanced signal processing• Acoustic emission test systems• Accessory materials• Factors affecting test equipment

selection

2 Equipment and Materials• Transducing processes• Sensors• Sensor attachments• Sensor utilization• Simulated acoustic emission sources• Cables



4 Interpretation and Evaluation• Data interpretation• Data evaluation• Reports

5 Procedures6 Safety and Health7 Applications• Laboratory studies (material-

characterization)• Structural applications

3 Techniques• Equipment calibration and set up for

test• Establishing loading procedures• Precautions against noise• Special test procedures• Data displays

Reference Catalog NumberNDT Handbook, Second Edition: Volume 5, Acoustic Emission Testing 130Acoustic Emission: Techniques and Applications 752


Fion Zhang at Shanghai10th September 2015

To all my dearest TeachersHappy Teacher Day!

敬爱的老师们,节日快乐!

Charlie Chong/ Fion Zhang Charlie Chong/ Fion Zhang

Greek Alphabet


Charlie Chong/ Fion Zhang http://greekhouseoffonts.com/

Chapter 2 PART 4. Acoustic Emission Transducers and TheirCalibration


PART 4. Acoustic Emission Transducers and TheirCalibration1.4.1 DefinitionAcoustic emission transducers are used on a test object’s surface to detectdynamic motion resulting from acoustic emission hits and to convert thedetected motion into a voltage-versus-time signal. This voltage-versus-timesignal is used for all subsequent steps in the acoustic emission method. Theelectrical signal is strongly influenced by characteristics of the transducer.Because the test results obtained from signal processing depend so stronglyon the electrical signal, the transducer’s characteristics are important to thesuccess and repeatability of acoustic emission testing.


1.4.2 Transducer TypesBasic transduction mechanisms can be used to achieve a transducer’sfunctions: the detection of surface motion and the subsequent generation ofan electrical signal.

Capacitive transducers have been successfully used as acoustic emissiontransducers for special laboratory tests. Such transducers can have good fidelity, so that the electrical signal very closely follows the actual dynamic surface displacement. However, the typical minimum displacement measured by a capacitive transducer is on the order of 10-10 m (4 × 10-9 in.). Such sensitivity is not enough for actual acoustic emission testing.

Laser interferometers have also been used as acoustic emission transducers for laboratory experiments. However, if this technique is used with a reasonable bandwidth, the technique lacks sufficient sensitivity for acoustic emission testing.


Piezoelectric TransducersAcoustic emission testing is nearly always performed with transducers thatuse piezoelectric elements for transduction. The element is usually a specialceramic such as lead zirconate titanate PZT is acoustically coupled to thesurface of the test item so that the dynamic surface motion propagates intothe piezoelectric element. The dynamic strain in the element produces avoltage-versus-time signal as the transducer output. Because virtually all acoustic emission testing is performed with a piezoelectric transducer, theterm acoustic emission transducer is here taken to mean a sensor with apiezoelectric transduction element.

Keywords:■ Capacitive transducers■ Laser interferometers■ Piezoelectric Transducers


Direction of Sensitivity to MotionSurface motion of a point on a test object may be the result of acousticemission. Such motion contains a component normal to the surface and twoorthogonal components tangential to the surface. Acoustic emissiontransducers can be designed to respond principally to any component ofmotion but virtually all commercial acoustic emission transducers aredesigned to respond to the component normal to the surface of the structure.Because waves traveling at the longitudinal, shear and rayleigh wave speedsall typically have a component of motion normal to the surface, acousticemission transducers can often detect the various wave arrivals.

Exam Question?longitudinal, shear and rayleigh wave speeds all typically have a component of motion normal to the surface, thus the AE transducer can be designed to respond principally to normal component.


Frequency RangeThe majority of acoustic emission testing is based on the processing of signals with frequency content in the range from30 kHz to about 1 MHz. In special applications, detection of acoustic emission at frequencies below 20kHz or near audio frequencies can improve testing and conventionalmicrophones or accelerometers are sometimes used.

Attenuation of the wave motion increases rapidly with frequency and, formaterials with higher attenuation (such asfiber reinforced plastic composites),it is necessary to sense lower frequencies to detect acoustic emission hits.

At higher frequencies, the background noise is lower; for materials with low attenuation, acoustic emission hits tend to be easier to detect at higher frequencies.


Acoustic emission transducers can be designed to sense a portion of thewhole frequency range of interest by choosing the appropriate dimensions ofthe piezoelectric element. This, along with its high sensitivity, accounts for thepopularity of this transduction mechanism. In fact, by proper design of thepiezoelectric transducer, motion in the frequency range from 30 kHz to 1 MHz(and more) can be transduced by a single transducer. This special type oftransducer has applications (1) in laboratory experiments, (2) in acousticemission transducer calibration and (3) in any tests where the actualdisplacement is to be measured with precision and accuracy.


30 kHz 1 MHz

a portion of the wholefrequency range of interest by choosing the appropriate dimensions of thepiezoelectric element

Comments:High frequency → High attenuationLow frequency → Lower attenuationHigh frequency → Lower noise contribution (mechanical & electrical)

Acoustic Testing transducer frequency → 30kHz ~ 1MHzSpecial case audio frequency is used → <20kHzSelective resonance frequency transducer → increase sensitivity


1.4.3 Transducer DesignFigure 10 is a schematic diagram of a typical acoustic emission transducermounted on a test object. The transducer is attached to the surface of the testobject and a thin intervening layer of couplant is usually used. The couplantfacilitates the transmission of acoustic waves from the test object to thetransducer. The transducer may also be attached with an adhesive bonddesigned to act as an acoustic couplant. An acoustic emission transducernormally consists of several parts. The active element is a piezoelectricceramic with electrodes on each face. One electrode is connected toelectrical ground and the other is connected to a signal lead. A wear plateprotects the active element.


FIGURE 10. Schematic diagram of a typical acoustic emission transducer mounted on a test object.


Behind the active element is usually a backing material, often made by curingepoxy containing high density tungsten particles. The backing is usuallydesigned so that acoustic waves easily propagate into it with little reflectionback to the active element. In the backing, much of the wave’s energy isattenuated by scattering and absorption. The backing also serves to load theactive element so that it is less resonant or more broad band (note that insome applications, a resonant transducer is desirable). Less resonance helpsthe transducer respond more evenly over a somewhat wider range of frequencies.

Comments:Resonant- selective frequencyBroadband transducer- backing to absorb resonance.


The typical acoustic emission transducer also has a case with a connector forsignal cable attachment. The case provides an integrated mechanicalpackage for the transducer components and may also serve as a shield toinimize electromagnetic interference.

There are many variations of this typical transducer design, including(1) designs for high temperature applications, (2) transducers with built-inpreamplifiers or line drive transformers, (3) transducers with more than oneactive element and (4) transducers with active elements whose geometry orpolarization is specifically shaped.

There are two principal characteristic dimensions associated with the typicalacoustic emission transducer: ■ the piezoelectric element thickness and■ the element diameter.


Typical Acoustic Emission Transducers

Charlie Chong/ Fion Zhang http://wins-ndt.com/bridge/acoustic-emission/

Element Thickness and Sensitivity ControlElement thickness controls the frequencies at which the acoustic emissiontransducer has the highest sensitivity, that is, the highest electrical output fora given input surface velocity. The half-wave resonant frequencies of thetransducer define the approximate frequencies where the transducer will havemaximum output. These are the frequencies for which t = 0.5λ, 1.5λ, 2.5λ,and so on, where t is time (second) (?) and λ is the wavelength (meter) of thewave in the element. The wavelength can be defined as the sound speed “c”in the piezoelectric element divided by the acoustic frequency “f”.

Comment: t = thickness?

Poisson coupling in the element can lead to radial resonances at other frequencies and can also lead to some sensitivity to in-plane motion. For common piezoelectric materials and acoustic emission test frequencies, active elements are typically several millimeters (0.1 or 0.2 in.) thick. A leadzirconate titanate PZT disk 4 mm (0.16 in.) thick would normally have a first half-wave resonance of about 0.5 MHz.


Acoustic emission transducers are usually made with backing and with activeelements having relatively high internal damping. Because of this design, thevariation in sensitivity from resonant to antiresonant (zero output) frequenciesis somewhat smoothed out, providing some sensitivity over a significantfrequency range.


DiscussionSubject: “Poisson coupling in the element can lead to radial resonances at other frequencies and can also lead to some sensitivity to in-plane motion. “

Discuss: on the above statement.

Hints:Poisson's ratio is the ratio of transverse contraction strain to longitudinal extension strain in the direction of stretching force. Tensile deformation is considered positive and compressive deformation is considered negative. The definition of Poisson's ratio contains a minus sign so that normal materials have a positive ratio. Poisson's ratio, also called Poisson ratio or the Poisson coefficient, or coefficient de Poisson, is usually represented as a lower case Greek nu, ʋ. ʋ = - ε trans / ε longitudinal

Strain ε is defined in elementary form as the change in length divided by the original length. ε = ∆L/L.


Active Element DiameterThe other principal characteristic of an acoustic emission transducer is theactive element diameter. Transducers have been designed with elementdiameters as small as 1 mm (0.04 in.). Larger diameters are more common.The element diameter defines the area over which the transducer averagessurface motion.

For waves resulting in uniform motion under the transducer (as is the case for a longitudinal wave propagating in a direction perpendicular to the surface), the diameter of the transducer element has little or no effect. (?)

However, for waves traveling along the surface, the element diameter strongly influences the transducer sensitivity as a function of wave frequency.If the displaced surface of the test object is a spatial sine wave, then there are occasions when one or more full wavelengths (in the object item) will match the diameter of the transducer element. When this occurs, the transduceraverages the positive and negative motions to give zero output. This so called aperture effect has been carefully measured and theoretically modeled. (?)


For transducers larger than the wavelengths of interest in the test object, thesensitivity will vary with the properties of the test material, depending stronglyon frequency and on the direction of wave propagation. Transducer sensitivityis also influenced somewhat by the nonplanar nature of the wave front.

Because of these complications, it is recommended that the transducerdiameter be as small as other constraints allow. For example, when testingsteel, a 3 mm (0.12 in.) diameter transducer works reasonably well below 0.5MHz.

Comment: Disadvantages of large transducer are the sensitivity;■ Vary with properties of test materials■ Strong dependency on frequency and direction of wave propagation■ Affected some what by non-planar nature of wave front


Because of these complications, it is recommended that the transducer diameter be as small as other constraints allow. For example, when testing steel, a 3 mm (0.12 in.) diameter transducer works reasonably well below 0.5 MHz.45


ASNT NDT Level III

Special Acoustic Emission Transducers and Transducer MountsAcoustic emission transducers are designed for various frequency rangesand are commercially available in a range of sizes with various piezoelectricmaterials. In addition, transducers or transducer mounts are available forspecial classes of applications as described below.

■ Severe Environments. Some acoustic emission transducers are designed for high temperatures and other harsh environments. Transducers are available in which all components are chosen and assembled for temperatures up to 550°C (1020°F). Transducers for use in harshenvironments are fully encapsulated and are available with integral cable.


■ Integral Preamplifiers. Some transducer models combine the transducer

and preamplifier functions into one package. These may be miniaturized tothe same size as conventional transducers. Transducers with integralpreamplifiers have the following advantages:

(1) reduced (combined) cost, (2) faster test setup, (3) compatibility with permanent installation for some industrial applications

and (4) lower noise levels (less sensitivity to electromagnetic interference).


■ Differential Transducers. Differential transducers may be constructed with two or more active elements (or special electrode design) and a positivesignal lead, a negative signal lead and a ground lead. The active elementsare connected in parallel so that the transducer is less sensitive toelectromagnetic interference. Generally, differential transducers are alsorelatively insensitive to longitudinal aves arriving at normal incidence to thetransducer face. The sensitivity of some models may heavily depend on thedirection of propagation in the plane of the surface. Differential transducersare designed for use with differential preamplifiers rather than the single-ended preamplifiers normally used with conventional transducers.

Keywords:- Two or more active elements- Connect in parallel- Generally, differential transducers are also relatively insensitive to

longitudinal aves arriving at normal incidence to the transducer face. (?)- Separate preamplifiers


■ Acoustic Waveguides. An acoustic waveguide is a special transducermount that provides a thermal and mechanical distance between thetransducer and the test object. A waveguide is typically a metal rod with oneend designed for acoustic coupling with the test object. The other end isconstructed to accommodate the mounting of an acoustic emissiontransducer. Waveguides are used for applications in which an acousticemission transducer cannot be in direct contact with the test object becauseof (1) temperature conditions or (2) limited access to the object’s surface.


1.4.4 Couplants and BondsFor an acoustic emission transducer, the purpose of a couplant is to provide agood acoustic path from the test material to the transducer. Without acouplant or a very large transducer hold-down force, only a few random spotsof the material-totransducer interface will be in good contact and little energywill arrive at the transducer.

For sensing normal motion, virtually any fluid (oil, water, glycerin) will act as agood couplant and the transducer output can often be thirty times higher thanwithout couplant. Note though that in some applications there are stringentchemical compatibility requirements between the couplant and the test object.A transducer hold-down force of several newtons (N) is normally used to ensure good contact and to minimize couplant thickness. For sensing tangential motion, a suitable couplant is more difficult to find because most liquids will not transmit shear forces. Some high viscosity liquids such as certain epoxy resins are reasonably efficient for sensing tangential motion.


An adhesive bond between the transducer and test surface serves tomechanically fix the transducer as well as to provide coupling. Most bondsefficiently transmit both normal and tangential motion. Depending on theapplication, bonds are sometimes inappropriate. If for example the testsurface deforms significantly because of test loads or if there is differentialthermal expansion between the surface, bond or transducer, then the bond orthe transducer may break and the coupling is lost. A standard has beenwritten for transducer mounting.


Typical AET Set-up

Charlie Chong/ Fion Zhang http://www.mdpi.com/1424-8220/13/5/6365

1.4.5 Temperature Effects on Acoustic EmissionTransducers

There can be a strong relation between temperature and the piezoelectriccharacteristics of the active element in an acoustic emission transducer.Some of these effects are important to acoustic emission transducers intesting at elevated or changing temperatures.


Effect of Curie TemperatureTypically, there is a temperature for piezoelectric ceramics at which theproperties of the ceramic change permanently and the ceramic element nolonger exhibits piezoelectricity. This temperature is known as the “curietemperature” and is the point at which a material moves from ferroelectric toparaelectric phase.

Piezoelectric ceramic elements have been used successfully within 50°C(122°F) of their curie temperature.

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 for lithium niobate at 1210°C (2210°F).

Testing limitations are therefore encountered in environments where staticelevated 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 Curie Temperature on Transducers Applicable within (below) 50°C (122°F) of their curie temperature. Lead zirconate titanate ceramics is 300 to 400°C (572 to 752°F) barium titanate at 120°C (258°F) lithium niobate at 1210°C (2210°F)


Ferro-electricity is a property of certain materials that have a spontaneous electric polarization that can be reversed by the application of an external electric field.[1][2] The term is used in analogy to ferromagnetism, in which a material exhibits a permanent magnetic moment. Ferromagnetism was already known when ferroelectricity was discovered in 1920 in Rochelle salt by Valasek.[3] Thus, the prefix ferro, meaning iron, was used to describe the property despite the fact that most ferroelectric materials do not contain iron.

Para-electricity is the ability of many materials (specifically ceramics) to become polarized under an applied electric field. Unlike ferroelectricity, this can happen even if there is no permanent electric dipole that exists in the material, and removal of the fields results in the polarization in the material returning to zero. The mechanisms that cause paraelectric behaviour are the distortion of individual ions (displacement of the electron cloud from the nucleus) and polarization of molecules or combinations of ions or defects.Paraelectricity can occur in crystal phases where electric dipoles are unaligned and thus have the potential to align in an external electric field and weaken it.

Charlie Chong/ Fion Zhang https://en.wikipedia.org/wiki/Dielectric#Paraelectricity

Effect of Fluctuating TemperatureSpecial problems are encountered when transducers are placed inenvironments with widely changing temperatures. Piezoelectric ceramicactive elements have small domains in which the electrical polarization is inone direction. Temperature changes can cause some of these domains to flip,resulting in a spurious electrical signal that is not easily distinguished from thesignal produced by an acoustic emission hit in the test object. In a leadzirconate titanate element, a temperature change of 100°C (212°F) can cause an appreciable number of these domain flips. 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.■ Acoustic waveguides may also be used to buffer the transducer from large temperature changes.


Magnetic Domain/ Spin

Charlie Chong/ Fion Zhang http://wps.prenhall.com/wps/media/objects/3311/3390683/blb0607.html

Keypoints: If acoustic emission testing must be done during large temperature

changes, then single-crystal piezoelectric materials such as quartz are recommended.

Acoustic waveguides may also be used to buffer the transducer from large temperature changes.

Use lithium niobate with Curie temperature at 1210°C (2210°F) for extreme temperature application


1.4.6 Transducer Calibration Terminology of TransducerCalibration

Calibration. The calibration of a transducer is the measurement of its voltageoutput into an established electrical load for a given mechanical input. Thesubject of what should be the mechanical input is discussed below.Calibration results may be expressed either as a frequency response or as animpulse response.

Keywords: Frequency response, 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 thisdiscussion, that solid object is called the test block.


■ Displacement. Displacement is the dynamic particle motion of a point inor on the test block. Displacement is a function of time and three positionvariables. Here, the word velocity or acceleration could replace displacement.Normal displacement is displacement perpendicular to the face of atransducer or displacement of the surface of a test block perpendicular to thatsurface. Tangential displacement is displacement in any directionperpendicular to the direction of normal displacement.


Normal displacement

Tangential displacement

Principles of Transducer CalibrationIf acoustic emission results are to be quantitative, then it is necessary to havea means for measuring the performance of a transducer. Techniques of doingthis have been the subject of much discussion. Because there are manytypes of transducers in use and because they may be called on to detectwaves of different kinds in different materials, it is not possible to have auniversal calibration procedure. A transducer calibration, appropriatelyapplied to the signal recorded from a transducer, should provide a record ofthe displacement of a point on the surface of the object being examined bythe transducer. There are several fundamental problems encountered duringcalibration, as listed below.


1. The displacement of a point on the surface of a test block is a threedimensional vector but the output of the transducer is a scalar.

2. Displacement is altered by the presence of the transducer. (damping effect?)

3. The face of the transducer covers an area on the surface of the test blockand displacement is a function not only of time but of the position within this area.

Because of these problems, transducer calibration is not feasible withoutmaking some simplifying assumptions. Various calibration approaches havebeen taken and they all make implicit 不直接言明的 assumptions.


scalar

3D-Vector

■ Calibration Assumptions. Regarding the vectoral nature of displacement (problem 1), it is usually assumed that the transducer is sensitive only to normal displacement. Naturally, errors will be introduced if the transducer is sensitive to tangential displacement. Calibrations for other directions of sensitivity are useful but are not routine.

The loading effect that the transducer has on the surface motion of a test block (problem 2) is significant but is not subject to any simple analysis. In general, the test block may be considered as having a mechanical impedance (source impedance) at the location of the transducer. The transducer also has a mechanical impedance at its face (load impedance). Interaction between the source and load impedances determines the displacement of the transducer face but both of these impedances are likely to be complex functions of frequency and no technique exists for measuring them.


For calibration purposes, the usual solution to this problem is to define theinput to the transducer as the unloaded (free) displacement of the test blockwith no transducer attached. The calibration is then practical because it is thedisplacement of the test block (and not the interactive effects) that are ofinterest. The function of the calibration scheme is to determine what thedisplacement of the surface of the test block would be in the absence of thetransducer. It must be noted, however, that when a transducer is attached todifferent test blocks having different mechanical impedances, it will havedifferent calibrations. Calibrations are transferable only when the test blockimpedances are the same.

Comments:■ Material impedance■ Mechanical impedance at transducer interface■ Face/ contact impedance of the transducer


For several calibration procedures, the test block approximates a semiinfinitehalf space of steel. Steel was chosen because it was expected that acousticemission transducers would be used more on steel than on any other material. The large size of the test block makes the mechanical impedance at its surface a property of the material only, and not of its dimensions within the usual acoustic emission working frequency range. It is demonstrated below that test blocks made of different materials produce significantly different calibrations of the same transducer.


Experiments have been done to determine how much effect the material ofthe test block has on calibration results. A commercial ultrasonic transducerand a conical transducer were calibrated on a steel block and then subjected to surface pulse waveforms in aluminum, glass and methyl methacrylate plastic.

The surface pulse waveforms were generated by a pencil break apparatushaving the provision for measuring the force. For each material, the surface pulse waveform was calculated at the transducer location, and modified by deconvolution to remove the source characteristics. The results are shown in Figs. 11 and 12. Analysis of the conical transducer has been carried out and the results are shown in Fig. 13. Because the blocks were smaller thanoptimal, these data are approximate.

The order of magnitude of the effect is clear and in the case of the conical transducer there is reasonable agreement between the theory and the experiment.


FIGURE 11. Approximate calibrations of a conical transducer.


FIGURE 12. Approximate calibrations of a transducer done on blocks of fourdifferent materials. A pencil graphite break was the source for all except the steel block calibration.


FIGURE 13. Calculated sensitivity of the conical transducer in Fig. 11 on the same four materials; calculations are based on the theory for the transducer


FIGURE 14. Straight line waves incident on a circular transducer.


The finite size of the transducer’s face (problem 3) is often ignored. This isequivalent to (1) assuming that the diameter of the face is small compared toall wavelengths of interest in the test block or (2) assuming that all motion isin phase over the face. The latter assumption is only true in the case of planewaves impinging on the transducer from a direction perpendicular to its face.In general, a transducer responds to a weighted average of the displacementover its face. This averaging or aperture effect may be considered a propertyof the transducer and grouped with all transducer properties in calibration.However, it must be observed that, as a consequence, the aperture effect andtherefore the calibration will differ depending on the type and speed of thewave motion in the test block.


■ Aperture Effect and Calibration. The aperture effect for an acousticemission transducer may be described as follows. Neglecting interactiveeffects between the test block and the transducer, the response of thetransducer may be written as follows:

where r (x,y) is the local sensitivity of the transducer face, S is the region(square meter) of the surface contacted by the transducer, A is the area(square meter) of region S and u(x,y,t) is the displacement (meter) of thesurface.


(7)

The x,y plane is the surface of the test block. As a special case, assume astraight line wave front incident on a circular transducer having radius a(meter) and uniform sensitivity r (x,y) = 1, over its face (see Fig. 14). Assumea wave of the form:

where k is ω∙c-1 and c is the Rayleigh wave speed (meter per second).


(8)

The transducer response (Eq. 7) then becomes:

which reduces to:

where J is the first order bessel function.Figure 15 shows this calculated bessel function response compared to thecalibration of an experimental capacitive circular disk transducer.


(9)

(10)

FIGURE 15. Results of the calculation of Eq. 10 compared with experimentalresults from a capacitive disk transducer. Source-to-receiver distance d = 0.1 m (4 in.); transducer radius a = 10 mm (0.4 in.); surface pulse is generated by a capillary break on a steel block.


■ Surface Calibration. Most calibration systems use a configuration inwhich the transducer under test and the source are both located on the sameplane surface of the test block. The result is known as a surface calibration orrayleigh calibration, so called because most of the propagating energy at thetransducer is traveling at the rayleigh speed. In this case, the transducer’scalibration is strongly influenced by the aperture effect.


surface calibration or rayleigh calibration

Aperture EffectIn plate objects such as vessel walls, it was shown that Rayleigh waves or Lamb waves are dominant . As for these wave modes transducer sensitivity is subject to the aperture effect. Figure 4 shows the mechanism of the effect, where the crests and troughs of the incident Rayleigh or lamb waves cancel out each other within the transducer aperture.

Charlie Chong/ Fion Zhang ASTM STP1353 Acoustic Emission: Standards and Technology Update

■ Through-Pulse Calibration. Other calibration systems use a configuration in which the transducer under test and the source are coaxially located on opposite parallel faces of the test block. All wave motion is in phase across the face of the transducer (except for a negligible curvature of the wave fronts at the transducer) and the calibration is essentially free of any aperture effect. The result is a through-pulse calibration or P wave calibration. Note that because of the axial symmetry of the through-pulse calibration, only normal displacement exists at the location of the transducer under test.


P wave calibration

Step Function Force CalibrationThe basis for the step function force calibration is that known, wellcharacterized displacements can be generated on a plane surface of a testblock. A step function force applied to a point on one surface of the test block initiates an elastic disturbance that travels through the block. The transducer under test is located either on the same surface (surface calibration) or on the opposite surface at the epicenter of the source (through-pulse calibration).

Given the step function source, the free displacement of the test block at thelocation of the transducer can be calculated by elasticity theory in both cases.The calculated block displacement function is the transfer function (mechanical transfer admittance, when expressed in the frequency domain)or the Green’s function for the block. The free displacement of the test block surface can also be measured using a capacitive transducer with a known absolute sensitivity. It is essential to the calibration that the calculateddisplacement and capacitive transducer measurement agree.


The calibration facility at the National Institute of Standards and Technologyhas used a cylindrical steel test block 0.9 m (36 in.) in diameter by 0.43 m (17in.) long with optically polished end faces. The step function force is made bybreaking a glass capillary (see Fig. 16). In the case of surface calibration, free normal displacement of the surface is measured by a capacitive sensor at a location symmetrical to that of the transducer under test with respect to the source location. The displacement is redundantly determined by elasticitytheory from a measurement of the force at which the capillary broke. Sourceand receiver are 0.1 m (4 in.) apart.


FIGURE 16. Schematic diagram of the surface pulse apparatus.


For through-pulse calibration, the free normal displacement is determinedonly by the elasticity theory calculation. Both calibrations are absolute: theresults are in output volts per meter of displacement of the (free) blocksurface. Following the initiation of the step function force, an interval of timeexists during which the displacement at the location of the transducer undertest is as predicted by the elastic theory for the semiinfinite solid (in the caseof the surface calibration) or for the infinite plate (in the case of the through-ulse calibration). However, as soon as any reflections arrive from thecylindrical surface of the block, the displacement deviates from the theory.The dimensions of the block are large enough to allow 100 μs of workingtime between the first arrival at the transducer and the arrival of the firstreflection. For most transducers, the 100 μs window is long enough tocapture most of the information in the output transient waveform in thefrequency range of 100 kHz to 1 MHz.


The transient time waveform from the transducer and that from the capacitivetransducer are captured by transient recorders and the information issubsequently processed to produce either a frequency response or animpulse response for the transducer under test. The frequency responsecontains both the magnitude and the phase information. It is generallyassumed that a transducer has only normal sensitivity because of its axialsymmetry (an assumption that may not always be justified). Calibration by thesurface pulse technique for a transducer having significant sensitivity totangential displacement will be in error because the surface pulse from thestep force contains a tangential component approximately as large as thenormal component. It could, however, be calibrated for the normal componentof sensitivity by the through-pulse technique because no tangentialdisplacement exists at the location of the transducer under test in the through-ulse configuration. It could also be calibrated (assuming no aperture effectexists) by averaging two surface calibrations with the transducer rotated 3.14rad (180 deg) axially between calibrations. By combining through-pulse andsurface calibration results judiciously, more information can be gained aboutthe magnitudes of all three components of sensitivity.


The certified frequency range of the calibration is 100 kHz to 1 MHz withinformation that is less accurate provided down to 10 kHz. The low end islimited by the fact that the 100 ms time window limits the certainty ofinformation about frequency content below 100 kHz. The expected lowfrequency errors depend on how well damped the transducer under test is.For transducers whose impulse response function damps to a negligiblevalue within 100 ms, the valid range of the calibration could be extendedlower than 100 kHz. The high frequency limitation of 1 MHz is determined bythe fact that frequency content of the test pulse becomes weak above 1 MHzand electronic front end noise becomes predominant at higher frequencies.This method of calibration is covered in published standards such as thosepublished elsewhere in this volume.


Reciprocity CalibrationReciprocity applies to a category of passive electromechanical transducersthat have two important characteristics: (1) they are purely electrostatic orpurely electromagnetic in nature and (2) they are reversible (can be used aseither a source or a receiver of mechanical energy). This category includes allknown commercial acoustic emission transducers without preamplifiers. Forsuch a transducer, reciprocity relates its source response and its receiverresponse in a specific way. If two exactly identical transducers are used, oneas a source and one as a receiver, both coupled to a common medium, and ifthe transfer function or Green’s function of the medium from the sourcelocation to the receiver location is known, then from purely electricalmeasurements of driving current in the source and output voltage at thereceiver, the response functions of the transducers can be determinedabsolutely.


With nonidentical transducers, three such measurements (using each of the three possible pairs of transducers) provides enough information to determine all of the response functions of the transducers absolutely. The primary advantage of the reciprocity calibration technique is that it avoids thenecessity of measuring or producing a known mechanical displacement orforce. All of the basic measurements made during the calibration are electrical.It is important to note, however, that the mechanical transfer function orGreen’s function for the transmission of signals from the source location tothe receiver location


must be known. This function is equivalent to the reciprocity parameter and isthe frequency domain representation of the elasticity theory solutionmentioned in the discussion on the step function force calibration. Theapplication of reciprocity techniques to the calibration of microphones,61-66hydrophones67 and accelerometers68 is well established. The reciprocitytechnique was proposed in 1976 for the calibration of acoustic emissiontransducers coupled to a solid and was subsequently implemented by onesteel producer as a commercial service.52,69,70 One steel producer’scalibration facility has used a cylindrical steel test block 1.1 m (44 in.) indiameter by 0.76 m (30 in. ) long to perform rayleigh calibration (analogous tosurface calibration) and P wave calibration (analogous to through-pulsecalibration). In the rayleigh calibration, the transducers are separated by 0.2m (8 in.) on the same surface of the block; for the P wave calibration, thetransducers are on opposite faces on epicenter.


The technique uses essentially continuous wave measurements but the signals are gated to eliminate reflections from the block walls. For a set of three transducers, the three electrical voltage transfer functions and the electrical impedances of all transducers are measured. From these data, receiving response (in volts of output per meter per second of input) andsource response (meters per second of output per volt of input) are calculated for the range of 100 kHz to 1 MHz. This source response applies to any point on the steel surface located 0.2 m (8 in.) from the source.

The same assumptions about direction of sensitivity that were mentionedunder step function force calibration apply to all transducers in a reciprocitycalibration. A violation of the assumption by any of the three transducerswould contaminate the results. The aperture effect also applies to lltransducers in a calibration and the considerations for mechanical loading ofthe test block are the same as for the step force calibration. A diffuse fieldeciprocity calibration has also been introduced. A broad band ultrasonictransducer and two resonant acoustic emission transducers were coupled toan aluminum block with all its corners sawed off at different angles to producea diffuse field.



Method of Reciprocity CalibrationFigure 5 shows the fundamental aspects of the method of reciprocity alibration. Three reversible transducers 1, 2, and 3 are prepared, and three independent transmission/reception pairs are configurated through a transfer medium.

The magnitudes of the transmission signal current and reception signal voltage, lij and Eij, respectively, are measured on each pair, where the subscript ij corresponds to transducer i for transmission and j for reception.

If the reciprocity parameter H, which is dependent not on the transducer design but on the mode of elastic waves, constants of medium, and definition of sensitivity, is given, absolute sensitivity is determined by purely electricalmeasurements.

ASTM STP1353 Acoustic Emission: Standards and Technology Update


FIG. 5- Three transmission/reception pairs for reciprocity calibration.

ASTM STP1353 Acoustic Emission: Standards and Technology Update

Secondary CalibrationThe secondary calibration of an acoustic emission transducer is performed ona system and with a technique that has logical links to the primary calibrationsystem and technique. It provides data of the same type as a primarycalibration but the data may be more limited. For example, there may be nophase information or a narrower range of frequencies or the calibration maybe applicable to a different material. Because of the logical link between thetechniques, the data may be compared if the source-to-transducer geometriesare taken into account. Standards have been published describing secondarycalibration. Transducer suppliers usually provide data on the sensitivity ofacoustic emission transducers over a range of frequencies. In some cases,these data are developed on a system very similar to a primary calibrationfacility and can be compared with primary calibration data. More often, thesupplied data provide relative response rather than absolute response; suchinformation cannot be logically linked to a primary calibration. It is useful forcomparing the response of similar transducers or for checking for changes intransducer response. This information is frequently based on a differentphysical unit (often pressure) than primary and secondary calibrations(displacement or velocity).


The development of secondary calibration techniques is an area of ongoingresearch. A secondary technique must offer compromises between systemcomplexity and accurate transducer characterization. There are listed belowsome tools for further developing secondary calibration procedures.


High Fidelity Transducers. Transducers that accurately measure surface motion with high sensitivity are helpful when developing calibration procedures. Such transducers may be used as transfer standards because of their stability and their uniform sensitivity over the frequency range of interest. There are transducers having flat frequency response over the range of 10 kHz to 1 MHz or higher. One such transducer (developed at the National Institute of Standards and Technology) has a small conical element backed up by a large brass block.Figure 17 shows the voltage-versus-time output of the conical transducer mounted on a large steel plate and responding to the displacement caused by breaking a glass capillary. The transducer’s output is compared with the theoretically predicted displacement of a point on a plate, a displacement caused by a point step function input. The favorable comparison indicates that the transducer accurately measures transient displacement.


FIGURE 17. Conical transducer’s output (lower curve) from a glass capillary breaking on a large steel plate compared to the output of a computer program’s calculation (upper curve) of the Green’s function of the steel plate.


Computer Programming. A second tool useful for secondary calibration isalso demonstrated by Fig. 17. A computer program provides a theoreticalprediction of surface motion for various (1) plate materials, (2) simulatedacoustic emission sources and (3) source-to-transducer geometries. Amechanical input of known force and time history (such as a breaking glasscapillary event51 or a pencil graphite break source54) is used and the sourceis modeled with the computer program. The predicted displacement timehistory can be used to determine the sensitivity of a transducer as a functionof frequency. Procedures for checking the response of acoustic emissiontransducers are relatively simple and can be used to check transducers fordegradation or to identify transducers that have similar performance. Theseprocedures are discussed in detail elsewhere. They are not capable ofproviding transducer calibration or of ensuring transferability of data setsbetween different groups.


Additional Acoustic Emission Transducer InformationA great deal of practical information on transducers is available in theliterature. Additional detailed information is available for acoustic emissiontransducers and their characterization. Much information about ultrasonic testtransducers is also valuable for acoustic emission transducers.



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Charlie Chong/ Fion Zhanghttps://www.yumpu.com/en/browse/user/charliechong

understanding acoustic emission testing reading 2 ndthb vol5 part 2

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