Acoustic Emission Revised Nov 2008

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 Acoustic Emission Testing

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The acoustic emission

phenomenon

 Acoustic emission is the elastic energy thatis spontaneously released by materialswhen they undergo deformation. It is alsocalled stress wave emission, microseismmicroseismic activity, and rock noise.

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Comparison with other NDT

techniques

1. The energy that is detected is released from

the test object, rather than being supplied

from source as in the case of RT, UT2. Crack growth and plastic deformation are the

major sources of AE

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 Application of acoustic

emission tests.

1. Mechanical properties and characterization

2. Preserves proof testing

3. Inservice testing4. On-line monitoring

5. In-process weld monitoring

6. Mechanical signature analysis7. Leak detection and location

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 Advantages of AE

1.  AE is a dynamic method in that it

provides a response to discontinuity

growth under an imposed structuralstress; static discontinuities will not 

generate AE signals.

2.  AE can detect and evaluate thesignificance of discontinuities throughout

an entire structure during a single test.

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 Advantages AE contd.

3.Since only limited access is required

discontinuities may be detected that are

inaccessible to the more traditionalmethods of NDT

4.Vessels and other pressure systems

can often be re-qualified during an in-service inspection that requires little or no

downtime

5. AE prevents catastrophic failure of 

systems.

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The Kaiser Effect

 An important feature affecting acousticemission applications is the generally

irreversible response from most metals. In practice it is often found that once a given

load has been applied and the AE fromaccommodating that stress has ceased,additional AE will not occur until that stresslevel is exceeded, even if the load iscompletely removed and then reapplied.

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 AE signal characterization Amplitude, A, is the greatest

measured voltage in a waveformand is measured in decibels (dB).

Rise time, R, is the time intervalbetween the first threshold

crossing and the signal peak. Duration, D, is the time

difference between the first andlast threshold crossings.

MARSE, E, sometimes referredto as energy counts, is the

measure of the area under theenvelope of the rectified linear voltage time signal from thetransducer .

Counts, N, refers to the number of pulses emitted by the measurement circuitry if 

the signal amplitude is greater than the threshold.

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 AE signal characterization

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 AE sensors

When the AE wave front impinges the surfaceof a test object, very minute movements of 

surface molecules occur. The sensor functionis to detect the minute movements on thesurface and convert into electrical signal,-theyare similar to UT probes, may be broad band

or resonance type. Natural occurring AEcontains complex wave modes.

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 AE test sensitivity

The sensitivity decreases with increase indistance from the source to sensors,

Wave mode conversions at the surfaces of thetest object and other acoustic interfaces,combined with the fact that different wavemodes propagate with different velocities, are

factors that complicate the analysis of AEresponse signals and produce uncertainties incalculating the AE source location.

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Sources acting in or on platesa. rays emanating from source and striking the

receiver.

b. the sum of direct and reflected waves seen byreceiver.  

Source a

b

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Noise

Noise in AE testing refers to any undesirable signals

detected by the sensors.

Examples of these signals include frictional sources(e.g. loose bolts or movable connectors that shift

when exposed to wind loads) and impact sources

(e.g. rain, flying objects or wind-driven dust) in

bridges.

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Noise

Some possible approaches involve

fabricating special sensors with electronic

gates for noise blocking, takingprecautions to place sensors as far away

as possible from noise sources, and

electronic filtering 

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Equipment

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Equipment

 Acoustic emission testing can be performed

in the field with portable instruments or in a

stationary laboratory setting.

Typically, systems contain a sensor,

preamplifier,

filter,

and amplifier, along with measurement, display,

and storage equipment (e.g. oscilloscopes, voltmeters, and personal

computers).

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Equipment

 As normal UT, signal is achieved through

transducers which convert mechanical

movement into an electrical voltage signal.

The transducer element in an AE sensor isalmost always a piezoelectric crystal, which

is commonly made from a ceramic such as

lead zirconate titanate (PZT).

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Equipment

Transducers are selected based on operating

frequency, sensitivity and environmental characteristics,

and are grouped into two classes:

resonant and broadband. operating frequency range of 30 kHz to 1 MHz.

For materials with high attenuation (e.g. plastic

composites), lower frequencies may be used to better 

distinguish AE signals. The opposite holds true as well.

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AE Source Location Techniques 

Point Location

In order for point location to be justified, signals must be

detected in a minimum number of sensors: two for linear,

three for planar, four for volumetric. Accurate arrival times

must also be available. Arrival times are often found by

using peak amplitude or the first threshold crossing.

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Thanks