Acoustic Emission Revised Nov 2008
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Transcript of Acoustic Emission Revised Nov 2008
7/27/2019 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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