Acoustic emission testing (AET) is a non-destructive testing method that detects stress waves generated by sudden changes in material properties, often used for monitoring structural integrity in dynamic environments. AET distinguishes itself by passively listening for natural energy releases from materials, allowing real-time failure detection and accurate localization of defects, though it requires specialized equipment and expert analysis. Its applications span various industries, including online monitoring of pressure vessels, weld quality assessment, and bridge inspections, capitalizing on its ability to identify growing discontinuities before catastrophic failures occur.

1. UNIT-V
• Acoustic emission techniques:
• Principles of acoustic emission techniques
• technique
• Instrumentation
• Sensitivity
• Applications.

2. Acoustic Emission Testing (AET) is a non destructive testing method that is
based on the generation of waves produced by a sudden redistribution of stress
in a material.
When a piece of equipment is subjected to an external stimulus, such as a
change in pressure, load or temperature, this triggers the release of energy in
the form of stress waves, which propagate to the surface and are recorded by
sensors.
Acoustic emissions can come from natural sources, such as earthquakes or rock
bursts or from the equipment itself such as melting, twinning, and phase
transformations in metals.
Acoustic emission testing involves the detection and analysis of AE signals using
specialized equipment and can provide information on the origin and
significance of discontinuities in a material.

3. Acoustic emission testing is different than other NDT techniques in two major ways:
1. Instead of supplying energy to the object under examination, AET listens for the
energy released by the object naturally. AE tests can be performed on structures
while they are in operation since this provides adequate loading for propagating
defects and triggering acoustic emissions.
2. AET deals with dynamic processes in a material. This is particularly useful because
only active features are highlighted during the examination. Thus, it is possible to
discern between developing and stagnant defects. However, it is possible for flaws to
go undetected if the loading isn’t high enough to cause an acoustic event that can be
detected by the system.

4. Acoustic emission testing is most often used in a dynamic test environment, meaning
that it is used to monitor for crack detection in pressure equipment when the
equipment is experiencing an increase in stress.
AET systems generally contain a sensor, preamplifier, filter, and amplifier, along with
measurement, display, and storage equipment.
Acoustic emission sensors respond to any dynamic motion caused by an acoustic
emission event. This is achieved through transducers that convert mechanical
movement into an electrical voltage signal.
The majority of AET equipment responds to movement in a range of 30 kHz to 1 MHz
in brittle and ductile material.
For materials with high attenuation, such as plastic composites, lower frequencies
may be used to better distinguish acoustic emission signals.

5. Because of its versatility, acoustic emission testing has many applications across a
variety of industries.
i) ONLINE MONITORING OF PRESSURE VESSELS, PIPELINES AND ENGINEERING
STRUCTURES.
ii) LEAKAGE DETECTION and location
iii) Quality control during fabrication.
iv) Monitoring underground pipelines
v) Online weld monitoring
In O&G and chemical processing industries, AET is often used to assess structural
integrity, test for leaks, monitor weld quality, and detect active flaws such as
corrosion in the bottom of aboveground storage tanks or creep damage in high
energy piping systems.

6. The genesis of today’s technology in AE was the work of Josep Kaiser. In 1950
Kaiser published his thesis where he reported the first comprehensive
investigation into the phenomenon of AE.
Kaiser used tensile tests of conventional engineering materials to determine :
1. Types of noises generated from within the specimen.
2. The acoustic processes involved.
3. The frequency ranges and amplitude levels found.
4. The relation between stress, strain and the frequencies recorded at various
stress levels to which specimens were subjected to.

7. • Acoustic emission (AE) is high frequency elastic Waves produced in a material
under load by the rapid release of strain energy during crack propagation,
plastic deformation or phase transformation.
• The Waves travel to the surface where they can be listened (detected).
• This technique is also called stress wave emission
• AE Can be considered as extension of ultrasonic testing, except that the
ultrasonic waves are replaced with acoustic waves.
• AE can give indications on the location and the relative severity of a defect.
• AE can be used to measure degree of stress (strain) of a material.

8. Principle of AET :
Acoustic emission inspection detects and analyses minute AE signals generated by
growing discontinuities in material under a stimulus such as stress and temperature.
Proper analysis of these signals can provide information concerning the detection and
location of these discontinuities and the structural integrity.
Another important feature of AE is its irreversibility.
If a material is loaded to a given stress level and then unloaded, usually no emission
will be observed upon immediate reloading until the previous stress has been
exceeded.
This is known as Kaiser effect and is due to the fact that AE is closely related to plastic
deformation and fracture.

10. Amplitude and Duration of AE Signal
Kaiser effect.

12. Advantages :
1. Real time detection of component failure.
2. Highly accurate results .
3. Sensitive
4. Economical
5. Early warning of crack growth
6. Exact location of defect can be found.
7. Possible to test pressure equipment during plant operation.
8. Can detect very small flaws (1 to 50 microns)
Disadvantages :
9. Sophisticated and high initial cost - as detected signal has low energy
10. Required skill person to operate and understand signals.
11. Difficult for outdoor use.

13. Weld Monitoring : - During the welding process,
temperature changes induce stresses between the weld
and the base metal.
These stresses are often relieved by heat treating the
weld.
However, in some cases tempering the weld is not
possible and minor cracking occurs.
Amazingly, cracking can continue for up to 10 days after
the weld has been completed.
Using stainless steel welds with known inclusions and
accelerometers for detection purposes and background
noise monitoring, it was found by W. D. Jolly (1969) that
low level signals and more sizeable bursts were related to
the growth of micro-fissures and larger cracks respectively.
ASTM E 749-96 is a standard practice of AE monitoring of
continuous welding.

14. Bucket Truck (Cherry Pickers) Integrity Evaluation : Accidents, overloads
and fatigue can all occur when operating bucket trucks or other aerial
equipment.
If a mechanical or structural defect is ignored, serious injury or fatality
can result. In 1976, the Georgia Power Company pioneered the aerial
man lift device inspection.
Testing by independent labs and electrical utilities followed. Although
originally intended to examine only the boom sections, the method is
now used for inspecting the pedestal, pins, and various other
components.
Normally, the AE tests are second in a chain of inspections which start
with visual checks.
If necessary, follow-up tests take the form of magnetic particle,
dye penetrant, or ultrasonic inspections. Experienced personnel can
perform five to ten tests per day, saving valuable time and money along
the way. ASTM F914 governs the procedures for examining insulated
aerial personnel devices.

15. Gas Trailer Tubes : - Acoustic emission testing on
pressurized jumbo tube trailers was authorized by the
Department of Transportation in 1983. Instead of using
hydrostatic retesting, where tubes must be removed from
service and disassembled, AET allows for in situ testing.
A 10% over-pressurization is performed at a normal filling
station with AE sensors attached to the tubes at each end.
A multichannel acoustic system is used to detection and
mapped source locations.
Suspect locations are further evaluated using ultrasonic
inspection, and when defects are confirmed the tube is
removed from use.
AET can detect subcritical flaws whereas hydrostatic
testing cannot detect cracks until they cause rupture of the
tube. Because of the high stresses in the circumferential
direction of the tubes, tests are geared toward
finding longitudinal fatigue cracks.

16. Bridges : - Bridges contain many welds, joints and connections, and a combination of load and
environmental factors heavily influence damage mechanisms such as fatigue cracking and metal
thinning due to corrosion.
Bridges receive a visual inspection about every two years and when damage is detected, the bridge is
either shut down, its weight capacity is lowered, or it is singled out for more frequent monitoring.
Acoustic Emission is increasingly being used for bridge monitoring applications because it can
continuously gather data and detect changes that may be due to damage without requiring lane
closures or bridge shutdown.
In fact, traffic flow is commonly used to load or stress the bridge for the AE testing.

17. Applications

18. Applications

19. Acoustic Emission - Technique
With the equipment configured and setup complete, AE testing may begin. The sensor is
coupled to the test surface and held in place with tape or adhesive.
An operator then monitors the signals which are excited by the induced stresses in the
object.
When a useful transient or burst signal is correctly obtained, parameters like amplitude,
counts, measured area under the rectified signal envelope (MARSE), duration, and rise time
can be gathered.
Each of the AE signal feature shown in the image is described below.

21. Ring down counts is the number of times the signal crosses the threshold level set for
eliminating background noise.
This could be used independently or as the cummlative counts with respect to time or any
other parameter.
Count rate is another parameter commonly used.
The most common ways in which AE signals can be processed are :
1. Ringdown counts, ringdown count rates, events.
2. Energy analysis : continuous and burst type emissions.
3. Amplitude analysis : characterise emissions from different processes.
4. Frequency analysis : identify different types of failures.
5. Advanced signal analysis concepts – pattern recognition, spectral analysis, etc.

22. On large pressure vessels – necessary not only to detect AE signals but also to locate their
sources.
By suitably placing several sensors over the surface of a pressure vessel and monitoring the time
of arrival of the signals to various sensor locations. Because of high velocity of sound and
relative closeness of sensors on a steel vessel, time resolutions in microsecond range must
be made in order to locate the source to within less than about 25mm.
AET is capable of locating one or more discontinuities while they are growing.
When the discontinuity approaches a critical size the AE count rate increases markedly thus
giving a warning for instability and failure of the component.
AE evaluation of structure depends on the ability to detect weak signals in noisy electrical and
mechanical environment.
Proper instrumentation and effective methods are needed to discriminate between wanted and
unwanted signals.

23. Instrumentation : AET consists of signal detection, data acquisition, processing and analysis units. The choice of analysing
unit depends on the purpose for which AET is employed, the level of sensitivity required and economic consideration.
1. Transducer for AET : Transducer for detection and recording of AE are piezoelectric transducer with frequency of the
order of 30kHz to 2 MHz. Resonant type are used for narrow band instrumentation and non-resonant type with wide
band instrumentation.
2. Pre-amplification and filtering : The pre-amplifier follows the transducer . It should have low noise , moderately high
power gain and input impedance matching the transducer. Filters are designed for different band widths in order to
meet specific requirements.
3. Post-amplification and threshold : The output of pre amplifier is very low and hence has to be amplified further before
handling with the processing circuitry. In order to eliminate the background noise from analysis only signals exceeding
certain threshold voltage are detected and analysed.
4. Data acquisition :2 types of systems are in common use for acquisition of raw AE data for off line analysis, namely video
recorder and analog magnetic recorders. When real time decision making is important with capability of on-line
processing and analysis, transient recorders with computer interface are used.
5. Processing and analysis : The processing instrumentation required for AET depends on the form and quality of raw data.
Its function is to convert analog data to digital form.
6. Display devices : Variety of display devices are used for displaying and recording analysed data. The simplest and most
commonly used recording device is the X-Y recorder.

24. Sensitivity : AE inspection is extremely sensitive with other NDT methods. The minimum
detectable crack for UT, RT and ECT methods is about 0.5mm if using ideal conditions. AET
can detect crack growth of the order of 25 microns.

25. Amplitude A, is the greatest measured voltage in a waveform and is measured in decibels (dB).
This is an important parameter in acoustic emission inspection because it determines the
detectability of the signal. Signals with amplitudes below the operator-defined,
minimum threshold will not be recorded.
Rise time, R, is the time interval between the first threshold crossing and the signal peak. This
parameter is related to the propagation of the wave between the source of the acoustic
emission event and the sensor. Therefore, rise time is used for qualification of signals and as a
criterion for noise filter.
Duration, D, is the time difference between the first and last threshold crossings. Duration can
be used to identify different types of sources and to filter out noise. This parameter relies upon
the magnitude of the signal and the acoustics of the material.

26. MARSE, E, sometimes referred to as energy counts, is the measure of the area under the
envelope of the rectified linear voltage time signal from the transducer.
This can be thought of as the relative signal amplitude and is useful because the energy of
the emission can be determined.
MARSE is also sensitive to the duration and amplitude of the signal, but does not use
counts or user defined thresholds and operating frequencies.
MARSE is regularly used in the measurements of acoustic emissions.
Counts, N, refers to the number of pulses emitted by the measurement circuitry if the
signal amplitude is greater than the threshold.
Depending on the magnitude of the AE event and the characteristics of the material, one hit
may produce one or many counts.
While this is a relatively simple parameter to collect, it usually needs to be combined
with amplitude and/or duration measurements to provide quality information about the
shape of a signal.