Acoustic emission
Acoustic emission (AE) is the phenomenon
of radiation of acoustic (elastic) waves in solids that occurs when a material
undergoes irreversible changes in its internal structure, for example as a
result of crack formation or plastic deformation due to aging, temperature
gradients or external mechanical forces. In particular, AE is occurring during
the processes of mechanical loading of materials and
structures accompanied by structural changes that generate local sources
of elastic waves. This results in small surface displacements of a
material produced by elastic or stress waves generated when the
accumulated elastic energy in a material or on its surface is released rapidly.The
waves generated by sources of AE are of practical interest in structural
health monitoring (SHM), quality control, system feedback, process
monitoring and other fields. In SHM applications, AE is typically used to
detect, locate and characterise] damage.
Contents
- 1Phenomena
- 2Uses
- 3See
also
Phenomena
Acoustic emission is the transient elastic waves within a
material, caused by the rapid release of localized stress energy. An event source
is the phenomenon which releases elastic energy into the material,
which then propagates as an elastic wave. Acoustic emissions can be detected in
frequency ranges under 1 kHz, and have been reported at frequencies up to
100 MHz, but most of the released energy is within the 1 kHz to
1 MHz range. Rapid stress-releasing events generate a spectrum of stress
waves starting at 0 Hz, and typically falling off at several MHz.
The three major applications of AE techniques are: 1) source
location – determine the locations where an event source
occurred; 2) material mechanical performance – evaluate and characterize
materials/structures; and 3) health monitoring – monitor the safe operation of
a structure, for example, bridges, pressure containers, and pipe lines, etc.
More recent research has focused on using AE to not only
locate but also to characterise the source mechanisms such as crack
growth, friction, delamination, matrix cracking, etc. This would give AE the
ability to tell the end user what source mechanism is present and allow them to
determine whether structural repairs are necessary.
AE can be related to an irreversible release of energy. It
can also be generated from sources not involving material failure,
including friction, cavitation and impact.
Uses
Main article: Non-destructive testing
The application of acoustic emission to non-destructive
testing of materials typically takes place between 100 kHz and 1
MHz. Unlike conventional ultrasonic testing, AE tools are designed for
monitoring acoustic emissions produced by the material during failure or
stress, and not on the material's effect on externally generated waves. Part
failure can be documented during unattended monitoring. The monitoring of the
level of AE activity during multiple load cycles forms the basis for many AE
safety inspection methods, that allow the parts undergoing inspection to remain
in service.
The technique is used, for example, to study the formation
of cracks during the welding process, as opposed to locating them after the
weld has been formed with the more familiar ultrasonic testing technique. In a
material under active stress, such as some components of an airplane during
flight, transducers mounted in an area can detect the formation of a crack at
the moment it begins propagating. A group of transducers can be used to record
signals, then locate the precise area of their origin by measuring the time for
the sound to reach different transducers. The technique is also valuable for
detecting cracks forming in pressure vessels and pipelines transporting
liquids under high pressures. Also, this technique is used for estimation of
corrosion in reinforced concrete structures.
In addition to non-destructive testing, acoustic emission
monitoring has applications in process monitoring. Applications where acoustic
emission monitoring has successfully been used include detecting anomalies in
fluidized beds, and end points in batch granulation.
Standards for the use of acoustic emission for
non-destructive testing of pressure vessels have been developed by the ASME, ISO and
the European Community.
See also
- Ultrasonic
homogenizer
- Ultrasonic
testing
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