Acoustic Emission (AE)
This technique is based on measuring acoustic sound waves
that are emitted during the growth of microscopic defects, such as stress
corrosion cracks. The sensors can thus essentially be viewed as microphones,
which are strategically positioned on structures. The sound waves are generated
from mechanical stresses generated during pressure or temperature changes.
Background noise effects have to be taken into consideration and can be
particularly troublesome in on-line measurements.
Acoustic Emission (AE) is a transient elastic wave
generated by the rapid release of energy accumulated in stressed materials. In
the early of 1960s it was recognized that growing cracks and discontinuities in
pressure vessels could be detected by their AE signals. A new nondestructive
testing technology was born as it was stated that small scale damage is
detectable long before failure. Formally, the AE may be defined as the class of
all physical phenomena where elastic waves are generated by localized sources.
Sources of AE include many different mechanisms of deformation and fracture of
materials, material corrosion, surface rubbing, micro earthquakes and rockbursts,
leaks from vessel, storage tanks, pipes, flanges, seals, etc. Sources in metals
have been identified like crack growth, moving dislocations, slip, twinning,
grain boundary sliding, fracture, decohesion of inclusions. In composites, the
well known sources are matrix cracking, breaking of fibers, delamination and
interface disbonding. These mechanisms typify the classical response of
materials to the applied load.
The AE method offers the following advantages over the
other NDT methods:
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AE is a dynamic inspection method in that it provides
a response to discontinuity growth under structural stress. |
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Static, not dangerous discontinuities will not
generate AE signals. |
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AE can detect and evaluate the significance of
discontinuities and of an entire structure during a single test. |
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Since only limited access is required, defects may be
detected that are inaccessible to the more traditional NDT methods. |
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Vessels and other pressure systems can often be
requalified during an in-service inspection that requires little or no
downtime. |
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The AE method may be used to prevent catastrophic
failure of system with unknown defects, and to limit the maximum pressure
during system operation. |
