Acoustic Emission listening while it is happening
Acoustic Emission A.E. is a non destructive technique to detect phenomena occurring inside any solid phase such as metals, concrete, wood or plastic.
Practical applications are generally focused on defect analysis (cracks in metals, disbanding or cracks in composites) or on quality assessment (mechanical, characterisation of concrete and plastic materials).
An A.E. pulse or burst is emitted whenever mechanical energy is released in a discontinuous pattern: when a crack increase its size, when a single component fails, when an internal surface is created by detachment of the solid matrix from an enclosed particle.
The A.E. pulse is detected by a piezoelectric transducer, converted into an oscillating voltage and analysis by an A.E. instrumentation.
Crack detection with A.E.
The progress of a crack is accompanied by release of mechanical energy, part
of which will appear as A.E. pulses. An A.E. analysis of a solid component can
evidence presence or absence of active cracks, can eventually locate defects
and can monitor their growth. A.E. can be applied either during hydrotesting
or in-service.
In the first case all those defects which can be set in motion by the applied
stress will be detect and can be localized; in the second case, cracks which
propagate under the prevailing environment conditions will be evidenced.
Among these latter, stress corrosion, fatigue, corrosion-fatigue and hydrogen
embrittlment cracks should be included.
Environmental
cracks
-
Stress corrosion
- Hydorgen embrittlment
- Corrosion fatigue
Environmental cracks propagate only as a consequence of an interaction between
a metal and a specific environment, an applied mechanical stress (as in the
case of hydrotesting or overpressurizing) does not in general activate such a
type of cracks.
This fact explains the need to detect A.E. during actual service conditions.
In-service monitoring is undoubtedly easier as it only require to “listen”
for a given length of time in different location and does not need
multichannel instrumentations and computers. It requires however a good signal/noise
ratio or an analysis of data able to distinguish a crack signal amid A.E.
signals generated by other phenomena.
The main advantage of the instrumentation developed by AETECH is precisely the
computation of a parameters, the mechanical energy associated to each A E.
pulse, which allows an easy distinction between fracture signals and other
signal or noise.
The direct relationship between Emitted Energy and increase in surface area of
cracks gives a semiquantitative estimation of the severity of cracking.