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Listening to Distress Sounds

Acoustic-emission inspection detects and analyzes acoustic signals emitted in structures under applied loads.

These are due to the propagation of transient elastic stress waves generated when there is a rapid release of stored strain energy in a material.

Release occurs because of local dynamic movements that happen where microscopic deformations are formed, as in discontinuities.

Among the detectable phenomena are initiation and propagation of cracks, twinning and slip.

Also grain boundary reorientation, martensitic phase transformation, bubble formation during boiling and fluid leaks.

If these Acoustic-emission signals are above a certain threshold level they can be detected and converted to voltage signals by sensitive piezoelectric transducers mounted on the structure's surface.

The frequency of the stress waves emitted is normally in the range
30 kHz to 1 MHz. Triangulation from multiple transducer and other techniques can give positional information and localize the sources of the emissions.

Inspection is performed upon loading by suitable means an instrumented structure and then detecting and analyzing the acoustic signals generated in the structure itself and emitted by it.

At each load level a certain measure of signals can be emitted. Upon unloading, no further emission will be generated until the previous load level is exceeded.

Acoustic-emission inspection is similar to seismology, the science studying earthquakes and telluric plate movements.

The study is done by recording and analyzing the energy released, the location and the depth of the source of the elastic waves propagated by the event.

In the manufactured structures industry, Acoustic-emission is used for
Non Destructive Testing.

As applied to welding processes, the results obtained showed that cracks and porosity forming during or after welding could be detected in real time as the method is very sensible to local transient instabilities.

Therefore Acoustic-emission methods can be implemented to monitor the performance to make sure that objectionable defects are not produced during manufacturing.

Acoustic-emission Standards

ASTM E749/E749M-12
Standard Practice for Acoustic Emission Monitoring During Continuous Welding
ASTM International / 15-Jun-2012 / 5 pages

Acoustic-emission inspection is non directional in that discontinuities are point source emitters, radiating acoustic energy in spherical wave fronts.

It is extremely sensitive to an active flaw during a change in the surrounding stress field, but cannot detect a static crack, at least as long as the applied stress is not high enough to cause an acoustic event.

This is different from other methods of Non Destructive Inspection where external energetic beams are directed to probable location of discontinuities, after the completion of welding operations.

Initiation and propagation of cracks during fatigue tests can be monitored continuously.

Another useful application of Acoustic-emission concerns the assessment of the integrity of pressure vessels and of pipelines during hydrostatic acceptance tests and also for in service monitoring.

This technique has become a reliable and standard method of non-destructive testing, but requires special equipment and inspectors instructed and especially trained to conduct the test and interpret the results.

Here is a list of European Standards for Acoustic-emission testing:

BS EN 1330-9:2009
Non-destructive testing. Terminology. Terms used in acoustic emission testing
British-Adopted European Standard / 31-Dec-2009 / 30 pages

BS EN 13477-1:2001
Non-destructive testing. Acoustic emission. Equipment characterization. Equipment description
British-Adopted European Standard / 15-Mar-2001 / 12 pages

DIN EN 13477-2
Non-destructive testing - Acoustic emission - Equipment characterisation
- Part 2: Verification of operating characteristic; German version EN 13477-2:2010
DIN-adopted European Standard / 01-Dec-2010 / 32 pages

BS EN 14584:2005
Non-destructive testing. Acoustic emission.
Examination of metallic pressure equipment during proof testing.
Planar location of AE sources

British-Adopted European Standard / 04-Oct-2005 / 18 pages

For further reading see:
Using Acoustic Emission in Fatigue and Fracture Materials Research

An Article on Wailing Structures, explaining why one should be listening, was published (3) in Issue 98 of Practical Welding Letter for October 2011. Click on PWL#098 to see it.

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Acoustic Emission Testing

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