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.[1] 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 [2] generated when the accumulated elastic energy in a material or on its surface is released rapidly.[3] The waves generated by sources of AE are of practical interest in the field of structural health monitoring (SHM), quality control, system feedback, process monitoring and others. In SHM applications, AE is typically used to detect, locate[4] and characterise[5] damage.

Acoustic emission phenomena

AE is commonly defined as transient elastic waves within a material, caused by the rapid release of localized stress energy. Hence, 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 safety operation of a structure, i.e. 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[5] i.e. crack growth, friction, delamination, matrix cracking, etc. This would give AE the ability to tell the end user what source mechanism is present and hence allow them to determine whether or not 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

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 within the material during failure or stress, rather than actively transmitting waves, then collecting them after they have traveled through the material. 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.[6]

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 [7][8] and pipelines transporting liquids under high pressures. Also, this technique is used for estimation of corrosion in reinforced concrete structures.[6][9]

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

References

  1. ↑ Miinshiou Huang, Liang Jiang, Peter K. Liaw, Charlie R. Brooks, Rodger Seeley, and Dwaine L. Klarstrom. tms.org website November 1998 (vol. 50, no. 11) JOM. Retrieved 2011-12-05.
  2. ↑ pacuk.co.uk website Archived December 27, 2011, at the Wayback Machine.. Retrieved 2011-12-05.
  3. ↑ Sotirios J. Vahaviolos (1999). Acoustic Emission: Standards and Technology Update. STP-1353. Philadelphia, PA: ASTM International (publishing). p. 81. ISBN 0-8031-2498-8.
  4. ↑ "Acoustic emission source location in composite materials using Delta T Mapping". Composites Part A: Applied Science and Manufacturing. 43: 856–863. doi:10.1016/j.compositesa.2012.01.023.
  5. 1 2 "Damage classification in carbon fibre composites using acoustic emission: A comparison of three techniques". Composites Part B: Engineering. 68: 424–430. doi:10.1016/j.compositesb.2014.08.046.
  6. 1 2 Blitz, Jack; G. Simpson (1991). Ultrasonic Methods of Non-Destructive Testing. Springer-Verlag New York, LLC. ISBN 978-0-412-60470-6.
  7. ↑ Stuart Hewerdine, ed. (1993). Plant Integrity Assessment by Acoustic Emission Testing (2 ed.). Rugby, UK: Institution of Chemical Engineers. ISBN 0-85295-316-X.
  8. ↑ A. A. Anastasopoulos; D. A. Kourousis; P.T. Cole (October 2008). Acoustic Emission Inspection of Spherical Metallic Pressure Vessels. The 2nd International Conference on Technical Inspection and NDT (TINDT2008). Tehran, Iran.
  9. ↑ Estimation of corrosion in reinforced concrete by electrochemical techniques and acoustic emission, journal of advanced concrete technology, vol. 3, No 1, 137-144, February 2005

External links and further reading

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