Acoustic Emission

 

What is it?

Acoustic Emission (AE) is a technique of non-destructive testing (NDT) for determining the integrity of a material, component or structure, and to qualitatively measure some of its characteristic properties.

 

Physical principle

The term AE refers to the generation of transient elastic waves produced by a rapid release of energy caused by the redistribution of stresses within a material.  When a part or structure is subjected to an external stimulus (change of pressure, load, temperature, etc.), internal changes can be produced such as: the growth of cracks, local plastic deformation, corrosion or phase changes, which usually start a release of energy in the form of elastic waves that propagate up to the surface.  Therefore, these waves contain information about the source that generated them, and the AE technique is based on the detection and analysis of these [1].

 

For example, if you take a wood pencil and gently bend it until it breaks, you can easily hear the fracture of wood fibers. The noise you hear is due to stress waves that propagate through the wood, in this case, the waves are generated at frequencies within the audible range. There are many examples of audible acoustic emission, though they are the least frequent, such as the splitting of wood, ice, paper, etc. Typically, AE signals are not audible because the waves are generated within the ultrasonic range.

 

AE signals are obtained by measuring the small displacements of the surface (movements of the order of 10-12 pm can be detected) with appropriate transducers, which convert these into electrical signals. These signals are processed to determine, characterize and locate sources of AE.

 

 

 

Signal detected by AE equipment during the contact fatigue testing of a steel sample.

 

Other NDT techniques (X-ray, ultrasound, induced currents, etc.) detect geometric discontinuities by injecting some form of energy in the element being studied.  With AE, however, the defect itself generates its own signal and this can be detected at some distance using the appropriate sensors. The source of the waves is one of its main advantages since it allows testing with minimal impact on the operation of the element under study.

 

There are multiple sources of AE, from natural events such as earthquakes, to defects in materials like metal, plastic or composite materials.

 

Applications

Industrial applications of AE are still limited, although in very specific applications it is used in a generalized way (for example, inspection of pressure tanks). Scientifically, many sources of AE are studied. In the following list is a compilation of some of these [2]:

 

  • Degradation of materials growing defects, propagation of cracks, plastic deformation, inclusions or precipitates rupture, surface degradation (eg: corrosion) [3].
  • Reversible processes: crystallographic phase transformations, solidifications, thermoelastic effects, surface friction, etc.
  • Manufacturing process: defects in welds, forging processes, rolling, turning, etc.
  • In fluids: detection of particles in suspension, leaks in pipes and/or valves, gas evolution, boiling point.

 

Projects completed

The LEAM works with the AE technique for the characterization of metallic and biologic material and compounds.

 

One application of AE currently being developed, together with the Technological Centre of Manresa, is the characterization of failure mechanisms in tool steel, with two purposes: in-depth study and knowledge of these, and designing a system of predictive maintenance for forming tools.  Application of AE in monotonic bending tests, bending fatigue, contact fatigue and nanoindentation [4].

 

Other projects being initiated, include the characterization of the mechanical properties of biological materials in both hard tissue (bone) and soft tissue (esophagus), in collaboration with the Biomechanics of Impact research group (UPC).

 

Monotonic bending test in specimens of tool steel.

 

[1] Vallen, H.;  Acoustic emission testing: fundamentals, equipment, applications. Informative booklets for non destructive testing, NDT compact and understable. Wuppertal (Germany). Castell Publication Inc., 2006. ISBN: 3-934255-26-4.

[2] Ono, K., 2005, Current understanding of mechanisms of acoustic emission, Journal of  Strain Analysis, Special Issue Paper, 40 (1), 1-15.

[3] Fukaura, K.; Ono, K. (2001): “Acoustic emission analysis of carbide cracking in tool steels”,  Journal of acoustic emission, vol.19, pp.91-99.

[4] Martinez, E.; Picas, I.; Casellas, D.; Romeu, J. (2010): “Analysis of fracture resistance of tool steels by means acoustic emission”, Journal of acoustic emission, vol.28, pp.163-169.