TWI Knowledge Summary

Acoustic Emission

How does it work?

When a load is applied to a solid structure (e.g. by internal pressure or by external mechanical means), it begins to deform elastically. Associated with this elastic deformation are changes in the structure's stress distribution and a storage of elastic strain energy. As the load increases further, some permanent microscopic deformation may occur, which is accompanied by a release of stored energy, partly in the form of propagating elastic waves termed 'Acoustic Emission' (AE). If these emissions 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.

A typical AE system consists of signal detection, amplification, data acquisition, processing and analysis. Various parameters are used in AE to identify the nature of the source, including: count, duration, amplitude, rise-time, energy, frequency and RMS (Root Mean Square).

An important aspect of AE testing is signal processing. There is a need to separate genuine stress-wave emissions, originating from within the material, from external signals, such as environmental noise (rain, wind with sand particles), mechanical noise (movement of the component during testing), electric noise, etc. Much of this is achieved by careful electronic filtering of the received AE data but best practice is still to identify and remove as many sources of extraneous noise as possible prior to testing.

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

Some European standards and codes of practice exist for AE testing: Acoustic Emission Terminology (EN1330-9); General Principles (EN 13544); Equipment Description (EN 13477-1); Equipment Characterisation (EN 13477-2); and Examination of Metallic Pressure Equipment during Proof Testing (prEN14584).

What will it find?

Sources of acoustic emission are:

Plastic deformation, dislocation motion, rupture of the inclusion, phase transformation, twin/slip deformation;
Different stages of crack propagation (static, fatigue, stress corrosion). AE is sensitive enough to detect newly formed crack surface down to a few hundred square micrometers and less.
The weld defects: lack of penetration and fusion, cracks, inclusion and porosity;
Corrosion: localised corrosion / pitting corrosion. Detecting and monitoring of active corrosion, hydrogen embrittlement, corrosion fatigue, and intergranular stress corrosion cracking. Hydrogen embrittlement; dissolution of metal; hydrogen gas evolution; the breakdown of thick surface-oxide films;
Friction;
Mechanical impact;
Leaks (liquid or gas);
External noise (mechanical, electrical, and environmental).

What kind of Materials can be monitored by AE?

Acoustic emission can be used in non-destructive monitoring of different kinds of materials such as:

Metals: steels, stainless steel, carbon steel, alloy, ferritic steel, aluminium, aluminium alloys, magnesium alloys, and others (e.g., copper and its alloys, uranium alloys, titanium, and zirconium alloys);
Composite materials and polymer: sandwich composite, glass-reinforced plastic (GRP) and carbon fibre;
Concrete, reinforced concrete;
Rocks;
Woods.

Where is it used?

Pressure equipment: Fundamental research and development efforts in the control of the damage in materials by acoustic emission have grown in the last twenty years. This technique has become a reliable and standard method of non-destructive testing for pressure vessels. AE is used to monitor flaws, corrosion, and leakage in pressure vessels, LPG, tanks, piping systems, steam generators;
Aircraft and aerospace: aerospace structures, wings, bulkhead, fuel tanks, Rocket engine, real time monitoring;
Petrochemical and chemical: storage tanks, reactor vessels, offshore and onshore platforms, drill pipe, pipeline;
Marine: corrosion, composite shell, engine and power plant;
Civil engineering: bridges, dams, suspension cable bridges, concrete structure reinforced by composite;
Research and development: acoustic emission is a good technique to monitor and study the damage in materials and their mechanical properties (new materials, smart materials, Shape memory alloys (SMA)).

What are its advantages?

Acoustic emission testing offers a distinct advantage over more conventional non-destructive techniques:

Real time monitoring in service structures;
Cost reduction;
Time reduction;
High sensitivity;
Defect localisation;
Global structures monitoring;
Control of non accessible zones;
No intentional injection of an acoustic signal into the component under test are needed;
Can be used with other destructive and non destructive techniques.

Further information

TWI offers training courses on acoustic emission.
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