METHODS

1. Principle

Acoustic emission is a phenomenon of releasing energy as transient elastic waves resulting from local micro-movements inside a material (cracks, inclusions, corrosion, delamination, etc.). This phenomenon appears in many materials when they are subjected to mechanical, thermal or chemical loading.

The application of a load and/or the presence of an aggressive environment produces internal modifications such as changes in a crack, local plastic distortions, corrosion and, in some cases, phase transformations that generate the acoustic emission. This therefore gives certain information on the internal behaviour of the materials in question. The waves are propagated through the structure then detected by specific probes that convert the surface movements of the material into electrical signals. These signals are processed by suitable instrumentation to indicate and locate the sources of acoustic emission.

2. Test method

The measurement of the acoustic emission generated by a structure subjected to loading can be described in two phases:

- Detection of the acoustic wave:

Piezo-electric sensors normally detect the acoustic wave which is converted into an electric signal. The acoustic emission signal is then conditioned by a pre-amplifier that performs the  impedance matching, the amplification and the filtering. The acoustic emission signal is then sent to the measuring system which digitises, records waveforms in real time and/or extracts a certain number of signal-related settings. The acoustic emission can be continuous or discrete. The discrete acoustic emission signal called burst has the general appearance of a damped sinusoid.

- Analysis of acoustic emission data in real time and off-line:

Once the acoustic emission data have been stored, appropriate processing is applied to assess the measurement results. This processing relates to the burst settings (historical and statistical analyses) but are also used to group the bursts detected by the various measurement channels to localise the acoustic emission sources.

As for the other test methods, the personnel required for the services, conducting of tests and interpretation of results must have the relevant training and experience in acoustic emission testing. The qualification and certification conditions required are specified in standard NF EN ISO 9712.

3. Scope

There are many areas where acoustic emission testing can be applied:

- Testing the integrity of pressure equipment. Acoustic emission testing is an alternative to hydraulic testing to requalify pressure equipment;

- testing industrial structures such as nuclear reactors;

- in-line testing of material manufacturing processes;

- monitoring of installations in operation;

- leak detection;

- corrosion detection;

- research and development;

- applicable to numerous materials (steels, composites, ceramics, etc.).

4. Advantages of the method

There are many advantages to acoustic emission testing, including:

- Global testing of the structure (volume);

- detection and localisation of advancing defects;

- dynamic testing in real time in service or loading conditions;

- diagnosis on the severity of the damage;

- monitoring over time;

- prevention of industrial risks;

- testing structures in service;

- minimising intervention times and immobilisation of installations.

5. Related standards

Standards currently in force

NF EN 13301-9 Non-destructive testing - Terminology - Part 9: Terms used in acoustic emission testing

NF EN 13477-1 Non-destructive testing - Acoustic emission testing - Equipment characterisation - Part 1: Equipment description

NF EN 13477-2 Non-destructive testing - Acoustic emission testing - Equipment characterisation - Part 2: Verification of operating characteristics

NF EN 13554 Non-destructive testing - Acoustic emission testing - General principles

NF EN 14584 Non-destructive testing - Acoustic emission testing - Examination of metallic pressure equipment during proof testing - Planar location of AE sources

NF EN 15495 Non-destructive testing - Acoustic emission testing - Verification of metallic pressure equipment during proof testing - Zone location of AE sources

Guide to good practices to acoustic emission testing of pressure equipment, 2nd edition 2009 and erratum no. 1 of 14 June 2011, AFIAP, GEA

Text prepared by COFREND in conjunction with Hasnae Zejli and Catherine Hervé (CETIM).

Photo credit: CETIM