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Physical principle of the Acoustic Emission Testing :

The Acoustic Emission Non Destructive Testing (AE) consists in detecting ultrasonic waves emitted by the noise generated by the release of energy in a structure under mechanical loading.

This technique needs to create a mechanical constraint on the test piece, a physical failure emitting an acoustic wave then the reception of acoustic signals by a network of ultrasonic transducers in contact with the component. Once recorded and processed, the signals allows to locate a flaw by using triangulation algorithm.

Several phenomena can be sources of acoustic emission :

• The propagation of a crack
• A delamination or a fiber breaking in composite structures
• A plastic deformation
• Some corrosion development
• The release of constraints in welds
• Some fluid or gas leakage

Application cases in different industry:

Acoustic Emission is particularly used for the control of pressure equipments as well as storage tanks or some large aeronautic structures.
Therefore, this technique is in use in various industrial businesses:

Transportation:

• Aeronautic structures in aluminium or composite: Wing surfaces, drifts, high pressure storage tanks in composite
• Boilers of nuclear submarines (pressure pipes)
• Ships: Control of composite embedded devices
• Offshore structures: Detection of corrosion in gas or oil carriers

Oil & Gas industry:

• Tanks: Large petrochemicals tanks, LPG tanks for private individuals, buried tanks, etc.
• Leakage detection in oil& gas pipelines
• Corrosion detection in storage vats

Power industry including nuclear generation: Waterproof check of the primary circuit in nuclear power plants (vessels, condensers, heat exchangers, etc.)

Civil Engineering:

• Concrete failure control
• Continuous monitoring of large construction works
• Detection of wire ruptures on cables (bridges stays)

Mechanical industry: For the monitoring of various manufacturing processes (pressing, machining, welding,etc.)

Advantages and current limits of the AE technique :

Advantages

Limits

- High sensitivity to events during cracking, thus very sensible to crack growth

- The test piece has to be put under load and has to be already damaged

- Global method, covers a full structure in one examination

- Difficult interpretation of signals: Difficult characterisation of flaws (position, type, dimension)

- Quite simple testing device, few access constraints

- Large uncertainties, which prevents this method to be really quantitative today

- Adapted to in service continuous monitoring (testing equipment can easily stay on the test piece)

- The disposal of sensors is done quite empirically today during the calibration

- Can replace Hydraulic Testing at lower cost, when this one can be harmful

- Difficult calibration (mainly to measure propagation velocities)

Current trends around acoustic emission:

Acoustic Emission Testing has expanded noticeably from the 90’s.
Several reasons can account for this evolution:

• A growing demand from manufacturers and end-users of pressure devices : The change in the rules that appeared from 1999 imposes that pressure devices must be controlled once before their commisionning, and a second time in service. This control generally leads to a hydraulic test that can be quite costly and sometimes harmful for the device (corrosion,etc.). Acoustic Emission can complete or replace hydraulic testing in a cost-effective way.
• The development (above all, in aerospace sector) of SHM technologies SHM (Structural Health Monitoring) proposing a continuous follow-up in service of the integrity of a structure. The equipment of an Acoustic Emission test as well as the very good sensitivity of this technique to an evolution of a defect makes it really adapted to this approach of a continuous control.
• A growing demand from users of composite materials, where this technique has an interesting sensitivity compared to other methods.
• The need for a control during the manufacturing allowing to guarantee the quality of a product and to protect the manufacturing devices.