Sensing Acoustic Emissions (AE)

Acoustic Emission, according to ASTM, refers to the generation of transient elastic waves during the rapid release of energy from localized sources within a material. The source of these emissions in metals is closely associated with the dislocation movement accompanying plastic deformation and the initiation and extension of cracks in a structure under stress. Other sources of Acoustic Emission are melting, phase transformation, thermal stresses, cool-down cracking, and stress build-up.

 The Acoustic Emission NDT technique is based on detecting and converting these high-frequency elastic waves to electrical signals. This is accomplished by directly coupling piezoelectric transducers on the structure's surface under test and loading the structure. Sensors are coupled to the structure using a fluid couplant and are secured with tape, adhesive bonds, or magnetic hold-downs. The output of each piezoelectric transducer (during structure loading) is amplified through a low-noise preamplifier, filtered to remove any extraneous noise, and further processed by suitable electronic equipment.

The instrumentation of Acoustic Emission must provide some measure of the total quantity of detected emission for correlation with time and/or load.

 Applications:

·       Transformers testing (Partial Discharge)

·       In-field inspection

·       Structural integrity evaluation

·       Vessels testing [ambient, hot or cryogenic, metallic and FRP, spheres]

·       Tank bottom testing

·       Nuclear components inspection (valves, lift beams, steam lines)

·       Corrosion detection

·       Pipeline testing

·       Railroad tank car testing

·       Tube trailers & high-pressure gas cylinders

·       Reactor & high energy piping testing

·       Aging aircraft evaluation

·       Advanced materials testing (composites, ceramics)

·       Production quality control

·       Rocket motor testing

·       Laboratory & R&D studies

Acoustic Emission for laboratory testing

Acoustic emission inspection is a powerful aid for testing materials and studying deformation, fracture, and corrosion. It gives an immediate indication of the response and behavior of a material under stress, intimately connected with strength, damage, and failure. Acoustic Emission also monitors chemical reactions, including the corrosion process, liquid-solid transformations, and phase transformations.

Acoustic Emission in Field Testing

Many codes and standards exist for the acoustic emission testing of vessels, from transportation gas cylinders and railroad tanks to storage tanks with thousands of tons. Because only active defects and deterioration produce Acoustic Emission, no time is wasted on latent defects that are not threatening structural integrity.

Global monitoring- 100% Inspection of the structure

A significant advantage of Acoustic Emission inspection is that it does not require access to the whole examination area. E.g. for covering a total area of a 16m-diameter sphere, 30-40 sensors are needed. Thus, the cost of the test is significantly less than that of inspection with conventional NDT methods (for 100% inspection and scanning of the whole area). Identified problem areas can be inspected using conventional NDT methods.

Testing with insulation / high-temperature processes

In insulation cases, only small holes are required for sensor mounting, resulting in more cost savings. In high-temperature processes, waveguides guide the Acoustic Emission waves from the hot surface to the edge where the sensor is mounted. Finally, permanent sensors are mounted under insulation in large cryogenic vessels for periodic inspection control.

On-line testing

As the method records defects in real-time, it offers the possibility of online inspection, e.g., during hydrostatic testing. Other online stress applications include introducing gas into the upper vapor space, temperature control, etc.

Rapid inspection

The Acoustic Emission test takes hours and, in some cases, even less. There is no comparable technique that can provide 100% volumetric inspection.

Cost Reduction

The use of Acoustic Emission results in a considerable reduction in plant maintenance costs while increasing the available information about plant integrity. Plant downtime for inspection is also minimized.

Permanent recording of test

Acoustic Emission data are digitized and stored on a PC, providing a permanent record of the test to be used at any time for re-evaluation and post-processing analysis.

Defect Location

When more than one sensor is used, an Acoustic Emission source can be located and, thus, the defective area. Location is based on the wave propagation principles within the materials and is effectuated by measuring the signal's arrival time to each sensor. By comparing the signal's arrival time at different sensors, the flaw’s location can be defined through triangulation.

Linear location is used on long gas cylinders, planar (2-dimensional) location for thick-walled and gas-filled vessels, while the 3-dimensional location is used for power transformers and concrete structures.

 Advantages compared to conventional inspection methods The benefits of the Acoustic Emission technique are:

·       High sensitivity.

·       Early and rapid detection of defects, flaws, cracks, etc.

·       Real-time monitoring

·       Cost Reduction

·       Defective area location: only critical defects provide sustainable Acoustic Emission sources.

·       Minimization of plant downtime for inspection; no need for scanning the whole structural surface.

·       Minor disturbance of insulation.