Acoustic Emission Process


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 the detection and conversion of these high frequency elastic waves to electrical signals. This is accomplished by directly coupling piezoelectric transducers on the surface of the structure under test and loading the structure. Sensors are coupled to the structure by means of a fluid couplant and are secured with tape, adhesive bonds or magnetic hold downs. The output of each piezoelectric sensor (during structure loading) is amplified through a low-noise preamplifier, filtered to remove any extraneous noise and furthered 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.



  • Laboratory & R&D studies

  • 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

  • Transformers testing (Partial Discharge)

  • 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.

Acoustic Emission for Laboratory Testing

Acoustic Emission inspection is a powerful aid to materials testing and the study of 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 is used also for monitoring chemical reactions including corrosion process, liquid solid transformations, phase transformations.


Acoustic Emission IN FIELD TESTING

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


Global monitoring- 100% Inspection of the structure

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


On-line testing

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


Rapid inspection

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


Cost Reduction

The use of Acoustic Emission results in 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 permanent recording of the test to be used at any time for re-evaluation and post processing analysis.


Defect Location

When more that one sensors are used, 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 3-dimensional location is used for power transformers and concrete structures.



Compared to conventional inspection methods the advantages 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.

  • Application of Artificial Intelligence (AI) and Technological Packages: Expert systems for evaluating the condition of metallic pressure systems and tank bottoms based on the acquired experience of a huge number of tests are world wide used.