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Acoustic Emission Technology
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ACOUSTIC EMISSION TECHNOLOGY

Did you know that materials "talk" when they are in trouble?

With Acoustic Emission, you can "listen" to the sounds of cracks growing, fibers breaking and many other modes of active damage in stressed materials. Read below to learn all about Acoustic Emission Technology.
Acoustic Emission Technology Overview

How Acoustic Emission works!

Acoustic Emission (AE) testing is a powerful method for examining behavior of materials deforming under stress.

The Acoustic Emission NDT technique is based on the detection and conversion of 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 and 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.
Acoustic Emission Defined

Definition: AE is the short rapid, release of energy in the form of a transient elastic wave.

Source: The Acoustic Emission source is created by a defect (crack) or discontinuity (e.g. high tension wire break) that becomes active due to changes in operating loads, overloading conditions or degradation of the material in (e.g. stress corrosion cracking, corrosion fatigue & hydrogen embrittlement).

Detection: The Acoustic Emission waveform has several parameters that can be used for characterization of the source defect or discontinuity.

Amplitude: How loud or strong the signal is, measured in volts or decibels.

Energy: Equal to the energy under the curve, one of the best ways to measure AE activity.

Duration: How long the source event lasts

Counts: The number of threshold crossings

Additional parameters can be used for (AE) Acoustic Emission source classification.
Acoustic Emission Detection
Acoustic Emission CapabilitiesBridge Inspector using AE Instrument.

Acoustic Emission Technology also offers many unique capabilities. At the frequencies we use, the distance between the AE source and the sensor could be as little as a few inches or as much as tens or even hundreds of feet. Sudden sub-critical local failures in materials under stress are the classic sources of Acoustic Emission.

Small-scale damage is detectable long before failure, so AE can be used as a non-destructive technique to find defects during structural proof tests and plant operation. AE also offers unique capabilities for materials research and development in the laboratory. Finally, AE equipment is adaptable to many forms of production testing, including weld monitoring and leak detection.

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 16 meter diameter sphere approx. 30 +/- 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 sensor mounting, resulting in increased 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 than one sensor is used, the AE 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.
The Acoustic Emission Advantage!
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.
  • Acoustic Emission Applications

    Acoustic Emission Applications

    Originally conceived as an NDT tool for pressure vessels, Acoustic Emission testing (AE) has become much wider in scope. We now apply it to all types of process monitoring as well as for its original purposes of flaw detection and structural integrity inspection.

    The technology is used to safeguard against catastrophic failures, to assess structural integrity and to enhance safety in a wide range of structures from fiberglass tanks to bucket trucks, from bridges and aircraft to high-pressure gas cylinders. On the process monitoring side, AE is used for a wide range of applications including leak detection, particle impacts, electrical discharges and a variety of friction-type processes, just to name the most common. Other areas of interest include higher-frequency machinery health monitoring and predictive diagnosis.

    Some typical applications of the Acoustic Emission principle in testing materials are as follows:

    Behavior of Materials (metals, ceramics, composites, rocks, concrete):
  • Crack propagation
  • Yielding
  • Fatigue
  • Corrosion, Stress corrosion
  • Creep
  • Fiber fracture, delamination
  • Nondestructive Testing During Manufacturing Processes:
  • Material processing
  • Phase transformation in metals and alloys (martensitic transformation)
  • Detection of defects such as pores, quenching cracks, inclusions, etc.
  • Fabrication
  • Deforming processes; rolling, forging, extruding
  • Welding and brazing; detects detection (inclusions, cracks, lack of penetration)
  • TIG, MIG, spot, electron beam, etc.
  • Weld monitoring for process control
  • Monitoring Structures:
  • Continuous monitoring (metallic structures, mines, etc.)
  • Periodic testing (pressure vessels, pipelines, bridges, cables)
  • Loose Part Detection
  • Leak Detection
  • Applications
  • 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
  • Other Special Applications:
  • (Petro) Chemical: storage tanks, reactor vessels, offshore, drill pipe, pipelines, valves, hydro-treaters.
  • Power: nuclear reactors, piping, steam generators, ceramic insulators, transformers, aerial devices.
  • Aircraft and aerospace: fatigue cracks, corrosion, composite structures, etc.
  • Electronics: loose particles in electronic components, bonding, substrate cracking.
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