Acoustic emission

Acoustic emission (AE) is the release of energy from a localized source within a material, creating transient elastic waves. It is a powerful non-destructive evaluation technique for monitoring damage during mechanical testing. Stress on a material can cause plastic deformation, flaws, or fractures, generating small stresses or ultrasonic waves. AE signals, typically in the 100 kHz to 1 MHz frequency range, are usually too faint or high-frequency to be heard and require piezoelectric sensors, often made from lead zirconate titanate (PZT), to detect and convert them into electrical signals.

A typical acoustic emission system consists of:

• Sensors used to detect AE events.
• Preamplifiers that amplify initial signal. Typical amplification gains are 40 or 60 dB.
• Cables that transfer signals on distances up to 300m to AE devices. Cables are typically of coaxial type.
• Data acquisition device that performs analog-to-digital conversion of signals, filtration, hits (useful signals) detection and it parameters evaluation, data analysis and charting.

Acoustic emission is a unique non-destructive test method that allows:

• Overall examination of large structures during operation, detection of flaws at their early stages, flaw typification and assessment.
• Study of dynamic material behavior, developing fracture and material properties.
• Control over manufacturing processes and production, machinery health monitoring.

Applications of Acoustic Emissions:

?? Structural Health Monitoring: Bridges, buildings, dams.

?? Aerospace Industry: Aircraft components, spacecraft.

?? Power Generation: Pressure vessels, boilers, turbines.

?? Petrochemical and chemical: Storage tanks, reactor vessels, offshore platforms, drill pipe, pipelines, valves, hydro-treater

?? Manufacturing: Material testing, weld inspection.

?? Mining Industry: Rock mechanics, ore processing.

?? Civil Engineering: Dam monitoring, retaining walls.

?? Railway Industry: Track monitoring, rolling stock.

?? Real-time leakage test and location within various components: small valves, steam lines, tank bottoms

Advantages of AE:

?? High sensitivity.

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

?? Real time monitoring

?? Cost Reduction

?? Minimization of plant downtime for inspection, no need for scanning the whole structural surface.

?? Other advantages of AET include fast and complete volumetric inspection using multiple sensors, permanent sensor mounting for process control, and no need to disassemble and clean a specimen.

Limitations of AE:

?? Limited to assessing structural integrity or machine health by locating issues, further inspection is usually required to fully diagnose issues.

?? Qualitative results only: AET can determine that flaws exist but cannot provide quantitative characteristics.

?? Limited to active flaws: It cannot detect stagnant flaws.

?? Can be slower than other non-destructive testing techniques.

?? Limited to materials that conduct sound waves well: Works best on metals with smooth surfaces.

?? Influence of ambient noise and attenuation of signals may result in poor output.