Acoustic emission (AE) testing is a non-destructive testing (NDT) technique that detects and monitors the release of ultrasonic stress waves from localised sources when a material deforms under stress.
Acoustic emission testing works by mounting small sensors onto a component under test. The sensors convert the stress waves into electrical signals, which are relayed to an acquisition PC for processing. The waves are captured when the component is submitted to an external stimulus, such as high pressures, loads or temperatures. As the damage grows in the component, there is a greater release of energy. The rates in which the acoustic emission is detected, the activity, and the intensity of the acoustic emission, the loudness, are monitored and used for assessing structural integrity and for health monitoring of components.
Acoustic emission can be thought of as tiny earthquakes that occur in the material. The technique globally monitors a component for defects, allowing large structures and machines to be monitored while in operation with minimal disruption, unlike destructive testing. By using multiple sensors, acoustic emission sources (and hence the damage) can be located. Through signal analysis, the presence of different source mechanisms can also be determined.
There are two AE testing methods: transient and continuous. The transient method captures AE bursts that exceed a threshold (loudness level) and extract features such as peak amplitude, signal energy and duration of the burst. These features are then used to assess the condition of the component under test. This method is well suited for testing structures for defects such as cracks.
The continuous method captures all AE within a set time period, for example 1/10th of a second. Then, features such as average signal level and root-mean squared (RMS) values are then extracted. This method is well suited to applications where there is a lot of background AE or AE amplitude is low, for example when testing gearboxes or detecting leaks.
Acoustic emission testing can be conducted in a laboratory, as well as in-field conditions, over both relatively short durations, such as a few hours, and longer durations, such as a few months. Wireless data relay methods make it possible to analyse the data remotely.
Acoustic emission has many advantages over other methods. These include:
- Ability to detect a range of damage mechanisms including, but not limited to, fibre breakages, friction, impacts, cracking, delamination and corrosion in their early stages, before they become significant issues
- Can be conducted during operation, during qualification (proof) testing or development testing
- Can locate damage sources and can be differentiate these based on acoustic signatures
- Global monitoring of a structure
- Assesses the structure or machine under real operational conditions
- A non-invasive method
- Operational in hazardous environments, including high temperatures, high pressures and corrosive and nuclear environments
- Can be conducted remotely
- Can detect damages in defects that are difficult to access with conventional non-destructive testing techniques
However, the method does also have some limitations:
- Limited to assessing structural integrity or machine health by locating issues, further inspection is usually required to fully diagnose issues
- Cannot detect defects that may be present, but that do not move or grow
- Can be slower than other non-destructive testing techniques
Acoustic emission can be applied to a range of applications and materials. These include:
- Concrete structures such as bridges and buildings
- Metallic structures such as pressure vessels, pipelines, storage tanks, aircraft structures and steel cables
- Composite structures such as aircraft structures, motorsport structures and composite beams
- Rotating machinery such as detecting early wear in bearings and gearboxes
- Electrical machinery such as detecting partial discharge in transformers and bushings
- Additive manufacturing for assessing build quality during build
- Leak detection in pipelines and pressure systems
- Particle impacts
- Frictional processes