Acoustic Emission Testing. A technique that makes use of transient elastic waves produced by the sudden redistribution of stress in a material. The material being tested receives an external stimulus (for example, a change in pressure, load, or temperature), and localised sources trigger the release of energy (in the form of stress waves) which can be detected at the surface and recorded by sensors. AET differs from other non-destructive testing (NDT) techniques in two ways: instead of supplying energy to the area being tested, it ‘listens’ for energy released by it; and it detects changes in materials, not static defects. The technique’s advantage is that it can provide an immediate indication of the risk of failure.