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Acoustic Emission Testing (AET)

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Acoustic Emission Testing (AET) is a nondestructive testing (NDT) method that is based on the generation of waves produced by a sudden redistribution of stress in a material. When a piece of equipment is subjected to an external stimulus, such as a change in pressure, load, or temperature, this triggers the release of energy in the form of stress waves, which propagate to the surface and are recorded by sensors. Acoustic emissions can come from natural sources, such as earthquakes or rockbursts, or from the equipment itself such as melting, twinning, and phase transformations in metals. Detection and analysis of AE signals can provide information on the origin and importance of discontinuities in a material.

AET is different than other NDT techniques in two major ways:

  1. Instead of supplying energy to the object under examination, AET listens for the energy released by the object naturally. AE tests can be, and often are, performed on structures while they are in operation since this provides adequate loading for propagating defects and triggering acoustic emissions.
  2. AET deals with dynamic processes in a material. This is particularly useful because only active features are highlighted during the examination. Thus, it is possible to discern between developing and stagnant defects. However, one should be aware that it is possible for flaws to go undetected if the loading isn’t high enough to cause an acoustic event that can be detected by the system.

AET is most often used in a dynamic test environment, meaning that it is used to monitor for crack detection in pressure equipment when the equipment is experiencing an increase in stress. AET systems generally contain a sensor, preamplifier, filter, and amplifier, along with measurement, display, and storage equipment. Acoustic emission sensors respond to any dynamic motion caused by an AE event. This is achieved through transducers that convert mechanical movement into an electrical voltage signal. The majority of AE equipment responds to movement in a range of 30 kHz to 1 MHz. For materials with high attenuation, such as plastic composites, lower frequencies may be used to better distinguish AE signals. The inverse is true as well.

Because of its versatility, AET has many applications within the industry, such as assessing structural integrity, detecting flaws, testing for leaks, or monitoring weld quality. Because of the diverse number of situations it can be applied to, it sees extensive use in several areas including the detection of active corrosion in the bottom of aboveground storage tanks, detecting creep damage in high energy piping (HEP) systems, pressure vessel inspection, and leak detection.

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