What it finds

  • Active damage: cracks growing, corrosion activity, fiber breakage or delamination in composites.
  • Leaks, in some setups, from the noise of escaping fluid.
  • The rough location of those events on a large structure, so that other methods know where to look.

How it works

Sensors are coupled to the surface of the structure, spread across it. The structure is then loaded, often by pressurizing a vessel or filling a tank, or by normal service loads. When something inside moves or breaks, stress waves travel through the material and arrive at the sensors at slightly different times. Comparing those times locates the source, much as seismologists locate an earthquake.

Because it listens to the whole structure at once, AE works well as a global screen or for continuous monitoring. It also tends to be quiet at loads the structure has already seen, so test planning has to consider the load history.

Where it is used

AE is used on pressure vessels, storage tanks and tank floors, piping, bridges, and fiber-reinforced plastic equipment. It is often chosen when taking equipment out of service for internal inspection would be costly. See our guides to refineries and petrochemical plants.

Limits

  • It needs load. With no stress on the structure, there is little to hear.
  • Only active sources show. A flaw that is not growing during the test may stay silent.
  • It does not size flaws. AE says where to look. UT or another method then has to say what the flaw is and how big it is.
  • Background noise. Pumps, wind, rain and rubbing supports can all interfere, and filtering them takes care.
  • Specialist work. Planning a test and interpreting the data take experience, and the test usually cannot be repeated straight away for the same result.

Who is allowed to do this work is set by a certification scheme. See How to Become an NDT Level I, II or III.

Frequently asked questions

Can AE replace an internal inspection?

Sometimes it can support a longer interval or reduced scope, but that depends on the governing code, the owner's program and the regulator. It does not automatically replace other inspections.

Does AE find old cracks?

Only if they are active during the test. A stable flaw may produce no signal, which is why AE is not proof that a structure is flaw-free.

Is AE a continuous monitoring method?

It can be. Sensors can stay in place and watch for activity during operation, though many tests are run as planned, one-time load tests.

How NDT LLC can help

Our team of Level IIIs helps owners decide where AE fits in an inspection program and how to follow up on the results. Contact us.

This article is general information for a business audience, not engineering, legal or regulatory advice. Always work to the current edition of the codes, standards, regulations and manufacturer data that apply to your equipment and contracts.

NDT LLC is an independent consulting firm. It is not affiliated with, endorsed by or sponsored by ASNT, ASME, ASTM International, or any other organization named in this article; names are used only to identify the organizations and documents referred to.