What it finds

  • Internal flaws such as cracks, lack of fusion in welds, inclusions and laminations.
  • Remaining thickness, so corrosion and erosion can be measured and trended from one side of a pipe or vessel.
  • Bond problems and delaminations in many composites and bonded structures.

How it works

A probe, or transducer, turns an electrical pulse into a burst of sound. A couplant, usually a gel or water, carries the sound into the part, because sound barely crosses an air gap. The sound travels until it reaches a boundary, such as the back wall or a crack, and some of it bounces back. The instrument times that echo. Since the sound speed in the material is known, time becomes distance, which gives the depth of the reflector or the thickness of the wall.

A straight-beam probe looks directly down into the part. An angle-beam probe sends sound in at a slant, which is how most weld inspection is done. Instruments are set up on reference blocks, so the inspector can compare a signal with a known reflector.

Two advanced techniques are worth knowing. Phased array (PAUT) uses many small elements, fired in sequence to steer the beam and build an image. Time of flight diffraction (TOFD) uses a pair of probes and picks up the faint signals from the tips of a flaw, which helps with sizing.

Where it is used

UT shows up on welds in pipelines, pressure vessels and structures, on forgings and plate, on aircraft structure and composites, on rail, and on thickness surveys across refineries and plants. See our guides to refineries, aerospace manufacturing and tube inspection.

Limits

  • Orientation matters. A flaw reflects best when the beam hits it squarely. One lying parallel to the beam can be missed, so the scan plan matters.
  • Surface and access. Rough surfaces, loose scale and tight geometry make coupling and scanning harder.
  • Difficult materials. Coarse-grained metals such as some castings and austenitic welds scatter sound and create noise.
  • Very thin or small parts can be hard to inspect, because near-surface signals crowd together.
  • Skill. Manual UT depends heavily on the operator, and it leaves no image unless the scan is recorded.

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

Is ultrasonic testing safe?

Yes. UT uses sound rather than radiation, so work can usually continue nearby. Normal site hazards such as height, heat and confined spaces still apply.

UT or radiography for welds?

UT is usually stronger on cracks and lack of fusion and needs access to one side, while radiography gives a picture and is good at porosity and slag. Codes and contracts often say which is required.

Can UT measure corrosion from the outside?

Yes. Thickness readings taken from the outside show the remaining wall, which is why UT is a mainstay of corrosion monitoring.

How NDT LLC can help

Our team of Level IIIs helps with UT procedures, technique selection and data review, and we take on field NDT and inspection work. 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.