Non Destructive Test / Non Destructive Evaluation

Many people use these two terms, NDT and NDE, interchangeably; the distinction is mostly in how the results will be used. When evaluating something, the goal is to seek information and understanding. When testing something, the goal is to determine whether it is acceptable for use or might need to be replaced.

Clearly, the inspection technology itself does not know the difference. Surprisingly, the software may not be the same. For NDE, the software aims to acquire as much information as possible and assist the user in sorting through it. For NDT, the software assists the user in comparing the measured data against rules and previous measurements.

What does Non Destructive mean?

In theory, it means that using this technique doesn't damage the part. While this is usually literally true, the preparation necessary to make the measurements may cause a small amount of damage to the part. After many inspections, over many years, that small damage can add up so that the part cannot be used and must be replaced. Perhaps a better term might be Least Destructive, but that would confuse people. There are four common ways to look for flaws in metal surfaces:

  1. Pour on a liquid that likes to seep into tiny holes, wipe off the excess and see what's left. The liquid usually contains a fluorescent dye that glows under ultraviolet light so that the tiny residue in the holes is easier to see. The problem with this technique is that paint and other coatings need to be removed from the surface first.
  2. Shine X-rays through and take a picture. In places where there is missing metal, more X-rays will get through and create a distinctive blob. In addition to the radiation hazard, if a crack is not aligned with the direction of the X-ray beam, the blob will be extremely faint and hard to see.
  3. Force high frequency sound (ultrasound) through the surface and use a special microphone to listen for reflections from interior damage. For everything other than really simple parts, there are lots of reflections from the other sides and corners that are hard to untangle. Also, the reflection from the surface (as the sound is going in) is much louder than the flaws, so flaws near the surface are hard to detect by this method.
  4. Pass electric currents through the material and notice places where the current cannot go (because there is a gap or hole). Depending on the shape of the part and the current path needed, the current can be applied using electrical contacts or by inductive coupling into the surface. It is difficult to make good electrical contact without modifying the surface, so the former is generally avoided whenever the latter is possible. Inductive coupling, also referred to as eddy current is the technique used for this system.

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