Another NDT technology

My last post addressed the advantage of eddy current for crack detection. It does have a couple of limitations though. The crack must be surface-breaking, or very near, and the material under inspection needs to be ferrous. So what if you need to look for flaws deeper under the surface?

Well the conventional options would be x-ray (expensive and raises safety concerns,) and ultrasound. Historically, ultrasound has been a single point pulse-echo technology, and so had to be scanned across a surface, making it slow. But I’m here to tell you that things are changing!

The new trend in ultrasound is the phased array. This “ … provides better imaging, higher repeatability and reproducibility and more angles and scan patterns over conventional ultrasonic methods.” That quote comes from “How Ultrasonic Phased Array is Shaping the Industry,” published on the NDTmag website July 28th, 2010. If you need to find flaws beneath a surface, this is something you ought to read.

Sometimes, you don’t want machine vision

Hard to believe, I know, but machine vision is not the best tool for every inspection job. Take crack detection for example. A crack has a lot of length but almost no width, so you need a ton of pixels to see the darn thing. Plus, lighting it can be awfully hard. If there’s no difference in height between the two sides darkfield lighting won’t work, so your only options are a Cloudy Day Illuminator (CDI) or Diffuse On-Axis Light (DOAL), both of which are bulky and still don’t guarantee success.

But if the object you’re inspecting is ferrous, eddy current works like a dream. Of course, it is a single point technology, so you’ll need to scan across the surface and it’s not as fast as just grabbing an image, but there again, it can be very repeatable and not too expensive.

For a quick eddy current primer, take a look at “Going Green with Eddy Current Testing” by Dan DeVries and Joe Jessop and published on the NDTmag website, July 28th 2010.