What is NDT

After any task, in any field, you check your work. In writing, you proof read, while in golf you watch the shot land. In manufacturing, you inspect what you made. This inspection can be performed at any step and can have several different types. Think dimensional checks with a CMM or a hardness check. Today we’re going to focus on a subsect of inspections; NDT or Non-Destructive Testing.

What is NDT:

Non-Destructive Testing covers the inspection of material, usually while searching for cracks or other defects like porosity. The goal is to determine if a manufacturing process (or usage of the part) has caused undue stress which might lead to failures. Most commonly used on metallic parts, some of these techniques can even work on plastics or ceramics. The most common NDT techniques are MPI, FPI and HFEC, while X-Ray and Ultrasonic inspections are usually more specialized.

MPI:

Magnetic Particle Inspection is a technique which uses a die mixed with suspended microscopic iron filings and a strong magnet to indicate cracks. This works because when a magnetic field is introduced through a part, it focuses on discontinuities in the material. This results in the iron filings in the die being attracted to any cracks present where the magnetic field is applied.

Since MPI relies on an applied magnetic field in the part, it only works on ferromagnetic parts. While great for mild steel or martensitic stainless steels, it does not work for anything else. It also works great for large parts since the die can be sprayed over a large area, and it reacts quickly with the magnetic fields introduced. The most common specification for this is ASTM E1444.

FPI:

Fluid Penetrant Inspection on the other hand can be used to find defects in almost all parts. While less sensitive than MPI, it still can be used to find all but the smallest defects. This system works by applying an ultra-thin die all over a part. The die is allowed to permeate the part, and then the part is cleaned as thoroughly as possible. The die soaks into even the smallest cracks or defects, and remains after the washing steps. After cleaning, a developer (usually talcum powder and isopropyl alcohol in a spray) is applied over the whole part. If any defects are present, which retained the die, then the developer soaks up the die, indicating a defect.

FPI can be a finicky process, relying on immaculate preparation to get accurate results. With inadequate die/penetrant soak times, the defects might not absorb enough to show when the developer is applied. Similarly, if the cleaning steps are too aggressive for removing excessive die, they could remove the die in the defect. Like MPI it is great for large parts due to the ease of applying the die and developer. ASTM E1417 is the most common FPI specification in use today.

HFEC:

High Frequency Eddy Current is the least utilized inspection technique we are going to look at today. It is extremely sensitive, and is most commonly used on small features. It relays on introducing a magnetic field into a part, and then measuring the field direction. Surprisingly it can introduce a magnetic field into any electrically conductive part, meaning aluminum, copper, etc are all possible. Due to the way it does this, the probes are small and work slowly. This means only small features can be inspected.

HFEC can, however, detect sub-surface defects, critical for some applications. This is often considered the most accurate form of commonly available NDT inspection, as it can detect cracks even before they open. This means that a HFEC inspection could catch defects not yet visible with an MPI or FPI. ASTM E2884 is the driving specification for this type of inspection.

Conclusion:

As you can see, NDT can be a complicated topic until you find the right technique for your application. Performing the inspections correctly requires a reliable process, optimized for accuracy. The benefit, though, of finding these minor defects before they grow, is huge. Many popular calamities could have been averted with a proper NDT inspection, such as the Arkansas and Pittsburg bridge collapses within the last few years. Let us review your processes and recommend the best NDT techniques for your shop today!

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