NDT Inspection at the Surface

Due to its wide-reaching nature, odds are that every person has interacted with something that has been inspected with non-destructive testing (NDT) from driving cars to flying on planes to riding roller coasters. You may have even stumbled across it on the internet or seen photos/videos or read an article about NDT and where it is used. But normally when NDT is promoted or discussed, they talk about methods that are exciting to look at. They show photos that are stimulating and seem to be on the edge of technology.

But what you do not normally see or hear about are methods that are used probably more than any others. That is likely because they are not flashy. They are not fun. They are not even clean, but they are necessary.

This article will focus on Magnetic Particle Testing and Liquid Penetrant Testing. These two methods are the first line of defense when looking for flaws open to the surface which are some of the most likely to grow under stressors leading to potentially catastrophic failures. However, as important as these methods are, they are commonly overlooked due to their simple application and presumed lack of importance.

Magnetic Particle Testing (MT, MPI)

 If you do any research on the internet around NDT, you will find a description of MT using buzz words like, “Magnetic particle inspection is a nondestructive testing process”, surface and subsurface inspection,” “ferromagnetic materials,” “fast,” “simple application.

What these statements fail to explain is that MT is an incredibly powerful tool that involves using the manipulation of electricity to create a magnetic field that can be controlled for direction and intensity by the operator. This involves careful measurements and correctly applying a magnetic field to the part that flows along its surface “looking” for flaws in its way. These flaws create a secondary magnetic field that attracts a medium to the edges of the flaw creating an outline for the operator to see and measure.

MT can be performed in a variety of environments from the most controlled laboratory to the most miserable field conditions. There are large powerful stationary units located in laboratories that are over 10 feet long and can inspect parts as large as a few feet in diameter all the way down to the most portable units for parts that are too large and impractical to perform in a laboratory allowing a near lab quality inspection to be performed at a customer’s site.

Quality is always a top priority. However, MT is an excellent tool for fast-paced inspections, which means the technician applies their knowledge to create an effective application to inspect parts quickly with the required level of assurance that they have met the standards set for their final use.

“In some cases, it may be beneficial to employ both methods in tandem, as they can complement each other and provide a more comprehensive evaluation of component integrity.”

Liquid Penetrant Testing (LPI, FPI, PT)

PT, much like MT has a boilerplate explanation that will include buzzwords like “low-cost inspection, surface-breaking defects, non-porous materials,” but these statements barely scratch the surface.

PT is one of the most sensitive methods used to locate extremely small surface discontinuities that would otherwise go unnoticed by the technician’s unaided eye or in some cases even its other surface NDT method counterparts. LPI can inspect large volumes of small parts like nuts, bolts, and gears to the largest of parts such as complex castings that must be moved with a multi-ton capacity crane. This is due to its ability to be applied in so many ways that it could be considered one of the most variable methods we have. LPI consists of five sensitivity levels, three types, four methods, and six forms. That is nearly 360 variations this application can be used for.

Each level of sensitivity, type, method of removal, and developer forms are designed to reach a desired result. Codes and standards will specify the requirements of performing an exam, but the correct application (in some cases a few seconds of variation at any given stage) can be the difference between the technician being able to find a discontinuity in the material or not. This is not because the technician failed in interpretation, but rather the process failed in allowing the indications to be viewed.

The process of LPI can be extremely simple with the use of a few spray cans and a rag or as elaborate as a series of vats with carefully controlled chemicals that the parts must be placed in for exact times measured by calibrated gauges and timers. Either of these applications requires the utmost attention of the technician to ensure that all the requirements are met, and procedures are followed to the letter. This is to ensure that the technician has the best chance to see the minute flaws that the method will display under the correct lighting conditions.

How do you decide what method is best for you?

Here are some questions to ask yourself:

Question: Did the specification require MT or PT?
Answer: Yes - Use that one. Did not specify which - Move to the next question. No - No need for the inspection.

Question: What is the part made of?
Answer: Meta - Either will potentially work. Non-metal/solids - LPI only.

Question: Can it be magnetized? If you are not sure, will a magnet stick to it?
Answer: Yes - It is eligible for MPI. No - It is not eligible for MPI so LPI must be used.

Question: Is the surface smooth or rough and porous?
Answer: Smooth - Either will work. Rough - MPI is advisable if not necessary.

How can Element help?

At Element, we offer a wide variety of surface inspection methods including Magnetic Particle and Liquid Penetrant Testing services. Our facility’s capabilities include multiple technicians certified in various NDT methods backed by numerous SMEs for quick problem-solving and technical assistance. Our equipment includes MPI equipment such as a 14-foot-long stationary bench for high production or exceptionally large parts as well as two LPI lines for both visible and fluorescent methods. Additionally, we offer a large variety of our services to be brought to the customer’s facilities allowing our technicians to use their expertise and provide accurate and reliable results, helping you make informed decisions quickly and efficiently.