Ultrasonic Testing vs. Magnetic Particle Inspection: When to Use UT Over MPI

The market for nondestructive testing (NDT) in North America is increasing rapidly, with similar growth around the world. There are two reasons for this: nondestructive testing works for nearly every industry, and there is a wide variety of NDT techniques for nearly every inspection need. 

With the use of NDT being so widespread, there are techniques that perform better in comparison to others. Two of the most popular are ultrasonic testing (UT) and magnetic particle inspection (MPI). 

In this article, we look at ultrasonic testing vs. magnetic particle inspection—the benefits, drawbacks, and when to use each. 

Magnetic Particle Inspection 

Magnetic Particle Inspection (sometimes referred to as Magnetic Particle Testing) is a very common and easy-to-use NDT technique. It works, as its name applies, by having the tester create a magnetic field around the object being inspected. This is done with a permanent or electromagnet, induced magnetic field (such as from a current-carrying coil) or by a direct electrical current run through the object. 

The object being tested is then coated with tiny iron particles. The current magnetizes the particles to the surface, revealing any flaws. 

The Benefits of Magnetic Particle Inspection

While this might seem complicated, it is actually pretty simple and easy-to-use. It is fast, can be inexpensive, and many MPI instruments are portable. 

MPI is considered much more accurate, effective, and efficient than inspections that use dye penetrants. MPI is excellent at detecting flaws on the surface of objects. 

The Drawbacks of Magnetic Particle Inspection

Likely the biggest drawback of MPI is that it has a much more limited use for subsurface flaws or cracks. It is not particularly effective when it comes to measuring the height or depth of a surface crack (length and width only). This leads to inspectors receiving only a partial view of the potential issue. 

Another drawback is that it is, essentially, subjective. It relies on the interpretation of the inspector, leaving room for human error. The readings can also be inaccurate if the magnetic field is not perfectly aligned. 

While MPI is easy-to-use, it can only be used on ferromagnetic materials—materials that can be magnetized. They have to be clean and unpainted, and then they have to be demagnetized afterward. 

In fact, due to these drawbacks, many are now looking at Eddy Current Array (ECA) testing as a replacement option vs. MPI for surface and subsurface flaw detection. ECA offers a fast, easy to use technique with no extensive surface preparation and a complete record of inspection.

Ultrasonic Testing

Ultrasonic testing is one of the original NDT techniques, dating back to the 1950s. Since then, technology has evolved to the point where the original UT techniques would likely be unrecognizable, but the principle remains the same. 

In ultrasonic testing, a transducer is manually moved over an object. The transducer introduces high-frequency sound waves into the material and hollow spots—tiny cracks, hairline fissures, microscopic pocks—create an echo. 

These soundwaves are used to create a full 3D image of the object, allowing the user to objectively see where the flaws are and how deep they might run. 

There are many types of UT, including phased array, straight beam, angled beam, time of flight diffraction (TOFD), and guided wave. Phased array and TOFD are the most common and trusted. The versatility and efficiency of UT have contributed greatly to the growth of NDT worldwide. 

The Benefits of Ultrasonic Testing

Speed. Reliability. Versatility. There’s a reason this is one of the most frequently used methods. Unlike some other methods, UT is portable, can go under the surface, penetrating objects dense or large, to find flaws that would be invisible to the naked eye and hidden from other types of testing. It is ideal for volumetric inspections and easily handles complex geometries.

Many of today’s phased array UT offerings can provide a true 3D image, created in real-time for immediate results. This allows inspectors to objectively analyze the full thickness of the object by looking at a screen; the image can be downloaded and shared for others to do the same. This allows someone to visualize the object of inspection without having to be physically present. 

Another benefit of UT is the ability to record inspection results and compare them over time. This allows inspectors to understand how different materials react to different environments and pressures. 

The Drawbacks of Ultrasonic Testing

While UT has a host of benefits, there is one thing to consider: the need for human guidance. The transducer still needs a human to perform. The incredibly accurate results are partly the result of the operator knowing just how to angle the path of the signal. While getting full coverage is easy, this still requires training. 

The reliability and ease of this system come from a few pieces of instrumentation, along with the scanners, the software, and the different probes and wedges. Accurate, 3D results require a certain level of complexity. 

Ultrasonic Testing vs. Magnetic Particle Inspection: Which Is Right For You? 

Over the last few years, more and more industries have begun to change their NDT techniques. For example, austenitic stainless steel makers have made the shift from radiography to ultrasonic. While many techniques may be able to detect flaws, not all technology is created equal. Like in the case of ultrasonic testing, magnetic particle inspection, and eddy current testing, one technique may work better than another depending on the application. 

When considering which technique best suits your needs, the key is to weigh the advantages and disadvantages, examine your budget, understand the costs of the most accurate tests, and then make a decision. When accuracy matters, NDT testing is more an investment than an expense. It is also a hedge against downtime, non-compliance, and potential safety hazards.

Zetec is a major provider of non-destructive, ultrasonic testing solutions for a wide variety of industries. Contact us today to ensure that you are equipped with the best possible testing capabilities.