FALL 2021

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100 THE DISTRIBUTOR’S LINK ROB LaPOINTE MAGNETIC PARTICLE TESTING VS. PENETRANT TESTING – WHICH TEST IS BEST? from page 8 FIGURE 3 - A CUTAWAY SECTION OF A FULLY THREADED STUD SHOWING A VOID OF MATERIAL KNOW AS A BURST ASTM F788, Standard Specification for Surface Discontinuities of Bolts, Screws, Studs, and Rivets, Inch and Metric Series, provides us with a list of and examples of surface discontinuities common to fasteners. F788 also provides tolerances for allowable discontinuities. Discontinuities that are not open to the surface such as internal voids, internal bursts, or inclusions are not able to be detected by PT and many are not detectable by MT. Magnetic particle testing does offer the ability to detect subsurface discontinuities provided they are close to the surface and the technician is skilled at spotting them. Figure 3 shows a subsurface discontinuity that would not be detectable by penetrant or magnetic particle testing. The burst in Figure 2 is not connected to the surface and is too deep below the surface for the possibility of detection by MT. For discontinuities like these, other NDT methods like ultrasonic or radiograph testing would be useful. These methods can image the inside of something without cutting it open. They are essentially the same as the medical techniques used to image unborn babies (ultrasound) or a broken leg (x-ray). It would also be worth our time to understand a little more about nondestructive testing. Nondestructive testing is just that – nondestructive. This testing does not destroy, mark, or otherwise injure the part to collect data. Specifically, nondestructive testing accesses the quality of the material and workmanship of the product to ensure that there are no hidden flaws that would render the part unfit for service. Also, since it’s nondestructive, test parts can be sold and used along with the production lot after testing. For highly controlled products, it is common for 100 % of the production lot to be tested for flaws using a nondestructive method. Nondestructive testing includes many detection methods such as Magnetic Particle Testing (MT), Penetrant Testing (PT), Visual Testing (VT), Ultrasonic Testing (UT), Radiographic Testing (RT) and many others. Personnel training, qualification and certification is typically an employer-based system through guidelines provided by the American Society for Nondestructive Testing (ASNT) using the SNT-TC-1A standard and the National Aerospace Standard NAS 410. It is a highly technical and regulated branch of materials testing. FIGURE 4 - THE RIGHT-HAND RULE FOR LOCATING THE MAGNETIC FIELD RELATING TO AN ELECTRICAL CURRENT Magnetic particle testing makes use of magnetic fields created by the flow of charges to detect breaks or discontinuities in conductive and magnetizable materials. To understand how this works, we need to be acquainted with some physics of electromagnetism. Don’t worry, we won’t be deriving Maxwell’s equations here, just some basic spatial relationships between electrical current and magnetic fields. When electrical current (electrons) flows through a metal, a magnetic field is created which incircles the flow. Physics students around the world use what’s called the “Right-hand Rule” for understanding this relationship. Figure 4 shows the right-hand rule. Notice that if your right thumb points in the direction of the current, then your curled fingers point in the direction of the magnetic field created by the flow. CONTINUED ON PAGE 158

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