From cannons to cars, MPI has proven to be effective and reliable NDT method for over a century.



Introduction

Magnetic particle inspection (MPI) is a method of nondestructive testing that detects surface (or near surface) flaws in ferromagnetic materials such as nickel, iron, and cobalt. A magnetic field is placed onto the test object and iron particles are drawn to where the flaw is located. Magnetic particle inspection is commonly used for detecting pores and cracks in welds. The method is used to inspect a variety of product forms including castings, forgings, and welds.



Origins



Magnetic particle inspection is based on the principle of magnetism. Magnetism is the aptitude of matter attracting other matter to itself. It is thought that the ancient Greeks were the first to discover this process in the mineral that the Greeks called magnetite. The earliest possible reference to magnetic testing of the element iron originated from an article within the Journal of Engineering in 1868. Within this article, it was stated that flaws within cannon barrels could be detected by sliding a magnetic compass across the length of the barrel.

US Military



The US military pioneered many of the nondestructive methods we are familiar with today during World War I and World War II. In the 1920's, an American military major, William Hoke, noticed magnetic particles could be used with magnetism to locate defects. Hoke observed that iron shavings collected where flaws were present in a steel sheet. He rightly concluded that these flaws were causing the field to distort and allow the particles to pool together. By the 1930s, magnetic particle inspection had quickly become an effective method for inspecting railroads, axles, steam engine boilers, tracks, and wheels.



Modern Use



Today, magnetic particle inspection is used for all kind of manufactured components such as vehicles, pressurized vessels, and boilers. MPI is also used on large structures such as bridges, ships, and storage tanks. MPI is frequently used in the automobile industry on parts and components to ensure that defective parts are removed from the assembly line.

Conclusion

Since its origins in the late nineteenth century, magnetic particle inspection has become a reliable method of nondestructive testing for a variety of assessments on components and structures. In terms of price and ease of use, MPI is a great option in many applications. Still, this method has its limitations in that it can only be used with ferromagnetic materials and can only detect surface to slightly subsurface flaws. Despite these limitations, magnetic particle inspection has proven to be effective and reliable method for over a century. The use of MPI makes our world a safer place today.



Citations

Lovejoy, D. (1993, January 1). The history and basis of the magnetic particle testing method. Retrieved from https://link.springer.com/chapter/10.1007/978-94-011-1536-0_1.

Magnetic Particle Inspection - Early History. (n.d.). Retrieved January 14, 2019, from http://www.tesndt.com/ndt-articles/magnetic-particle-inspection-early-history/.

(n.d.). Retrieved January 14, 2019, from https://www.nde-ed.org/EducationResources/CommunityCollege/MagParticle/Introduction/history.htm.