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Introduction to Magnetic Particle Inspection


Introduction to Magnetic Particle Inspection

Magnetic particle inspection (MPI) is a nondestructive testing method used for defect detection. MPI is fast and relatively easy to apply, and part surface preparation is not as critical as it is for some other NDT methods. These characteristics make MPI one of the most widely utilized nondestructive testing methods.
magnetic particle testing
MPI uses magnetic fields and small magnetic particles (i.e.iron filings) to detect flaws in components. The only requirement from an inspectability standpoint is that the component being inspected must be made of a ferromagnetic material such as iron, nickel, cobalt, or some of their alloys. Ferromagnetic materials are materials that can be magnetized to a level that will allow the inspection to be effective.
magnetic particle testing yoke
The method is used to inspect a variety of product forms including castings, forgings, and weldments. Many different industries use magnetic particle inspection for determining a component's fitness-for-use. Some examples of industries that use magnetic particle inspection are the structural steel, automotive, petrochemical, power generation, and aerospace industries. Underwater inspection is another area where magnetic particle inspection may be used to test items such as offshore structures and underwater pipelines.

Basic Principles

In theory, magnetic particle inspection (MPI) is a relatively simple concept. It can be considered as a combination of two nondestructive testing methods: magnetic flux leakage testing and visual testing.Consider the case of a bar magnet. It has a magnetic field in and around the magnet. Any place that a magnetic line of force exits or enters the magnet is called a pole. A pole where a magnetic line of force exits the magnet is called a north pole and a pole where a line of force enters the magnet is called a south pole.
magnetic particle inspection
When a bar magnet is broken in the center of its length, two complete bar magnets with magnetic poles on each end of each piece will result. If the magnet is just cracked but not broken completely in two, a north and south pole will form at each edge of the crack. The magnetic field exits the north pole and reenters at the south pole.The magnetic field spreads out when it encounters the small air gap created by the crack because the air cannot support as much magnetic field per unit volume as the magnet can. When the field spreads out, it appears to leak out of the material and, thus is called a flux leakage field.
magneticl particle testing
If iron particles are sprinkled on a cracked magnet, the particles will be attracted to and cluster not only at the poles at the ends of the magnet, but also at the poles at the edges of the crack. This cluster of particles is much easier to see than the actual crack and this is the basis for magnetic particle inspection.
magnetic particle inspection
The first step in a magnetic particle inspection is to magnetize the component that is to be inspected. If any defects on or near the surface are present, the defects will create a leakage field. After the component has been magnetized, iron particles, either in a dry or wet suspended form, are applied to the surface of the magnetized part. The particles will be attracted and cluster at the flux leakage fields, thus forming a visible indication that the inspector can detect.
magnetic particle testing
History of Magnetic Particle Inspection

Magnetism is the ability of matter to attract other matter to itself. The ancient Greeks were the first to discover this phenomenon in a mineral they named magnetite. Later on Bergmann, Becquerel, and Faraday discovered that all matter including liquids and gasses were affected by magnetism, but only a few responded to a noticeable extent.

The earliest known use of magnetism to inspect an object took place as early as 1868. Cannon barrels were checked for defects by magnetizing the barrel then sliding a magnetic compass along the barrel's length. These early inspectors were able to locate flaws in the barrels by monitoring the needle of the compass. This was a form of nondestructive testing but the term was not commonly used until some time after World War I.

In the early 1920’s, William Hoke realized that magnetic particles (colored metal shavings) could be used with magnetism as a means of locating defects. Hoke discovered that a surface or subsurface flaw in a magnetized material caused the magnetic field to distort and extend beyond the part. This discovery was brought to his attention in the machine shop. He noticed that the metallic grindings from hard steel parts (held by a magnetic chuck while being ground) formed patterns on the face of the parts which corresponded to the cracks in the surface. Applying a fineferromagnetic powder to the parts caused a build up of powder over flaws and formed a visible indication. 
magnetic particle testing equipment
In the early 1930’s, magnetic particle inspection was quickly replacing the oil-and-whiting method (an early form of the liquid penetrant inspection) as the method of choice by the railroad industry to inspect steam engine boilers, wheels, axles, and tracks. Today, the MPI inspection method is used extensively to check for flaws in a large variety of manufactured materials and components. MPI is used to check materials such as steel bar stock for seams and other flaws prior to investing machining time during the manufacturing of a component. Critical automotive components are inspected for flaws after fabrication to ensure that defective parts are not placed into service. MPI is used to inspect some highly loaded components that have been in-service for a period of time. For example, many components of high performance racecars are inspected whenever the engine, drive train or another system undergoes an overhaul. MPI is also used to evaluate the integrity of structural welds on bridges, storage tanks, and other safety critical structures. 

----From NDT resource center