Magnetic Particle

Automotive Engineering

Phd Eng. Marcin Korzeniowski

Non Destructive Evaluation in Contemporary Manufacturing Systems

Lecture 3Magnetic Particle Testing

Magnetic Testing (MT)MAGNETIC PARTICLE INSPECTION

Method of locating surface and subsurface discontinuities in ferromagnetic materials. When the material or part under test is magnetized, magnetic discontinuities that lie in a direction generally transverse to the direction of the magnetic field cause a leakage field to be formed at and above the surface of the part.The discontinuity, is detected by the use of finely divided ferromagnetic particles applied over the surface, with some of the particles being gathered and held by the leakage field.

Limitation for Magnetic Particle Testing:The method can be used only on ferromagnetic materials,For best results, the magnetic field must be in a direction that will intercept the principal plane of the discontinuity; this sometimes requires two or more sequential inspections with different magnetizations,Demagnetization following inspection is often necessary,Post cleaning to remove remnants of the magnetic particles clinging to the surface may sometimes be required after testing and demagnetization,Exceedingly large currents are sometimes needed for very large parts,Care is necessary to avoid local heating and burning of finished parts or surfaces at the points of electrical contactAlthough magnetic particle indications are easily seen, experience and skill are sometimes needed to judge their significance.

Magnetic Testing (MT)

Magnetic Testing (MT)FERROMAGNETIC MATERIALSMetals which conduct magnetic field:elements:iron,nickel,cobalt,alloys:cast ironcarbon steel

Ferromagnetic MaterialsA material is considered ferromagnetic if it can be magnetized. Materials with a significant Iron, nickel or cobalt content are generally ferromagnetic. Ferromagnetic materials are made up of many regions in which the magnetic fields of atoms are aligned. These regions are call magnetic domains. Magnetic domains point randomly in demagnetized material, but can be aligned using electrical current or an external magnetic field to magnetize the material.

Magnetic Testing (MT)The physical background

Magnetic Testing (MT)Directions of the discontinuities

Magnetic Testing (MT)

Magnetic Testing (MT)The ways of magnetizationcircular

longitudal

Producing a Longitudinal Magnetic Field Using a CoilA longitudinal magnetic field is usually established by placing the part near the inside or a coils annulus. This produces magnetic lines of force that are parallel to the long axis of the test part. Coil on Wet Horizontal Inspection Unit Portable Coil

Magnetic Testing (MT)Magnetized ringWhen a magnetic material is placed across the poles of a horseshoe magnet having square ends, forming a closed or ringlike assembly, the lines of force flow from the north pole through the magnetic material to the south pole

Magnetic Testing (MT)Magnetizing currentBoth direct current (dc) and alternating current (ac) are suitable for magnetizing parts for magnetic particle inspection.

The strength, direction, and distribution of magnetic fields are greatly affected by the type of current used for magnetization.

The important difference with regard to magnetic particle inspection is that the fields produced by direct current generally penetrate the cross section of the part, while the fields produced by alternating current are confined to the metal at or near the surface of the part, a phenomenon known as the skin effect. Therefore, alternating current should not be used in searching for subsurface discontinuities.

Magnetic Testing (MT)Magnetizing currentalternate current

rectifiedthree phase ACrectifiedone phase AC

Magnetic Testing (MT)YokesTwo basic types of yokes that are commonly used for magnetizing purposes: permanent-magnet,electromagnetic yokes.

Both are hand held and therefore quite mobile.

Permanent-magnet yokes are used for applications where a source of electric power is not available or where arcing is not permissible (as in an explosive atmosphere). The limitations of permanent-magnet yokes include the following:Large areas or masses cannot be magnetized with enough strength to produce satisfactory crack indications,Flux density cannot be varied at will,If the magnet is very strong, it may be difficult to separate from a part,Particles may cling to the magnet, possibly obscuring indications.

Magnetic Testing (MT)Yokes

Electromagnetic yokesConsist of a coil wound around a U-shaped core of soft iron. The legs of the yoke can be either fixed or adjustable. Adjustable legs permit changing the contact spacing and the relative angle of contact to accommodate irregularly-shaped parts. Unlike a permanent-magnet yoke, an electromagnetic yoke can readily be switched on or off. This feature makes it convenient to apply and remove the yoke from the testpiece.

Magnetic Testing (MT)Yokes

Electromagnetic yokesMagnetic Testing (MT)

Application of Magnetic Media (Wet Versus Dry)MPI can be performed using either dry particles, or particles suspended in a liquid. With the dry method, the particles are lightly dusted on to the surface. With the wet method, the part is flooded with a solution carrying the particles.

The dry method is more portable. The wet method is generally more sensitive since the liquid carrier gives the magnetic particles additional mobility.

Dry Magnetic Particles Magnetic particles come in a variety of colors. A color that produces a high level of contrast against the background should be used.

Wet Magnetic ParticlesWet particles are typically supplied as visible or fluorescent. Visible particles are viewed under normal white light and fluorescent particles are viewed under ultraviolet light.

Crane Hook with Service Induced Crack Fluorescent, Wet Particle Method

Gear with Service Induced Crack Fluorescent, Wet Particle Method

Drive Shaft with Heat Treatment Induced Cracks Fluorescent, Wet Particle Method

Splined Shaft with Service Induced CracksFluorescent, Wet Particle Method

Threaded Shaft withService Induced CrackFluorescent, Wet Particle Method

Large Bolt with Service Induced CrackFluorescent, Wet Particle Method

Crank Shaft with Service Induced Crack Near Lube HoleFluorescent, Wet Particle Method

Lack of Fusion in SMAW WeldVisible, Dry Powder MethodIndication

Toe Crack in SMAW WeldVisible, Dry Powder Method

Advantages of Magnetic Particle Inspection Can detect both surface and near sub-surface defects.Can inspect parts with irregular shapes easily.Precleaning of components is not as critical as it is for some other inspection methods. Most contaminants within a flaw will not hinder flaw detectability.Fast method of inspection and indications are visible directly on the specimen surface.Considered low cost compared to many other NDT methods.Is a very portable inspection method especially when used with battery powered equipment.

Limitations of Magnetic Particle InspectionCannot inspect non-ferrous materials such as aluminum, magnesium or most stainless steels.Inspection of large parts may require use of equipment with special power requirements.Some parts may require removal of coating or plating to achieve desired inspection sensitivity.Limited subsurface discontinuity detection capabilities. Maximum depth sensitivity is approximately 0.6 (under ideal conditions).Post cleaning, and post demagnetization is often necessary.Alignment between magnetic flux and defect is important