Magnetic Particle Inspection

Magnetic particle inspection (often abbreviated MT or MPI) is a nondestructive inspection method that provides detection of linear flaws located at or near the surface of ferromagnetic materials. It is viewed primarily as a surface examination method.

Magnetic Particle Inspection (MPI) is a very effective method for location of surface breaking and slight sub-surface defects such as cracking, pores, cold lap, lack of sidewall fusion in welds etc in magnetic materials.

There are many different techniques. The most versatile technique is using a 110v AC hand held electromagnetic yoke magnet, a white strippable paint as contrast background and a magnetic "ink" composed of iron powder particles in a liquid carrier base.

The area is magnetised with the yoke magnet. In the event of a surface or slightly sub surface defect being present, the lines of magnetic force will deform around the defect.

The magnetic ink is applied and the iron powder particles will bridge the gap caused by the defect and give a visible indication against the white contrast background.

Magnetic Particle Inspection (MPI) provides very good defect resolution and is used extensively on..
Welded fabrications in magnetic material, Castings, Locating fatigue cracks in items subject to cyclical stress

Schematic of flux lines flowing past both a perpendicular and parallel defect, and flux leakage occurring around the perpendicular flaw

Magnetic Particle Inspection is performed in four steps..

Induce a magnetic field in the specimen
Apply magnetic particles to the specimen's surface
View the surface, looking for particle groupings that are caused by defects
Demagnetize and clean the specimen

Advantages of Magnetic Particle Inspection

Can find both surface and near sub-surface defects
Some inspection formats are extremely portable and low cost
Rapid inspection with immediate results
Indications are visible to the inspector directly on the specimen surface
Can detect defects that have been smeared over
Can inspect parts with irregular shapes (external splines, crankshafts, connecting rods, etc.)

Limitations of Magnetic Particle Inspection

The specimen must be ferromagnetic (e.g. steel, cast iron)
Paint thicker than about 0.005" must be removed before inspection
Post cleaning and post demagnetization is often necessary
Maximum depth sensitivity is typically quoted as 0.100" (deeper under perfect conditions)
Alignment between magnetic flux and defect is important

Standards

StandardsInternational Organization for Standardization (ISO)

ISO 3059, Non-destructive testing - Penetrant testing and magnetic particle testing - Viewing conditions
ISO 9934-1, Non-destructive testing - Magnetic particle testing - Part 1.. General principles
ISO 9934-2, Non-destructive testing - Magnetic particle testing - Part 2.. Detection media
ISO 9934-3, Non-destructive testing - Magnetic particle testing - Part 3.. Equipment
ISO 17638, Non-destructive testing of welds - Magnetic particle testing
ISO 23279, Non-destructive testing of welds - Magnetic particle testing of welds - Acceptance levels

European Committee for Standardization (CEN)

EN 1290, Surface Crack Testing
EN 1330-7, Non-destructive testing - Terminology - Part 7.. Terms used in magnetic particle testing
EN 1369, Founding - Magnetic particle inspection
N 10228-1, Non-destructive testing of steel forgings - Part 1.. Magnetic particle inspection
EN 10246-12, Non-destructive testing of steel tubes - Part 12.. Magnetic particle inspection of seamless and welded ferromagnetic steel tubes for the detection of surface imperfections
EN 10246-18, Non-destructive testing of steel tubes - Part 18.. Magnetic particle inspection of the tube ends of seamless and welded ferromagnetic steel tubes for the detection of laminar imperfections

American Society of Testing and Materials (ASTM)

ASTM E1444-05
ASTM A 275/A 275M Test Method for Magnetic Particle Examination of Steel Forgings
ASTM A456 Specification for Magnetic Particle Inspection of Large Crankshaft Forgings
ASTM E543 Practice Standard Specification for Evaluating Agencies that Performing Nondestructive Testing
ASTM E 709 Guide for Magnetic Particle Testing Examination
ASTM E 1316 Terminology for Nondestructive Examinations
ASTM E 2297 Standard Guide for Use of UV-A and Visible Light source and Meters used in the Liquid Penetrant and Magnetic Particle Methods