Guide to Magnetic Particle Inspection (MPI)

Magnetic Particle Inspection (MPI) stands as a crucial nondestructive testing process utilized to detect surface and shallow subsurface discontinuities in ferromagnetic materials. These materials encompass iron, nickel, cobalt, and certain alloys susceptible to magnetization. The method involves establishing a magnetic field within the test object, achieved through direct or indirect magnetization. Direct magnetization passes electric current through the test object, forming a magnetic field within the material. In contrast, indirect magnetization applies a magnetic field externally without passing electric current through the test object. This process induces a condition where any presence of surface or subsurface discontinuity allows magnetic flux to leak, attracting ferrous particles—either dry or suspended in a wet medium—to form an indication. This indication is then assessed to determine its nature, cause, and necessary action.

Electrical Current Variants

Several electrical currents are employed in magnetic particle inspection, selected based on considerations like part geometry, material, desired discontinuity type, and required magnetic field penetration depth.

Alternating current (AC): Primarily used to detect surface discontinuities due to its limited penetration depth caused by the skin effect—a phenomenon where the current runs along the part’s surface. The frequency of AC determines its depth of penetration.

Full wave DC: Utilized to detect subsurface discontinuities beyond the reach of AC. The depth of penetration depends on the current passing through the part.
Half wave DC (HWDC, pulsating DC): Similar to full wave DC but offers enhanced mobility of magnetic particles, aiding in detecting surface-breaking indications and penetrating the part more effectively than full wave DC.
AC electromagnet: Preferred for finding surface-breaking indications. However, detecting subsurface indications with an AC electromagnet poses challenges.

Steps of Magnetic Particle Inspection

Magnetization:

The first step in MPI is to magnetize the test specimen. This is done by either using a magnetic yoke, a permanent magnet, or an electromagnet. The magnetization induces a magnetic field in the material being tested.

Application of Magnetic Particles:

Magnetic particles in the form of a dry powder or a wet suspension (containing finely divided iron or iron oxide particles) are applied to the surface of the material. These particles are typically colored with a fluorescent or contrasting dye for better visibility.

Inspection Under Ultraviolet (UV) or White Light:

The magnetized part is inspected under ultraviolet (UV) light or white light, depending on the type of magnetic particles used. UV light is often used when fluorescent particles are applied. The presence of a magnetic field causes the magnetic particles to align along the lines of magnetic flux, creating a visible indication at the location of a defect.

MAGNETIC PARTICLE INSPECTION MPT — MAGNETIC PARTICLE INSPECTION (MPI)

Indication Interpretation:

Trained inspectors examine the surface for indications, which appear as lines or clusters of magnetic particles. These indications are called “magnetic particle indications” or “magnetic indications.” The size, shape, and location of these indications are analyzed to determine their significance and whether they meet specified acceptance criteria.

Demagnetization:

After the inspection is complete, the test specimen is usually demagnetized to remove any residual magnetism. This can be done using techniques like AC demagnetization or decreasing the magnetic field gradually.

Reporting:

The results of the Magnetic Particle Inspection are documented in a report, which includes information about the inspection process, the location and characteristics of any indications found, and the disposition of each indication (acceptable or reject).

Advantages & Limitations of Magnetic Particle Inspection

Criteria	Advantages of MPI	Limitations of MPI
Defect Detection	Can find both surface and near sub-surface defects	The specimen must be ferromagnetic (e.g., steel, cast iron)
Portability and Cost	Some formats are extremely portable and low cost	Paint thicker than about 0.005″ must be removed before inspection
Speed and Immediate Results	Rapid inspection with immediate results	Post cleaning and post demagnetization are often necessary
Visibility of Indications	Indications are visible to the inspector directly on the specimen surface	Maximum depth sensitivity is typically quoted as 0.100″ (deeper under perfect conditions)
Defect Detection Capability	Can detect defects that have been smeared over	Alignment between magnetic flux and defect is important
Versatility in Part Shapes	Can inspect parts with irregular shapes (external splines, crankshafts, connecting rods, etc.)	–

Magnetic Particle Inspection is commonly used in industries such as aerospace, automotive, construction, and manufacturing to detect surface and near-surface defects in materials, particularly in welds, castings, forgings, and other critical components made of ferromagnetic materials. It is a relatively quick and cost-effective NDT method, making it suitable for production line testing and field inspections. Proper training and adherence to industry standards are essential for the accurate and reliable application of MPI.

Standards

Standards International 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