Magnetic Particle Testing for Surface and Near-Surface Cracks

Cracks that lie at or just under the surface of a ferrous component are what magnetic particle inspection finds. It picks up tight defects that penetrant testing would miss, such as in-service fatigue cracks that have closed under load and creep cracking in high-temperature piping, because it does not rely on the defect being open enough to admit liquid. itmad provides magnetic particle testing (MT/MPI) using yoke, prod and bench techniques, visible and fluorescent, to ASTM E1444, ISO 9934 and ASME V Article 7. itmad operates across the Middle East, Asia and Europe. Last reviewed: October 2026.

How the method works

Magnetising the component, whether with an applied external field or by passing current through it, sets up magnetic flux in the material. Any discontinuity distorts the flux and pushes part of it out through the surface as a leakage field. Fine ferrous particles applied to the surface are pulled to that field and build up along the edges of the discontinuity, so the indication is far larger than the defect itself.

A usable leakage field only forms when the magnetic field crosses the defect at right angles, so each area is examined twice, in directions about ninety degrees apart. Examining in just one direction misses cracks that run parallel to the field, and that is the commonest reason a competent operator misses a defect.

Magnetising techniques

The electromagnetic yoke is the most widely used field technique: an articulated yoke is placed on the surface and produces a longitudinal field between its legs. It is portable, needs no electrical contact with the component and leaves no arc strike risk, which is why it is preferred on finished and in-service equipment, and its lifting power is checked against the standard before use. In the prod technique, current is passed directly through the component between two electrodes to produce a circular field. It is effective on heavy sections and large weldments but carries a risk of arc strike and local heating, so it is restricted or prohibited on some materials and finished surfaces.

Bench techniques, namely head shot through contact heads, encircling coil and central conductor or threaded bar magnetising, are carried out on a magnetic particle bench and are used for manufactured components where controlled, repeatable magnetisation in both directions is required. Permanent magnets and coils are used where power is unavailable or the geometry suits, with field strength verified using a flux indicator, artificial flaw shim or Hall effect meter as the procedure requires.

Visible and fluorescent particles

Colour contrast MPI uses visibly coloured particles against a contrasting background lacquer and is examined under white light, with a minimum of 500 lux at the surface. Fluorescent MPI uses particles coated with fluorescent dye, examined under UV-A with a minimum of 1000 µW/cm² at the surface in a darkened area, and it offers substantially higher sensitivity for fine cracks. Both dry powder and wet suspension methods are used, with wet fluorescent generally applied where the smallest detectable defect matters, in aerospace and rotating component work in particular.

Field strength verification and demagnetisation

Field adequacy is verified and not assumed, using a flux indicator, artificial flaw shim or Hall effect meter as the procedure requires. Components are demagnetised after examination where residual magnetism would interfere with machining, welding, instrumentation or service performance, and the residual field is measured to confirm it.

Standards applied

ASTM E1444 is the standard practice for magnetic particle testing and ASTM E709 the guide, with the ISO 9934 series covering general principles, detection media and equipment. ASME Boiler and Pressure Vessel Code Section V, Article 7, applies with acceptance to the relevant construction code section, alongside AWS D1.1 for structural steel welding, and client, project and manufacturer specifications apply where they are more stringent.

Personnel qualification

The technicians who examine and interpret hold ISO 9712, ASNT SNT-TC-1A Level II and PCN qualifications as the job demands, with NAS 410 for aerospace components, and Level III oversight covers procedure approval and technique validation.

Typical applications

Welded joints in pipework and structures; pressure vessel welds and nozzles; crankshafts, camshafts and connecting rods; engine blocks, bearing caps and gears; landing gear and aerospace components; crane hooks, pins and lifting components; motor and drive shafts; studs, bolts and threaded bar; castings and forgings; and steam piping and high-temperature components. The work is done on site, in fabrication yards, offshore or at itmad premises.

Limitations

The method works only on ferromagnetic materials. It cannot be used on aluminium, copper, titanium or austenitic stainless steel, for which penetrant or eddy current examination applies instead. Detection is limited to surface and near-surface defects, so deeper flaws need ultrasonic or radiographic methods, and coatings must generally be removed or their thickness accounted for, since surface condition affects sensitivity. Where MT is the wrong method for the defect to be found, itmad says so.

What the client receives

A report that identifies the components examined by weld number or part reference, with the magnetising technique, particle type and field direction applied. Field strength verification records and equipment calibration traceability are included, and indications are recorded with location, dimension and character, supported by photographs. The report assesses the findings against the acceptance criteria in the specification, confirms demagnetisation with residual field readings where applicable, and gives the technician's qualification level and certification reference.

Frequently asked questions

What is magnetic particle testing?

A non-destructive method for detecting surface and near-surface cracks in ferromagnetic materials. The component is magnetised, and fine ferrous particles gather at the leakage fields created by discontinuities, forming indications larger than the defect itself.

Which materials can be tested?

Only ferromagnetic materials, such as carbon steel, low-alloy steel, martensitic stainless steel and cast iron. Aluminium, copper, titanium and austenitic stainless steel are non-magnetic, so penetrant or eddy current testing applies to them instead.

Why is the test done in two directions?

The field must be perpendicular to the defect to produce a usable leakage field, and a defect parallel to the field gives no indication. Every area is therefore examined in two directions at about ninety degrees.

What magnetising techniques are used?

The electromagnetic yoke, preferred on finished and in-service equipment because it leaves no arc strike risk, the prod technique for heavy sections, bench techniques for manufactured components, and permanent magnets and coils where power is unavailable. Field strength is verified in each case.

What is the difference between visible and fluorescent MPI?

Visible MPI uses coloured particles under white light of at least 500 lux, while fluorescent MPI uses dyed particles under UV-A of at least 1000 µW/cm² in a darkened area and is substantially more sensitive to fine cracks. Wet fluorescent is generally used where the smallest detectable defect matters.

Which standards apply?

ASTM E1444, ASTM E709, the ISO 9934 series, ASME Section V Article 7 and AWS D1.1, with client, project and manufacturer specifications where they are more stringent. Acceptance is to the relevant construction code section.