INTRODUCTION TO MAGNETIC PARTICLE AND FLUX LEAKAGE TESTING

Magnetic Particle Testing (MPT) is an NDT method used to detect surface and near surface flaws in ferromagnetic materials such as steel and iron. The technique uses the principle that magnetic lines of force (flux) are distorted by the presence of a flaw in a manner that will reveal it's presence. The flaw (for example, a crack) is located from the "flux leakage", following the application of fine iron particles, to the area under examination.

The iron particles can be applied dry or wet; suspended in a liquid and coloured. For the most sensitive applications, Fluorescent coated particles are used, and inspection is carried out under an Ultra Violet light. This enhances the detection even more. For near surface defects, the effectiveness quickly diminishes depending on the flaw depth and type. The image is more sharp if the flaw is closer to the surface. Surface irregularities and scratches can give misleading indications. Therefore, it is necessary to ensure careful preparation of the surface before MPT is undertaken. Defects which are perpendicular to the lines of force are detected efficiently.


Magnetisation Methods

For magnetisation of components, A.C, D.C. and HWDC are used. While AC methods are ideal for detection of shallow surface defects and DC or HWDC methods are preferred for detection of near-surface defects. Different methods of magnetisation are :
� Longitudinal magnetisation (coil wrapping over component, detects radial cracks)
� Circular magnetisation (passing current through component, detects longitudinal cracks)
� Yoke magnetisation (longitudinal magnetisation, adjustable legs, portable)
� Prods (Circular magnetisation, inspection of welds, burning/damage of surface)

A component is usually magnetised in more than one direction because detection of sensitivity of each method maximum along one direction. Indications of discontinuities are preserved by photography or video recording or by the use of peel off transparent adhesive films.

The Detectables

MPT can be used for detection of cracks, blowholes, laps, non-metallic inclusions, and segregation etc. Under optimal conditions, and with very good surfaces, detection of defects of about 0.5mm long can be achieved (depths from about 0.02mm). The sensitivity of MPT depends on the magnetisation method and on the electromagnetic properties of the material tested as well as on the size, shape and orientation of the defect.

Demagnetisation

Demagnetistion of the component is often specified after MPT to avoid electromagnetic interference, arc deflection, arc blow and other build up of particles. Demagnetisation is carried out by subjecting the component to continuously reversing and reducing magnetic field.

Required Care

In MPT, utmost attention is paid for reliable detection of defects due to the underlying fact - a defect detected is almost characterised to the maximum possible extent. In other words, scope does not exist in MPT to apply signal processing methods for enhanced detection and accurate characterisation of defects as practised in ultrasonic, eddy current and other NDT methods. In light of this, magnetisation methods, amperage, powders, carrier fluids, sprinkling methods, viewing conditions and recording methods etc. are carefully tailored such that an existing defect (within the detection limit of the test procedure) does not go undetected. For example, dry powder methods are employed if large discontinuities (>1 mm), especially the sub-surface ones are expected. Red coloured powders are preferred on dark surfaces and black coated powders on hot objects (up to 400� C). On the contrary, to detect small and shallow surface defects such as tight fatigue cracks, wet fluorescent methods with black light illumination are resorted to. The size of powder has to be small in both dry (upto 150 microns) and wet (upto 25 microns) methods to enable detection of smaller discontinuities by easy migration and build up of powder particles.

Typical Aplication

Wet fluorescent MPT method is routinely applied as part of in-service inspection programme of low-pressure (LP) side turbines for detection of fatigue cracks, corrosion damage in rotors and blades.

Caution

It is commonly thought that MPT is relatively a simple method and training is usually overlooked. The consequences of such an assumption are missing of harmful defects due to improper magnetisation/demagnetisation, inaccurate calibration of equipment, inadequate illumination, inaccurate particle concentration, and misinterpretation. It is all the more essential to use Gauss meters for measurement of magnetic fields, quality indicators (shims) for controlling the field strength and verifying field direction and more importantly, the Ketos ring for establishing the detection sensitivity.

MPT Limitations

One major limitation of MPT is that only ferromagnetic materials can be tested. Another limitation of MPT is the impossibility to characterise depth and orientation of defects. A large near-surface defect and a shallow surface defect may give identical indications causing uncertainty. To classify such indications into surface and near-surface, other NDT methods such as visual testing are necessary.


MAGNETIC FLUX LEAKAGE (MFL) TESTING

In contrast to MPT, localised magnetic leakage fields are detected in MFL testing using sensors such as inductive coils, Hall elements, magnetometers and magetodiodes. Use of sensors in MFL testing enables automatic testing and quantitative evaluation without human inspectors. The sensor output depends on the size and orientation of the defects as well as on the level of magnetisation and the inspection speed. MFL testing is widely used for inspection of oil storage tank floors and pipes (internal/external), steel wire ropes under water structures and highly irregular components.

Unlike in MPT, the magnetisation levels are usually low and high strength rare earth magnets are commonly used for magnetisation. Since magnetisation is local, demagnetisation is usually not required. The amount of leakage flux is dependant on depth, orientation, type and position (topside or bottom-side) of the defect, material permeability and magnetisation level.

In general, the MFL unit comprising of magnets and sensors is scanned at uniform speed and the sensor output is recorded continuously. MFL units can be portable, battery-powered, and compact. For inspection of long oil pipelines, which run a few hundreds of kilometres, pipe inspection gauges (PIGs) housing the MFL units are widely employed for detection and evaluation of corrosion damage. Recently one such PIG has been developed at BARC, India for the inspection of oil pipelines. PIGs consist of MFL unit, stand-alone battery supply, data analysis and processing computers and other supporting electronics for acquiring and transmitting data to a remote log station, where evaluation is carried out.

MFL method is applicable for inspection of tankfloors involving thickness upto 15 mm. Selection of sensor is important as it decides the success of MFL testing. Though Hall sensors are undeniably more sensitive than inductive coils for measurement of leakage fields, they are too sensitive to surface conditions and this results in an unreliable inspection and the generation of significant false calls. Hence, for example, for the inspection of tubes, the preferred sensor is the traditional humble coil due to stability and reliability.

Further Reading

Magnetic Particle Inspection: A practical guide, D.J. Lovejoy, Chapman & Hall, 1993.
Practical NDT testing, Baldev Raj, T. Jayakumar T and M.Thavasimuthu, Narosha, New Delhi, 1997