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

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Introduction

Visual techniques are widely used to ensure product reliability during manufacturing and to examine any gross discrepancies on the surface of operating components. These techniques involve illumination of object surface with light and examination of the reflected light using visual aids, usually at magnification (McIntire P and Moore P O, 1996). Visual examination can reveal gross surface defects, cleanliness, foreign objects, surface condition, mismatches and any other discrepancies (Baldev Raj et al 1997). Visual techniques are easy to apply and are considered to be the most effective and the least expensive NDT techniques.

Visual techniques are as old as the telescopic devices used for human organs without operative procedures. Phillip Bozzini was the first to develop cytoscopes way back in 1806 for the purpose of medical research. They were modified for examination of holes such as gun bores and hence, called borescopes. Since then, a variety of flexible and rigid borescopes and more powerful and efficient optical aids have been developed for quick examination of products during manufacturing and other real life situations. Pioneering work in this direction was carried out by John Lang, who developed closed circuit television based borescopes for inspecting inner surface of helicopter blades, jet engines, wings, turbine blades, etc (McIntire P and Moore P O 1996). Recent advances in microelectronics, computer technology and artificial intelligence have popularised the concepts such as machine vision for realising automated visual examination techniques and unmanned inspection stations. This article discusses the details of visual techniques for examination of surfaces. Typical instruments, testing methods, industrial applications and latest developments are also covered.

1. Instruments for Visual Testing

The human eye is an excellent sensor and with that, it is possible to easily perceive many material characteristics such as shapes, colours, gloss, shades, speeds, perspective etc. and discontinuities in them. The human eye is an important component for performing visual NDT. Visual examination carried out by an experienced inspector can reveal the general condition of the component. Usually, visual techniques are used for examining cleanliness, misalignments and other mismatches, foreign objects etc.

Optical aids are usually recommended for visual examination, essentially for magnification purpose and also for inspecting the inaccessible areas. For the examination of inside surfaces of tubes, bores and chambers, boroscopes, endoscopes, telescopes are used (Baldev Raj et al 1997). The length and diameter of the borescope can be varied depending on the dimensions of the object. Extension sections are available in 1, 2, 3 m lengths, permitting assembly of borescopes upto 10 m. Various designs of borescopes are used for different conditions. These include angulated, calibrated, panoramic, wide field, ultraviolet, waterproof, and gas cooled designs.

In recent times, with the availability of flexible fibre-optic borescopes, charge coupled device (CCD) cameras, and computer based image processing software, it is possible to examine corners, bent surfaces, and inaccessible surfaces. Using these instruments, it is possible to take sharp and clear images of parts and interior surfaces and make quantitative evaluations. Most of the flexiscopes possess a wide-angle objective lens that provides a 100? filed of view, and adjustable focus. Usually, for industrial use, they are more ruggedly constructed by wrapping the fibre optic systems with flexible steel lining. The diameter and length of the flexiscopes are usually adapted depending on the requirements. Selection of a visual instrument mainly depends on factors such as the object geometry and the access, expected defect size and resolution requirements.

The five basic elements in a visual test are the test object, the inspector, the optical instrument, illumination and recording. Each of these elements interacts with the others and affects the test results. The objective distance, object size, discontinuity size, reflectivity, entry port size, object thickness and direction of view are all critical aspects of the test object that affect the visual test. Reflectivity is another factor affecting illumination. Dark surfaces such as those coated with carbon deposits require higher levels of illumination than light surfaces do.

In many situations, in order to aid vision, magnification with power ranging from 1.5X to 2000X is employed. Depending on the working distance and the field of view various lower, medium and high power magnification systems (microscopes) are used. With high power systems, it would be possible to achieve resolution of a few microns. The defect size usually determines the magnification and resolution required for visual testing. For example, greater resolution is required to detect hairline cracks in welds than to detect an undercut.

2. Surface Examination Using Visual Techniques

Visual techniques are probably the simplest, quick, and widely used NDT techniques for the examination of material surfaces (McIntire P and Moore P O 1996). They are also used to verify the presence or absence of cracks, corrosion and other forms of in-service material degradation. Typical industrial applications of visual techniques are given in Table 1. In many situations, quantitative evaluation as regards to type, location and orientation are also possible. The unique advantage of many visual techniques is their ability to yield quantitative information more readily than any other NDT tests.

Visual testing is performed in accordance with applicable codes, standards, specifications and procedures. For example, visual testing of a nuclear reactor vessel and its internal components is performed according to the rules of the plant’s in-service test program and special requirements of regulatory agencies, e.g. American nuclear regulatory commission. Majority of the tests meet the requirements of the ASME Boiler and Pressure Vessel code, which forms a part of the in-service inspection program. For example, Section XI recommends visual testing for the examination of condition of a part, component or surface, for identification of leaks and for the examination of mechanical and structural conditions. The code also gives detailed test procedures. Qualified personnel are required to carry out the visual tests.

In most situations, it is specified that the test surface should be free of slag, dirt, grease, weld spatter or other contaminants. Before visual testing, personnel are usually given basic near vision acuity and colour recognition screening tests. Near vision measurements are recorded for each eye and for both eyes. Similarly, the angle of view is very important during visual testing, especially when quantitative information is to be obtained. It is essential that the inspectors attempt to observe the object surface at the center axis of the eye. The angle of view should not be more than 45? from the normal. Similarly, the period of time during which a human inspector is permitted to work is usually limited to about 2 hours on continuous basis to avoid errors concerning visual reliability and discrimination. There are several integrated visual testing variables beyond equating near vision acuity to performance including lighting, knowledge of crack pattern recognition, orientation of test object, psychological factors and test instructions.

The data produced from almost all types of NDT tests are usually recorded and interpreted visually. For this reason, almost any NDT test could be considered a visual test, particularly at the detection or interpretation stages. With magnetic particle, liquid penetrant, radiography, and some leak tests, the link is easily evident. The same is the case with the in-situ metallography and other microscopy methods. Visibility criteria are specified for magnetic particle tests and liquid penetrant tests, especially when fluorescent systems that use black light are used for achieving enhanced detection sensitivities.

3. Latest Developments

The basic design of the borescopes, which has been in use for many decades, has been modified accommodating the state-of-the-art advances in video, illumination, robotic, optical and computer technologies. Developments in image processing, artificial intelligence, video technology and other related fields have significantly improved the capability of visual techniques (Forsyth et al. 1998). Figure 2 shows typical application of visual technique to examination of wheels for detection of very fine cracks originate due to residual stresses. Liquid penetrant tests could not detect secondary cracks. Visual technique using video-microscope with imaging processing capability has clearly revealed a secondary crack as depicted in Fig. 2.

Present day demand for higher performance and faster production exceed the abilities of visual tests by humans. Consequently, visual tests made by human eye are being replaced by automated visual testing using optical instruments and unstaffed inspection stations. Such aspects are usually referred to as machine vision. In essence, the machine vision acquires processes and analyses images to reach conclusion automatically. A typical machine vision system consists of a light source, a video camera, digitiser, a computer and an image display. Usually, the test object is illuminated and the image is captured using a video camera for processing by computer. The computer first enhances the contrast of the image with a procedure called image enhancement (Gonzalez R G and Wintz P, 1987) and later, the image is segmented for feature extraction and finally for classification using the power of artificial intelligence. Fourier analysis, multivariant analysis and statistics are increasingly being applied to evaluate invariant parameters from the image data for their use in automated object recognition and machine vision (Dougherty E R, Giardina C R 1988). Further, laser based scanning systems are being developed for on-line measurement and evaluation of volume and average size of wood chips or iron ore pellets and the detection of cracks in asphalt or wood planks. Other recent developments include D-sight, edge of light (Forsyth et al. 1998) techniques. While later is yet in developmental stages, the former method has already found its way to practical use.

Concluding Remarks

Visual techniques are the simple, quick, and widely used NDT techniques to examine the material surfaces for qualitative as well as quantitative assessment of gross discrepancies. Surface examination using visual techniques encompassing not only optical considerations and image processing but also the peripheral technologies such as electronics, computers, process control management (refer Fig.1). Driven by the demand for higher performance and faster industrial production, advancing trends in automated visual testing are expected to continue into the future.

Bibliography

McIntire P and Moore P O, 1996, Visual and optical testing, Vol 8, ASNT

Baldev Raj, Jayakumar T and M.Thavasimuthu, 1997, Practical NDT testing, Narosha, New Delhi

Forsyth DS, Komorowski J P, Gould R W and Marincak A, 1998, Automation of enhanced visual NDT techniques, Proc. 1st Pan-American Conf. for NDT, Toronto, Canada, pp 107-117

Gonzalez R G and Wintz P, 1987, Digital image processing, Addison-Wesley Publishing Co.

Dougherty E R, Giardina C R, 1988, Mathematical methods for artificial intelligence and autonomous systems, Prentice Hall Inc.

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