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Non-destructive Examination (NDE) Part 1 Liquid Penetrant and Magnetic Particle Inspection

Fig. 1 Principles of LPI

In achieving high quality defect free welds there is no substitute for experienced and qualified welders and competent supervision. However, no matter how skilled the welder, during the process of depositing weld metal, imperfections of various types may be formed. It is therefore necessary to have methods of ensuring that the weld is of an acceptable quality, hence the development of a range of non-destructive inspection techniques capable of both detecting and sizing buried and surface breaking imperfections, enabling a decision to be made regarding acceptance or otherwise.

Note that both the ISO specifications and the ASME codes differentiate between an ‘imperfection’ (ISO) or a ‘discontinuity’ (ASME) and a ‘defect’. It is accepted that all welds contain features or imperfections but it is only when an imperfection exceeds the relevant acceptance standard does an imperfection become a defect – as ASME states “..... this term designates rejectability...”   

Liquid penetrant (LPI) and magnetic particle (MPI) inspection techniques are methods that supplement visual inspection, revealing defects such as fine cracks or micro-porosity that would be invisible or difficult to detect by the naked eye.

LPI is a simple, cheap and easily portable inspection method that requires no equipment apart from spray cans. It can detect surface breaking imperfections only and relies on a coloured or fluorescent dye, sprayed on the surface and penetrating the imperfection. About15 minutes is generally specified to enable the dye to penetrate any very fine imperfections. After cleaning the excess the dye is drawn to the surface by spraying on a developer in the case of the colour contrast dye or exposing the surface to ultra-violet light in the case of a fluorescent dye, the imperfection being revealed by the dye staining the developer or fluorescing, as shown schematically in Fig. 1 and in Fig. 2, a liquation crack in an aluminium alloy.

The fluorescent dye gives greater sensitivity than the colour contrast dye and does not require the use of a developer. It does however require the use of an ultra-violet light source and preferably a darkened room which makes it a less portable inspection method than the contrast dye technique.

The dye used as a penetrant must be able to penetrate tight cracks but must not be capable of being removed from more open imperfections during the cleaning operation carried out prior to applying the developer.

Fig. 2 LPI test result – cracking in the HAZ of an aluminium alloy weld

Careful surface preparation and thorough cleaning of the item before applying the penetrant is important. Swabbing with or immersion of the item in a proprietary degreasant is generally sufficient – cleaning in an ultrasonic bath is the best method but can be used only for small portable components. Grinding or wire brushing, particularly of materials such as copper and aluminium alloys, should be avoided if possible as such cleaning methods can smear over imperfections, making them undetectable. If such is the case an acid etch may be required to remove the smeared metal and enable the dye to penetrate the imperfection
Inspection in positions other than flat can be a problem but penetrants are available with a jelly like consistency that can be used in the vertical and overhead positions. It is possible to automate the process with small components loaded into baskets and processed on a conveyor line. Fluorescent dyes are better than contrast dyes in this application due to their greater sensitivity.

Although a simple inspection process to use, interpretation can be a problem if the surface is naturally rough – coarsely ground or rough machined for example - or contains acceptable geometric features that trap the dye. Training of operatives to recognise genuine and spurious indications is therefore essential. Other limitations are that it can be used at room temperature only and it is not possible to indefinitely retest components as the imperfection becomes filled with dry dye. Health and safety may also be an issue with irritation of unprotected skin and fumes from some of the cleaning and solvent chemicals, particularly when the process is used in confined spaces.

The process can however be used to inspect both ferrous and non-ferrous metals, large areas can be examined very quickly and it can be used on components with complex geometry.

Magnetic particle inspection (MPI) is also a simple-to-use inspection method but, as the name suggests, is limited in use to magnetic materials- in other words ferritic (NOT austenitic) steels. The basic principle is that the component is magnetised, producing a flux within the metal as shown in Fig 3 An imperfection is non-magnetic and therefore cuts the lines of flux producing a leakage field around the imperfection –a localised “magnet”. A magnetic powder sprayed or dusted onto the surface will be attracted to this “magnet” forming a line of powder. The strength of the magnetising current should be specified in a written examination procedure and the adequacy of the magnetic field verified by the equipment being capable of lifting a specified weight. 

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Fig. 3 Principles of MPI

Conventionally the item to be inspected is spray painted with a thin coat of rapid drying white paint, the magnetic field is applied and a black magnetic ink is sprayed onto the surface, forming a black indication against the white background. The maximum imperfection sensitivity is when the imperfection cuts the magnetic flux at 90O. In order that both longitudinal and transverse oriented imperfections are detected, examination of a weld must therefore be carried out with the magnet applied twice at 90O along and across the weld . 

Magnetisation may be by prods, electromagnetic yokes as shown in Fig 4, or permanent magnets. Inspection by the use of prods supplied with high amperage low voltage alternating or direct current is often used in the workshop, the local magnetisation being achieved by two prods connected to a transformer or transformer/rectifier. The prods are pressed onto the metal surface, a trigger pulled to initiate a current in the component and the magnetic ink applied. This is generally a two man operation. Care must be taken not to initiate the current before the prods are in firm contact with the surface as arcing between the prod tips and the component can occur, resulting in a feature similar to a welding arc strike. The use of rectified half wave single phase direct current has an advantage over alternating current and the yoke in that the process is capable of detecting imperfections up to perhaps 1mm below the surface, depending on their size and orientation.

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Fig. 4 MPI using an AC yoke

The yoke method, illustrated in Fig 4, has several advantages over the prod method. The equipment is relatively small and lightweight; can be battery powered and is readily portable, making it ideal for site inspection. The yoke can be operated in one hand and the magnetic ink sprayed on from the other making this a one man operation. In addition no electrical current is transferred into the component.

Spurious indications can be produced where there is a difference in magnetic properties within the joint, perhaps in the HAZ where it may be mistaken for unacceptable undercut. Two metals of different magnetic characteristics when joined together can give a well defined indication suggesting the presence of a crack – a careful dressing of the surface followed by LPI is helpful in deciding whether or not the indication is genuine. Ferritic/austenitic dissimilar metal joints cannot be MPI’d. Residual magnetism can also cause problems in interpretation. As with LPI rough uneven surfaces may also give rise to spurious indications.

Neither technique gives a permanent record of the inspection but where this is necessary photographs of the affected area are very useful. Conveniently positioned reference markers and a scale are helpful for the accurate recording of the size and position of indications, particularly if repairs are required. It is also possible to transfer the indication onto transparent sticky tape by carefully pressing the tape onto the surface and then applying this to a sheet of white paper.

This article was written by Gene Mathers.