Residual technique

The residual technique uses only DC or rectified forms of AC to magnetise a component because it is the residual flux density which is relied upon to attract magnetic particles to the flux leakages created by defects. Direct current and rectified AC produce a full cross section magnetisation, whereas AC will only create an effective flux density in the skin, hence skin effect. Thus, the residual field from AC is not considered adequate for the residual technique.

Also the residual technique is only applicable on components which have high retentivity, that is high carbon equivalent steels. It usually follows that components suitable for the residual technique are high tensile machine parts, often when we are looking for flaws in corners or thread roots etc. If the continuous technique is used on these parts there will be a high build up of detecting media across such features and these non-relevant indications are likely to mask an actual defect beneath them. For best defect sensitivity the detecting media is applied after magnetisation and to allow time for the particles to migrate. Inspection takes place a short time after that.

Table 6 lists the steps in a one shot residual technique. The magnetising values should be the higher ones recommended for aerospace, using the appropriate electrical current waveform. Again, circular magnetism shots should be done before longitudinal, as invariably demagnetisation will be necessary.

1. Demagnetise 2. Clean

3. Affix magnetising contacts

4. Apply magnetising force, not AC, 2-3 sec 5. Apply detecting media, spray or dip 6. Wait, 30 sec-1 min

7. Inspect 8. Demagnetise 9. Clean

10. Protect

Table 6. Residual technique 6.5 Fluorescence and the Electromagnetic Spectrum

Fluorescence is the property of some materials to absorb

electromagnetic energy of one wavelength and re-emit the energy at another . The ultraviolet and visible light section of the spectrum, which is of interest in MPI, lies between 100 and 800 nm, see Fig.27.

(A nanometre (nm) is 1 millionth of a mm.) In MPI long wavelength ultraviolet (black light) light sources are used having a waveband between 315 and 400nm. This is UV-A radiation. Fluorescent inks absorb energy at approximately 365nm and re-emit at about 550nm.

10^-10 10^-8 10-6 10^-4 10-2 1cm 102 10⁴ 106 10⁸ Wavelength

Electric Waves

TV Microwaves

Infra red Ultra

violet Industrial

radiography

Figure 27 : Ultraviolet spectrum 6.5.1 Types of UV-A Lamp

By far the most common type of light source used to inspect components tested with fluorescent ink is the mercury vapour arc lamp. In fact, the mercury arc lamp is a street or

workshop lamp which has a filter over it to reduce the visible light to a minimum but allow the UV-A to be transmitted.

The filter is called a Woods in the UK and a Kopp in the USA.

On the Philips type of lamp the filter is integral with the outer envelope but on the Magnaflux unit, using either a GE or Westinghouse lamp, it is separate.

The mercury arc is drawn between electrodes enclosed in a quartz tube. The resistor limits the amount of current in the starting electrode. The quartz tube is mounted and enclosed in the outer glass envelope which serves to protect it and filter out any possible hazardous radiations.

400W mercury vapour arc flood-lamps can be used where very large components are tested or to give a background illumination in an inspection area. However, background light in a darkened area can be more economically provided by UV strip lights.

6.5.2 Safety Precautions and Operating Instructions

Under normal working conditions, there are no known long term harmful effects arising from the use of UV-A (black light) sources, providing simple safety precautions and operating instructions are observed. The precautions and instructions in these notes are general. For full advice the manufacturers' data on a particular light should be followed.

Safety precautions when using a UV-A mercury vapour arc lamp

a) Avoid looking directly at the light source

b) The light must not be used without a correctly fitted filter c) Do not operate the light with a chipped or cracked filter d) Avoid contact with the lamp housing as it becomes hot e) Keep the light cables away from liquids, to avoid

contamination or shorting

f) Ensure that regular electrical earth continuity checks are carried out on the lamp unit

6.5.3 Operating instructions for a UV-A lamp

a) Allow 5 minutes warm up period after switch on, before inspecting with the lamp

b) If the lamp is switched off and then immediately switched on again, allow a minimum of 10 minutes before recommencing inspection. The bulb will not re-light until its temperature reduces.

c) Avoid repeated switching on and off, as this will reduce bulb life significantly

d) Angle the light with respect to the specimen being inspected, to avoid reflections which reduce inspection efficiency

e) Clean the lamp filter regularly, with lint-free material moistened with a mild detergent/water solution f) Check the light output of the lamp regularly. This

should be done in accordance with paragraph 6.1 of BS4489. The lamp must achieve a UV-A irradiance level of 0.8mW/cm² at the testing surface.

7. DEMAGNETISATION

British Standard 6072 recommends that demagnetisation should only be carried out if specifically requested. In certain industries the consequences of not demagnetising can be catastrophic.

Demagnetisation should be carried out:

1. before testing, if residual fields could affect test results

2. between tests except for when a similar shot is to be applied but at a higher amperage. An exception can be made if a subsequent shot is to be applied at 90° to the original and the original field strength is to be exceeded

3. after testing, when applicable Post demagnetising must be done:

1. on aircraft parts, where magnetic compasses and electronic equipment may be affected

2. on rotating parts, where magnetic debris might adhere and cause excess wear

3. where automatic arc or electron beam welding is to be carried out and arc wander may be caused by residual magnetic fields

4. if residual magnetic fields may affect subsequent machining processes. Reamers and taps become magnetic as well and thus can break in use, if swarf is not cleared from flutes.

5. when a high quality finish, such as electro-plating is to be applied. The particles attracted will prevent or reduce adhesion.

It is not usually necessary to demagnetise specimens which are to be heat treated. This is provided that the heat treatment is beyond the Curie point, about 700°c. At and above the Curie point, ferromagnetic materials

become paramagnetic.

Often it is not possible or practical to demagnetise a specimen completely, therefore a maximum residual field level must be agreed. An agreed

deflection on a calibrated or uncalibrated magnetic field indicator is the most common.

For critical situations a compass test is recommended. The component under test is positioned at an agreed distance from a suitable compass and rotated through 360°. The compass needle must deflect by less than 1°.