Page 84 - Quick Guide to Welding and Weld Inspection by S.E. Hughes, Clifford Matthews

Non-destructive and Destructive Testing
2
in a darkened room with a light intensity of 1000 W/cm on
the surface being viewed.
Advantages
. Can test most materials including non-magnetic ones.
. Cheap and simple to use.
Disadvantages
. Can only find surface breaking defects.
. Good surface preparation and cleaning is required.
Magnetic particle testing (MT)
Magnetic particle testing is used to find mainly surface
breaking defects in ferromagnetic materials. Sometimes it is
possible to find slightly subsurface defects when used with a
permanent magnet or d.c. electromagnet. A magnetic flux (or
field) is introduced into the material and any defects cutting
across the magnetic flux can be detected when ink or powders
containing ferromagnetic particles (iron filings) are applied to
the material. What happens is that a flux leakage occurs at
the defect, which effectively makes the defect a magnet in its
own right. This ‘magnet’ attracts the ferromagnetic particles,
which take the shape of the defect. Figure 6.2 shows the
arrangement.
The magnetic flux can be introduced from:
. a permanent magnet;
. an electromagnet (either a.c. or d.c.);
. electric prods (either a.c. or d.c.) between which a current
flows (and the current flow is surrounded by a magnetic
field).
Ferromagnetic particles can be applied to the material as:
. a black ink (viewed against a pre-applied white contrast
paint);
. fluorescent ink (viewed under UV light conditions);
. red or blue dry powders (used at higher temperatures).

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Woodhead Publishing Ltd – A Quick Guide to Welding and Weld Inspection
Data Standards Ltd, Frome, Somerset – 17/9/200906QG Welding chap6.3d Page 69 of 87