Magnetic Particle Testing - Tecsud

Magnetisation techniques
in accordance with DIN EN ISO 9934-1
Yoke magnetisation
Alternating current yoke
Open W-yoke – A magnetic circuit, determined by the
mains frequency of 50/60 Hz is completed through air;
there is a constant field polarity reversal and with open
yokes the maximum clamping length = 1000 mm.
Closed W-yoke – A closed iron core is used and when
the test piece is clamped the majority of the magnetic
field lines are constrained within the iron core; an alternating
field can therefore be maintained over a larger
distance.
Maximum clamping length = 1200 mm
D.C. yoke
Arrangement for test units with D.C. yokes
Crack indications: Transverse cracks
Advantages:
• Robust construction
• Longest possible clamping lengths
Crack indication: Transverse cracks
Advantages:
• Uniform field distribution even with complicated
shaped parts with large cross-sectional changes.
• Can be adapted for induction fluxing techniques.
• Simple and rapid de-magnetisation of the test
piece.
Disadvantages:
• Expensive construction
• Limited clamping length
Disadvantages:
• Stray field formation at the rounded ends of
workpieces.
• Considerable field reduction with large
cross-sectional changes of the test piece.
• Expensive to de-magnetise the test piece.
Magnetisation with
current flowing conductor
Coil magnetisation (solid or moveable coil)
Crack indications: Transverse cracks
Indications at 90° to the magnetic field or in the
current direction i.e. in accordance with the
direction of the coil windings.
Criteria:
• When testing tubes in conjunction with alternating
current, only the outside surface is magnetised.
• Because the magnetic field lines are introduced at
right-angles to the plane surface of the material,
circular cracks may be indicated with reduced
sensitivity.
Straight-through-conductor
(Threading bar technique)
Crack indications: Longitudinal cracks
Internal and external, star-like cracks running from
the centre to the outside on the plane surface.
Advantages:
• Contactless magnetisation, and therefore no
danger of burning.
• When A.C. is used the tube is only internally
magnetised.
Direct fluxing
Axial direct fluxing
Crack indications: Longitudinal cracks
Indications at 90° to the magnetic field or in the
direction of the current i.e. in a direct line between
both contact poles.
Criteria:
• Crack indications at the contact points not possible.
• With dirty contact points there is a danger
of burning.
• With very high current strengths the test piece
heats up.
Induction fluxing
Requirements for carrying out induction fluxing
• Only closed ring or tube-shaped test pieces can
be inspected by means of the circulating induction
current.
• A dynamic field is necessary for the generation of an
induction current.
i.e. alternating current can only be used for induction
fluxing.
Crack indication: Circulating defects
All circulating defects, internal, external and at
both plane surfaces.
Advantages:
• Contactless magnetisation and therefore no
danger of burning.
• Good possiblities of combining with indirect
fluxing, for contactless detection of defects of all
directions (finished part testing).
11/02 · g+r
Tiede GmbH + Co · Bahnhofstr. 94-98 · D-73457 Essingen · Tel. +49 (0) 73 65/81-0 · Fax +49 (0) 73 65/81-449 · info@tiede.de · www.tiede.de