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A Tale of Caution

Magnetic flux indicator strips (Castrol or Ely Strips) are widely used to indicate the presence of induced magnetic fields during the magnetic particle inspection method for inspection of ferromagnetic materials. Flux Indicators give evidence of an external field in the air above the magnetised surface and in some circumstances can be used to obtain a semi-qualitative estimate of the tangential field strength H.

 

When any method of magnetisation which uses a magnetic field induced in the air around the workpiece is used, no estimate of field strength can be made since the flux indicator will respond to airborne flux whether a ferromagnetic body is present or not.

This excludes the possibility of estimating field strength when any of the following methods of magnetisation are used:

  • Encircling coil
  • magnetic flow
  • yoke magnetisation
  • induced current
  • central conductor
  • adjacent cable
  • permanent magnet

A method for determining field adequacy is the use of a Standard Test Bar (AS1171). This is essentially a feromagnetic component that has a discontinuity that provides minimal flux leakage. However as the field strength of the equipment is confirmed on a test piece rather than the part, confirmation of the test materials suitable permability is required.

 

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