Surtest magnetic flux indicator 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.
Type 1 indicators are typically used for general engineering applications and Type 11 are used for aerospace applications. Both types consist of three laminations measuring SOx 12 mm which are fixed together to form a sandwich structure wh ich is nominally 0.15 mm thick. The outer lamination has 3 interruptions which are parallel to the long side, The width of these interruptions and the material of the outer laminations means that Type 1 indicators respond to a weaker field than Type 11 strips . Both types are protected by a polymeric layer and can be differentiated by the Roman numeral next to the Ely logo.
Flux Indicators have the advantage of being flexible enough so that they can be bent to fit the contours of a workpiece, but robust enough to enable them to be used many t imes. They may also be cut into smaller pieces to allow them to be fixed in recesses and otherwise inaccessible locations on the workpiece.
Attach one flux indicator to a vertical surface of the workpiece under test so that it’s length is at right angles to the direction of the applied current and attach a second at right angles to the first. Magnetise the workpiece by the use of the same electric current wave form as will be used for the inspection and by the same technique.
The magnetisation time will vary according to the self inductance of the workpiece, but should not be less than 3 seconds. While the workpiece is being magnetised apply magnetic particles. It is essential that the application of magnetic particles stops before the end of magnetisation.