Description
Flexible Laminated Strips (Type II) are used to indicate the presence of induced magnetic fields during magnetic particle inspection of aerospace components. Flux indicators give excellent evidence of the direction(s) and strength of a magnetic field. They are designed to respond to the tangential component of the field ‘H’ in the air above the surfaces of magnetized workpieces. The indicators can be used to give a semi-quantitive estimate of field strength only when the workpiece is magnetized by the current flow technique and the using alternating current. When other methods of magnetization are used (e.g. a coil), a magnetic field is created in the air and whether there is a ferromagnetic item present or not, the indicator will respond to such a field. Correct use of the indicators can be described in the following four steps:
- Attach one indicator to the workpiece so that its long axis is in the same direction as that of any expected defects.
- Attach a second indicator at a right angle to the first.
- Magnetize the workpiece in the normal way and apply magnetic particles.
- A minimum of 3 seconds of magnetization is needed to allow for particle migration before the indicators are inspected.
These foil type indicators are designed for the use in both critical and general engineering applications. They are used to give a clear guide on the direction of a magnetic field and can be used to give an estimate of the field strength across surfaces that are magnetized. These indicators have been designed for the application of Magnetic Particle inspection under applicable customer standards.
Interpretation
If one indicator shows indications clearly and the other none at all, then the direction of the induced magnetic field is at a right angle to the indicator which shows indications.
If both indicators show indications then the direction of the induced magnetic field lies somewhere between the directions of the two indicators. It is usually possible to deduce the direction of the field by the diffuse nature of the indications at the ends of the indicators, in such a case the indicators should be re-sited in the direction which will lead to one indicator showing indications and the other not (after a re-application of magnetic particles and magnetization).
If neither indicator shows any indications then there are several possibilities, the most common of which are;
a) that there is insufficient magnetic field induced in the material for magnetic particle inspection to proceed.
b) that there is no external field, which may occur in circular magnetization.
Interpretation of field strength can only be made when alternating current is used. The indicators must be mounted on a vertical surface so that the magnetic ink may drain off them readily, then it can be estimated that when all three indications are clearly visible the induced field in the air above the surfaces of the workpieces is greater than 2000 Amp/metre. When the indicators are mounted horizontally no assessment of the field strength can be made.
