In Magnetic particle testing, to detect a flaw these should be sufficient flux leakage. Greater the flux leakage at the flaw area better is the detectability. For sufficient flux leakage to happen, there should be adequate current to be present. Therefore, to detect flaw of a certain size, current needs to be properly calculated.
Just adequately distorted flux can attract magnetic particle to get accumulated at the flaw location. If too much current is applied, the part will be over filled with flux and starts enormous leakages from everywhere. This leads to over magnetization and thus causing heavy accumulation of powder. Contrast is lost and flaws may go undetected.
If the part is undermagnetized, there is no adequate flux to get distorted. Power may not be able to get accumulated at the flaw location. Magnetic particle Indication can be too faint to get attention of the inspector. Such indications may not be detectable.
So, for best detection in MPI testing, current should be sufficient to cause accumulation of ferro magnetic powder. This will bring to the attention of inspectors.
How do you verify that the magnetic field is adequate?
There are various methods and techniques to verify the magnetic field adequacy. This is done using any one of the following:
- Lifting dead weights
- ASTM Field Indicator
- Hall Effect gauss meter
Lifting Dead weights
In case of Permanent magnets and electromagnetic yokes, lifting dead weight is required before using the equipment.
Permanent Magnets
Permanent magnets are suitable for applications where other techniques are restricted to use. It has limited application due to lack of sufficient detectability. However, application like MPI testing of welds, structures in petroleum refineries, mines etc where fire hazard is existing, this technique is most suitable. Before using this techniques there should be an agreement about the usage of permanent magnet among the contracting parties.
Aerospace standards does not permit using permanent magnets for MPI testing of aerospace components. As the particle movement is not to the extent of electromagnetic yokes, use of this technique is restricted.
Current Calculation as per ASTM E709 and ASTM E1444
Find below guide lines on current calculations as per ASTM E709 or ASTM E1444. As these are just guide lines, we advice you follow the same. However, you need not stricly adhere to these formula. Verify the field adequacy in any case to verify the detection level on your job.
Head Shot and Central Conductor techniques – current calculations
For both head shot and central conductor techniques: Current required is about 12 to 31A/mm. For example, if the component is about 100mm diameter, the current required will be 1200A to 3100A based on permeability.
You can start magnetizing using 1200A for the material and check the field strength using Hall Effect gauss meter. If the strength is above 30 Gauss it should be adequate and can be up to 60 Gauss depending on standard you use.
Initially apply current of 1200A and slowly increase it so that you can get the field strength required. Note the final current and prepare an MPI technique sheet.