Case Windmills Kinderdijk
Preservation of Unesco World Heritage Kinderdijk Windmills
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Magnetic Particle Inspection (MPI) or Magnetic Testing (MT) is an NDT method for checking the surface integrity of ferromagnetic materials. The material is magnetized using a handheld yoke or a horizontal MPI bench setup. Defects in the surface and shallow subsurface cause magnetic field fluxes to "leak". When a liquid containing tiny magnetic particles is applied, these particles accumulate around those discontinuities making them visible to the naked eye. If necessary a white contrast agent is applied to the material before testing. For high sensitivity, a fluorescent liquid and blacklight can be used. This requires a dark testing environment.
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A simple way to establish if a part can be tested using magnetic particle inspection is by using a permanent magnet to see if it is ferromagnetic. If it is, the magnet will "stick" to the part and we can inspect it using MPI.
Depending on the material and location requirements, we can choose from the following liquids:
Time
MT: fast inspection technique
PT: relatively slow process
Materials
MT: can only be applied in examination of ferromagnetic materials
PT: provides optimal results in all metals and alloys
Discontinuities
MT: detects not only surface cracks but also imperfections that are very near to the surface
PT: only detects discontinuities which are on surface and open for the dye to penetrate
Surface conditions
MT: no pre-cleaning and post-cleaning processes are necessary because surface pollutants don’t have adverse effect on inspection results
PT: Strict pre-cleaning is required
Temperature
MT: is applicable up to extreme temperatures (normally below Curie temperature)
PT: limited temperature conditions (5-60 degree Celsius)
Equipment
MT: power supply is required to operate yoke magnet
PT: no power supply needed
