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 Basic Knowledge of Crack Detection with Magnetic Particles
During the production of steel (casting), during the processing (rolling, forging, welding) and also during the utilization of steel components (bending and tensile loads) cracks can occur. Most often the crack formation begins at the surface and then the crack grows into the material due to stress concentrations at the crack tip. Finally, this can lead to a fatigue fracture of the component.
Therefore cracks are the most crucial defect type and must be eliminated by grinding out the crack or scrapping the complete component. The most sensitive method for the detection of surface cracks in ferromagnetic parts is magnetic particle testing (MT).

 Main Applications
  • Safety relevant components in the automotive industry. Those are steering, axle and motor components in the state of half finished forged or cast products or in the final machined state.
  • Welds on pressure vessels and pipelines
 Principle
Magnetic particle testing of magnetizable metals (mostly steel, less often nickel or cobalt), is well acknowledged by experts. For the inspection the object must be magnetized. In case of large components, which cannot be magnetized completely, only the inspection zone is magnetized.

Magnetic field lines which are generated by the magnetization process flow parallel to the surface in the component. Cracks with a perpendicular orientation in relation to the field lines disturb these and create a leakage flux: The field lines exit on one side and enter into the material on the other side, thus creating magnetic poles on the two side of the crack. When iron powder is dispersed on this area, it conglomerates above the crack because it is drawn by the magnetic poles.

Cracks with a parallel orientation in relation to the field lines cannot be detected because they do not generate a leakage flux. Covered cracks below the surface can be localized down to a certain depth of approximately 1 mm.

The magnetization is achieved either by magnetic field penetration or by feeding an electric current through the component to be tested. Both methods can be combined, to enable a crack localization in all orientations.

Magnetic Field Penetration: In this case the object is clamped into a yoke which is magnetized by means of electrical coils. Thus, a magnetic field is generated in the longitudinal direction of the object and cracks which are oriented perpendicularly to the field (i.e. transverse cracks) produce a leakage flux and are indicated.

Electric Current Magnetization: Depending on the inspection task also longitudinal cracks must be detected. For this purpose a second type of magnetization is applied: An electric current flows through the component and generates an annular magnetic field. This method enables the localization of longitudinal cracks.

Combined Magnetization: Often the cracks do not have a predominant direction. Mass products, e.g. in the automotive industry are usually inspected in stationary testing machines, which permit either the magnetic field penetration, the electric current feeding or a combination of both at the same time, so that cracks of all orientations can be detected.



Test medium (also called ink or test agent):
The magnetic particles (iron or iron oxide powder) are applied in a liquid suspension during the magnetization process. Very small granulated ferromagnetic iron oxide particles enable the detection of even hairline cracks.

In order to achieve a contrast enhancement the iron oxide powder particles are lacquered with fluorescent pigments. When illuminated with ultraviolet light (UV light) in a dark room, the cracks are indicated brightly with a high contrast in yellow-green. But also other colors are possible. (We know this method from banknote testing, where also UV light is applied.)

 Advantages of Crack Testing with Magnetic Particles
  • Highest sensitivity for the detection of surface cracks, also for complex object geometries
  • Clear and reproducible crack indications
  • Short testing time (approx. 10 s)
  • In general, no surface preparation required (oily surfaces must be cleaned before testing with water-based test media or tested in oil-based test media)
  • Detection of covered cracks below the surface
  • No particular radiation protection required
  • No special requirements concerning the operator’s expertise and skill. However, carefulness and permanent attention is needed.

 Further Information

Knowledge Floater "Crack Detection with Magnetic Particles"

You can get further extensive information on magnetic particle testing by our following Knowledge Floater video “Crack detection with magnetic particles”.



Note: This video frame links to Youtube. Please ensure that no internal browser setting prevents external loading or click here for direct access to our NDTChannel on Youtube.
Alternative: The Knowledge Floater can be downloaded as zipped Powerpoint ® file here (size 42 MB approx.). The zip-file also contains the necessary powerpoint viewer. Unpack the archive into a separate directory and start the presentation by opening the file "play.bat".


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