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The penetrant liquid test is a non-destructive testing method (NDT) used to look for discontinuities on the materials' surface. The method is based on the ability of some liquid substances (generally organic based) to penetrate by capillarity, within small discontinuities present on the surface under examination. After an adequate contact time, in which the liquid penetrates into any of these discontinuities, the excess penetrant is removed.
Subsequently, with the aid of a particular detector or developer product, the penetrant contained in the discontinuity tends to resurface indicating the presence and location of the discontinuity itself. The penetrant, after the application of the detector, is invisible to the naked eye, but requires the use of a black light or Wood lamp to highlight its fluorescence.
The penetrant liquid method allows to highlight only discontinuities open to the surface such as microcracks, spinning, overlapping and porosity. It can be applied almost on all machinable metals and their alloys, on glass, ceramics and plastics in general.
Advantages of this method are:
Non-destructive inspection by means of magnetic particles allows any type of discontinuity to be detected with high sensitivity in ferromagnetic materials, even if not open to the surface but close to it.
The control is based on the fact that when the object to be tested is magnetized, the discontinuities that are arranged transversely to the magnetic field determine a deviation of the flow lines of the magnetic field itself.
To highlight the defect, the surfaces are sprayed with suitable suspensions of ferromagnetic, colored or fluorescent powders. The particles end up lining up along the flow lines of the magnetic field, forming a "profile" of the discontinuity, which generally indicates its position, size, shape and extent. The particles thus concentrated on the defectiveness are made visible by lighting with a Wood lamp.