VT method is used as classical NDT inspection focused on the detection of cracks, undercuts, and surface porosity, shape irregularities (e.g. linear misalignment, weld root concavity, casting die misalignment), and last, but not least general surface condition. The VT is full-value NDT method comparable with other (PT, MT, UT) and in its extent actually exceeds the scope of abilities of other methods. Visual testing represents one of the NDT methods that shall be performed before another NDT method is applied.
The ferromagnetic material becomes magnetically saturated if it is magnetized in a proper way. The material discontinuity has different magnetic properties (usually is non-magnetic - air inside of the crack, slug or gas in porosity). This inhomogeneity of the magnetic properties deforms magnetic field, its force lines step off the material to surface, and create the scatter magnetic flow. This flow holds the information about magnetic inhomogenity. The magnetic powder is applied on the surface of the material. This powder is in place of the material magnetic properties fitted to the surface by the scattered magnetic flow. This fitted cluster of magnetic powder creates an indication. The indications have bright or color contrast and could be easily seen on the background material. The MT is able to detect the imperfections from width of a few microns.
Penetrant (the liquid with suitable properties) is deposited on the clean and dry surface of the part to be examined. Penetrant needs some "penetration" time to penetrate the surface opened imperfections. After this dwell time the excess penetrant shall be removed from surface and the developer is deposited on the surface. The developer is strong absorber, and withdraws the penetrant from discontinuities to the surface. After developing time it is possible to inspect presence of two-dimensional indications. Advantage is, that imperfections that are so small to be visible by human eyes, create larger indications. These indications create high contrast (color or brightness) on the background. Imperfection is detectable approximately from the width of microns.
We use one of the new approaches, the PHASED ARRAY technique. This method is based on usage of a probe set (Sonatest PRISMA UT/PA apparatus), in combination with special electronic circuits, which could shape the beam. PHASED ARRAY probe consist of the set of miniature piezoelectric transducers. Setting of output parameters for each transducer separately forms the beam. This equipment has standard output signal in shape of the A-scan.
One of the basic advantages is, that only one probe could generate the beam with different output angle. This allows covering much bigger area of the examined part without any move with probe. It results into the spatial resolution improvement and finally into more exact evaluation of the imperfection.