The defect is a discontinuity in the material. Defects can have various causes: congenital, arise during the material casting, during heat treatment and hot working, can arise due to mechanical fatigue (fatigue cracks). Discontinuity is the interruption of physical structure of the material, which, however, does not affect the use of the product. Only after the evaluation of the defect and the acceptability criterion, it can be determined whether the product is usable. The defects can be cracks, porosity, inclusions, crystallizations, splits, segregations, cavities and they can have different shapes.
The fault evaluation can be performed by different methods, including:
UT ultrasound control – exploits the phenomenon of reflection of an ultrasound wave within a piece. The ultrasounds emitted by the probe pass through the piece, reaching the bottom wall and then going back. During this path, if the ultrasound beam encounters a defect, there is an early reflection of the signal, which therefore highlights the discontinuity. The ultrasound check allows the identification of both surface and depth defects. This type of control is used in many fields, such as metallurgical industry, welding, aerospace industry, shipbuilding industry and in all sectors where non-destructive controls are essential.
The control with PT Penetrating Liquids method – is used for the detection of surface defects and exploits the penetration of a highly capillary liquid.
After applying a detector liquid, which creates a contrast, the defect can be identified.
The control with MT Magnetic particle detection method – exploits the phenomenon of the magnetic field and is used for the identification of superficial or sub superficial defects with liquid containing magnetic particles. The presence of cracks or other defects causes an interruption of the magnetic field that is created in the artefact; the magnetized metal particles group together at the discontinuity.
This type of control is particularly employed in welds and castings control. It is possible to identify superficial or sub superficial defects in the order of a few millimeters.