Non-destructive testing (NDT) is a general term for any method that determines material properties without damaging the object being tested. Most commonly it is used to measure cracks and pores in materials that may be subject to a brittle failure.
Because these defects could act as crack initiation sites, the size and frequency of the defects indicates the strength of the material. NDT is, therefore, very important for aluminium aircraft components, welds, cast parts and additive manufactured (AM) parts. It is also used to determine if delamination has occurred in composites and for regular inspection where fatigue may cause crack formation.
Common forms of NDT include:
- Visual inspection is used to identify cracks and defects on the surface of a part, may be enhanced using digital or optical magnification. A borescope may also be used for confined spaces.
- Liquid penetrant die, applied to a part before visual inspection, can significantly increase the contrast of small cracks and pores, greatly increasing their probability of visual detection. Capillary action draws the die into small defects and the excess penetrant is then removed from the surface, making the defects clearly visible. This method is widely used for castings, forgings, welds and AM.
- Ultrasonic testing (UT) uses a contact probe to send short pulses of ultrasonic vibration into a part and records the time for the reflected wave to be returned to the probe. This gives the distance to the nearest free edge of the material. If a defect is present inside the material, this distance will be less than the material thickness. UT can therefore be used to detect cracks and pores in welds, castings and AM, delamination in composites, and overall thickness for applications such as pipe corrosion.
- Industrial radiography uses X-rays or gamma rays to view inside a material and produce 2D images (radiography) or 3D images (computed tomography or CT).
- Eddy-current testing generates a magnetic field and observes the eddy currents induced by a conductive material placed within the field. Changes in the eddy currents can indicate material thickness and defects, as well as measuring the conductivity of the material.
- Magnetic-particle inspection observes how iron filings accumulate on the surface of a ferromagnetic part subjected to a magnetic field. A crack or pore on or close to the surface will cause the magnetic flux to leak and therefore attract more of the magnetic particles. This allows visual identification of the defects.