The term non-destructive testing (NDT) covers a range of methods used to verify the integrity of parts, components, materials and structures without damaging them. Non-destructive testing supplements destructive testing, which is carried out to determine the strength of structures by methods such as breaking, peeling, etc.
Main types of NDT
X ray radiography
X ray testing provides access to the internal structure of objects and materials. The resulting images are displayed in conventional (photographic film) or digital format. The technique is used, among other things, to check weld integrity, corrosion and cracks in mechanical parts, etc.
Gamma ray (γ ray) radiography
Gamma ray testing is similar to conventional x ray testing but uses a higher-energy gamma radiation source. Gamma ray testing is performed in areas that are difficult to access and on very thick parts and components.
The variation in the speed at which ultrasonic waves propagate through a material or part is used to detect anomalies in the structure being analysed. This technique is comparable to the ultrasound tests used in medicine. Ultrasonic NDT identifies both surface defects and defects present within the material (internal inspection).
Dye penetrant testing
Dye penetrant testing is used to detect defects (down to 1µm) on the surface of a part of component. The operation consists in applying a layer of dye (primarily red) or fluorescent liquid to the surface to be tested. Following cleaning and the application of a developer, the colour is retained above the defects. Dye penetrant testing is used to inspect sheet metal, welds, pipes, etc.
A variety of leak tests can be performed, depending on the field of application, including helium (accumulation, vacuum, sniffer probe, dye penetrant), pressure variation, vacuum chamber and ammonia tests. They are especially used to verify the integrity of storage equipment and infrastructure (LNG carrier tanks, onshore tanks, pipe systems, exchangers, etc.).
Magnetic particle testing
In magnetic particle testing, a strong magnetic current is applied to the surface of a part or component containing iron. If a defect is present, the magnetic field is disrupted, causing accumulated particles above the defect.
Sound wave propagation is measured within a part being tested to locate any areas of weakness such as cracks and/or surface defects.
Eddy currents are induced in a closed circuit within a conductive material placed near an electromagnetic winding. Any structural defects in the material will disrupt the current flow.
Visual and video inspection
Careful visual inspection of the outside of a part provides crucial information regarding its wear status and the nature of any observable defects. When the areas to be inspected are not readily accessible, a probe or robot fitted with a camera is used. The images obtained are displayed on a screen and/or digital device.
The computed tomography technique, which is comparable to CT scans used in medical imaging, generates three-dimensional images of the object being tested. It is used to analyse the material both externally and internally.