The aim of this section is to provide a general overview of the fundamental nature of NDT, including its role in the engineering, construction and maintenance of in-service industrial systems and components. It has been written to acquaint non NDT personnel looking for a new career. The emphasis will be on the more common forms of NDT and especially those techniques used in conjunction with rope access. It will cover the basic physical concepts of various testing techniques, how they are selected, who uses them, their capabilities and their limitations.
Inspection of materials, components, and assemblies is often required in order to determine if they comply with design specifications or are defective. Of utmost concern for newly manufactured or constructed items is their ability to withstand the mechanical stresses and chemical attack to which they are expected to be exposed. For items that are already in service, the question is whether or not the item will continue to function satisfactorily in its service environment until the next scheduled inspection. NDT by virtue of its ability to detect flaws, measure dimensions, and assess material characteristics, has become a primary tool in addressing the predictions of component performance and remaining service life.
The selection of an appropriate NDT method for a specific application is based upon the physical characteristics of the material and the discontinuities of most interest, including the probable location, orientation, size, and morphology. The assumption is that if an item is free from unacceptable flawed conditions, it should meet performance expectations, that is, carry the load, withstand the vibration, or maintain dimensional integrity.
The NDT approach to materials testing is non-destructive in that each inspected item is expected to be placed in service. This is in stark contrast to the destructive methods of materials evaluation where sacrificial specimens are required. In destructive testing, specially prepared specimens are stressed in a manner that simulates the stresses to which the item may be subjected and at levels exceeding the design limits.
The destructive approach results in specimens fracture, distortion, or some other form of degradation, and thus is applied on only a sampled basis.
NDT is most commonly used where component failure may have catastrophic consequences, such as in aircraft, electric power plants and petrochemical plants, as well as gas lines and offshore installations. The periodic inspection of components in these fields determines if they are suitable for continued service. The advantages of NDT for in service applications include safety evaluation with a minimum of system down time, early detection of potentially dangerous and costly unscheduled service interruption and the ability to monitor degradation processes from which realistic and cost effective re-inspection schedules can be developed.
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Magnetic Particle Inspection (MT) is one of the best-known and commonly used methods of NDT. Its aim is to detect the presence of surface braking discontinuities (cracks) in the part under inspection.
Liquid Penetrant Inspection (PT) is another common method of NDT, and is solely used to detect surface breaking discontinuities, which are free from debris that can limit the entrance of the dye (oil, grease and paint).
Ultrasonic Inspection (UT) is a method of NDT that is used to detect internal anomalies in a part, which may contain welding, or stress defects that can be detrimental to the integrity of the component.
Eddy Current Inspection (ET) is one of the most widely applicable of the Non-Destructive Inspection methods.
Radiography (RT) and Radiographic Interpretation (RI) uses X-rays or gamma-rays to produce an image of an object on film.
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