Introduction to Non-Destructive Testing
Non-destructive testing (NDT) may be defined as the application of an inspection method to a component or structure in which the testpiece is not adversely affected by the testing method. Generally, NDT is normally carried out in one of the following circumstances:
- As a control check and an aid to the monitoring of quality during manufacture. In this case NDT is used to check the maintenance of required quality levels during manufacture. The presence of manufacturing imperfections and defects may be detected. The physical characteristics and the severity of such imperfections and defects may also be assessed.
- During in-service inspection. In this case NDT methods are used in order to detect any physical deterioration in a component or structure that might occur under service conditions. NDT methods are most usually employed to detect cracking, or crack propagation from existing defects. Such cracking is commonly associated with some form of fatigue.
- Critical defect assessment. This use of NDT normally involves an in-depth examination of known defects in a structure in order to provide detailed information for "fitness for purpose" assessments and fracture mechanics calculations.
Figure 1. Penetrant Inspection
There are six major NDT methods used in engineering.
| Visual | Surface inspection method |
| Penetrant | Surface inspection method |
| Magnetic Particle Inspection | Surface & slightly subsurface inspection |
| Ultrasonics | Volumetric inspection method |
| Eddy Current | Primarily surface inspection method |
| Radiography | Volumetric inspection method |
Figure 2. Defects in a surface can be
detected through the magnetic
field
Whatever the circumstances under which a non-destructive test is carried out and whatever method is applied there are a number of essential prerequisites that must be met if the engineer is to have confidence in the accuracy of the reported results.
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