Definition of NDT
The use of noninvasive techniques to determine the integrity of material, compound, structure or quantitatively measure some characteristics of an object i.e. inspect or measure without harm.
Nondestructive testing (NDT) is the process of inspecting, testing, or evaluating materials, components or assemblies for discontinuities, or differences in characteristics without destroying the serviceability of the part or system. In other words, when the inspection or test is completed the part can still be used.
There are 6 most frequently used NDT methods, and also many others not so used methods:

To provide abrasive wheels training course participants with the knowledge, skills and attitude to enable operative use of the abrasive wheel safely.
TIG welding is an electric arc welding process in which the fusion energy is produced by an electric arc burning between the workpiece and the tungsten electrode.
Polyethylene Welding is used within the Construction industry in the Gas and Water fields as well as other areas such as Electric and Waste Treatment industries.
The Metal Inert Gas (MIG) or Metal Active Gas (MAG) welding is a semi-automated process, widely used in many sectors of industry, such as Fabrication, Structural, Shipbuilding, Railroad construction and production of heavy plants and machinery.