Non-destructive materials testing is a collective term for various testing methods that can be used to test the quality of a component with regard to its material properties on the surface or inside the component without damaging or permanently changing the test specimen itself. 
In contrast, destructive materials testing tests the physical and chemical properties of materials by destroying the test specimen or permanently changing it at least on the surface, which is why this method can only be used for random samples or individual examinations.
Non-destructive materials testing can be used for 100% inspection for material defects. This includes the detection of internal defects such as blowholes, inclusions and loosening as well as surface defects such as cracks and open pores. It is also possible to assess the microstructural properties in terms of hardness, hardening depth, tempering or grinding burn, and to detect layer thicknesses and internal dimensions. Volume-oriented methods include X-ray and ultrasonic testing. Surface-oriented methods include eddy current testing, magnetic particle testing and dye penetrant testing.

Non-destructive materials testing includes a range of test methods with different operating principles for specific application areas and materials. Each test method has its limits. Often it is sufficient to state whether certain defects are present or not. Furthermore, it may be necessary to combine different test methods in order to achieve sufficient test reliability. Likewise, it can happen that defects cannot be detected with sufficient certainty, e.g. due to the construction of the component, despite the use of different test methods. 
One group of test methods is based on the use of waves. Material defects, cracks, separation points, boundary layers etc. cause scattering, absorption or a changed reflection behaviour of the waves, which can be measured and evaluated. Radiographic testing is based on the use of X-rays or gamma radiation, thermography on thermal radiation or infrared waves, and ultrasonic testing on ultrasonic waves.
Other methods are based on the detection of the course of magnetic field lines, which are generated by magnetising the component or by inducing eddy currents. In principle, material defects lead to a changed course of the magnetic field lines, which is measured. Eddy current and flux leakage testing, electromagnetic microstructure characterisation and magnetic particle testing function on this basis. 
Another method is the visualisation of surface-open defects with the help of colour contrasts in dye penetrant testing.