Microwaves are electromagnetic waves with a frequency from approx. 300 MHz up to some 10 GHz. They can be used for non-destructive testing (NDT) of components. Different materials within the component, which indicate flaws, such as inclusions and pores, are characterized by various dielectric permittivities. By analyzing the reflecting or transmitting microwave radiation, which is interacting with the material, the sample features can be concluded.
Electrically isolating materials, such as glass fiber reinforced plastics, are widely transparent for microwaves. In case of conductors, such as carbon fiber reinforced plastics or metal inlays, radiation is reflected.
In principle, the microwave method is a non-contact procedure. Due to the great wavelength, they are often operated in near field to improve lateral resolution. Therefore, the distance between transducer and test specimen has to be as small as possible.