Electromagnetic analysis is a term which describes the broad spectrum of electronic test methods involving the intersection of magnetic fields and circulatory currents. The most widely used technique is the eddy current.
Eddy currents are composed of free electrons, which are made to "drift" through metal, under the influence of an induced electromagnetic field.
Eddy current is used in aircraft maintenance to inspect jet engine turbine shaft and vanes, wing skins, wheels, bolt holes, and spark plug bores for cracks, heat or frame damage. In aircraft manufacturing plants, eddy current is used to inspect castings, stampings, machine parts, forgings, and extrusions.
When an alternating current is passed through a coil it develops a magnetic field around the coil which in turn induces a voltage of opposite polarity in the coil and opposes the flow of original current. If this coil is placed so that the magnetic field passes through an electrically conducting specimen, eddy currents will be induced into the specimen. The eddy currents create their own field which varies the original field's opposition to the flow of original current. Thus the specimen's susceptibility to eddy currents determine the current flow through the coil (see figure 10-19).
The magnitude and phase of this counter field is dependent primarily upon the resistivity and permeability of the specimen under consideration, and it is this fact that enables us to make a qualitative determination of various physical properties of the test material. The interaction of the eddy current field with the original field results is a power change that can be measured by utilizing electronic circuitry similar to a wheatstone bridge.
The specimen is either placed in or passed through the field of an electromagnetic induction coil, and its effect on the impedance of the coil or on the voltage output of one or more test coils is observed. The process - whereby electric fields are made to explore a test piece for various conditions - involves the transmission of energy through the specimen much like the transmission of X-rays, heat, or ultrasound. In the transmission of X-rays, heat, or ultrasound, the energy flows in beams having a recognizable direction and intensity and obeys the laws of absorption, reflection, diffraction, and diffusion. Receiver elements can be placed into the beams and a direct measurement of energy flow is possible. However, in electromagnetic tests, the energy distributes itself in a vaguely known manner and undergoes a transformation in the process, from magnetic to electric energy, and subsequently, back to magnetic energy. Since the induced currents flow in closed circuits, it is neither convenient nor generally possible, to intercept them at the specimen boundaries.