What results from the motion of the test specimen through a magnetic field in eddy current testing?

In eddy current testing, the motion of the test specimen through a magnetic field results in the induction of currents. When a conductive test specimen is exposed to a changing magnetic field, it generates circulating currents within the material due to electromagnetic induction. These currents, known as eddy currents, flow in loops perpendicular to the direction of the magnetic field and are affected by the material's properties, such as its conductivity and permeability.

The presence and characteristics of these eddy currents can provide valuable information about the test specimen, including its electrical conductivity, shape, and any potential flaws or discontinuities. Measuring the impedance of the probe or coil used in eddy current testing can reveal changes in the induced currents, which can then be analyzed to assess the quality of the material.

The other options do not directly relate to the primary phenomenon observed in eddy current testing. Thermal expansion is a result of heat generated by electrical resistance, mechanical vibration would not typically occur solely from motion through a magnetic field, and phase shift pertains more to the relationship between voltage and current in an AC circuit rather than directly due to the motion through a magnetic field.