Secondary Electromagnetic Field.
Eddy currents also generate an electromagnetic field. This field, called the secondary electromagnetic field, opposes
the primary electromagnetic field (see Figure 4-3) and is a consequence of Lenzs Law. Lenzs Law, as applied to this
case, states that induced currents (eddy currents) act to reduce the magnitude of the inducing current. The opposition
of the secondary field to the primary field decreases the overall electromagnetic field strength and reduces both the
current flowing through the coil and the resultant eddy currents. Changes to the properties of the inspection article
produce changes to the eddy currents and thus their secondary magnetic fields. In this manner, changes in the
inspection article produce effects that can be detected by monitoring either the source of the primary electromagnetic
field or the overall electromagnetic field.
Variables Affecting Eddy Currents.
The generation and detection of eddy currents in a part are dependent upon the design of the inspection system (coil
assemblies), material properties of the part, and the test conditions. The inspection systems, including coil assemblies,
are discussed in Section 4.4. Material properties and inspection conditions that influence eddy current response are
summarized in Table 4-2.
Table 4-2. Material Properties and Inspection Conditions Influencing Generation of Eddy Currents.
Coupling or Lift-Off
Electrical conductivity is a measure of the ease with which electrons (and thus eddy currents) can move within a
material (see paragraph 4.1.5). Good conductors of electricity have electrons that are not tightly bound in the atomic
lattice or crystal structure and are relatively free of obstacles to the movement of those electrons. Metals have greater
conductivity than nonmetals, but even within metals there is a wide range of conductivity.
126.96.36.199.1.1 Factors Affecting Electrical Conductivity.
A perfect lattice is one in which there is no interruption in the orderly arrangement of the atoms making up the
material. This situation offers the fewest obstacles to electron flow and therefore the highest conductivity. Any
irregularity or distortion of the atomic lattice impedes the flow of electrons. Sources of such impediments include
atoms of alloying elements and grain boundaries (where lattice mismatches occur because of differing crystalline
orientations). Further impediments are created when heat treat processes precipitate alloying elements at grain
boundaries to increase strength. Cold working also creates impediments to electron flow by its disruption of the lattice
structure. Most importantly for NDI, cracks and other discontinuities also impede electron flow.
188.8.131.52.1.2 Measurement of Resistivity.
Electrical resistance is a measure of the resistance to the flow of electric current in a conductor. Resistance depends on
the length and area of the current path, and the conductivity of the conductor. Resistance is commonly measured in
ohms. If a material allows one volt (electric potential) of driving force to push one ampere of current through a
conductor, the electrical resistance of the conductor is defined as one ohm of resistance. Resistivity is a material
parameter independent of the size of a material sample and is related to resistance as shown in the second equation of
paragraph 184.108.40.206.1.3 below. Resistivity is defined as ohms per unit of length per unit of cross-sectional area.