In the case of a long straight conductor carrying current, the lines of magnetic force (or flux) exist which are closed circular paths concentric with the axis of the conductor.
The relative permeability of air and non-magnetic materials for all practical purposes is considered to be 1. In case of ferromagnetic materials the relative permeability is not constant but is a function of flux density. However, for eddy current probe energized by low levels of magnetization, the permeability can be considered reasonably constant. Now when the straight wire is wound into a coil (many loops of wire), the lines of the force encircling the wire form a magnetic field inside and outside the loop as illustrated in FIG. 3.11.
FIG.3.11. Magnetic field of a coil.
The field thus created is similar to the field of a bar magnet. The strength of this field is dependent upon two factors: the number of turns in the coil and the magnitude of the current.
The field strength Hz along the axis of a current carrying coil of radius r meters at a point z meters from the center, and having N turns, is given by:
) (
2 22 3
2
z r
r
Hz NI
=
+
(3.3)where
Hz = field strength r = coil radii N = number of turns I = current
z = distance from centre
3.2.1 Eddy current path in a part according to its position relative to inductor coil
Eddy currents are closed loops of induced currents circulating in a plane perpendicular to the direction of the magnetic flux. Their normal direction of travel is parallel to coil’s winding and parallel to the surface. Eddy current flow is limited to the area of the inducing magnetic field. For detection of flaws it is essential that the eddy current flow be perpendicular to the crack to obtain maximum response. If the eddy currents flow is parallel to the defect there will be no disruption of current and hence no change in coil’s impedance. FIG. 3.12.a illustrates the sensitivity of a surface probe to discontinuities relative to their position in the test piece.
A surface probe such as pancake type will have poor sensitivity to laminations, bonding of coatings and those discontinuities lying parallel to the surface of the test sample.
For defects lying parallel to coil’s winding a horseshoe (U- shaped) probe with wide gap may have reasonable sensitivity. A gap probe uses ferromagnetic material to shape the magnetic
FIG. 3.12.a Directional properties of a surface probe.
Zero Sensitivity Low Sensitivity Maximum Sensitivity
FIG. 3.12.b Directional properties of a surface probe for a given crack size.
3.2.2 Distance influence on coupling in various shapes
Many practical eddy current test systems are arranged with some spacing between the coil and the test material so that test objects can be handled and moved within the coil fields. The effects of such spacing on reactance and the induced eddy currents are however significant and should be taken into account when designing the probes. For surface coils or feed through coils the effect of spacing affects in a way on the coupling between the coil’s field and material under test.
When an eddy current coil is lifted away from the surface of nonmagnetic conducting material by some distance, a portion of the magnetic flux created by the test coil current fails to reach the test material. If the coil is lifted so far above the material surface that none of its magnetic flux lines reaches the test material, the coil exhibits its empty coil inductive reactance. This is the highest value attainable during tests of nonmagnetic materials. If the coil then approaches the surface of the test material, more of its magnetic flux lines intercept the test material inducing eddy currents that oppose a change in the coil's magnetic field. As the eddy current reaction field strength increases, the total magnetic flux linkage with the exciting coil is reduced.
Coil
Surface Crack
Eddy
Currents Lamination
Test Piece
As the eddy current reaction field increases with close proximity of the coil to the test material surface, the coil inductance and inductive reactance are accordingly reduced. The limit of this reduction is attained when the face of the coil assembly is placed in firm contact with the test material surface.
The eddy current test sensitivity to material properties is greatest when the eddy current resistance losses are maximized. Maximum probe sensitivity is attained when the coil is in direct contact with the flat surface of a nonmagnetic test material. Increase in lift-off always reduces the sensitivity of eddy current tests.
3.2.3 Focusing method
Shielding of eddy currents is required for focusing purpose i.e. larger part of the available flux may be concentrated below the probe. Use of shielded eddy current probes may also be necessary to prevent the field generated by the probe from interacting with certain objects in the vicinity of the probe. The primary concern is the interaction with conducting and magnetic bodies that are not part of test but lie in close proximity and may produce false indications or mask the signal from discontinuities in the vicinity. Testing for discontinuities near edges (such as testing fastener holes) is an example. Shielding of eddy current probes can be done by three ways:
(a) Magnetic shielding.
(b) Active shielding.
(c) Eddy current shielding.
Magnetic shielding is achieved by creating a low reluctance path for field lines within the area required and away from unwanted region. A very simple shielded probe could be built by covering the coil (with or without a ferrite core) using a sleeve of high permeability, low conductivity material such as ferrite. In active shielding the generation of an active field is employed by means of a coil or system of coils to cancel part of the original field in specific area. Eddy current shielding employs the skin effect to prevent the magnetic field from extending to its normal limit. In this case, shielding is achieved through attenuation rather than changing the magnetic path.
3.3. Reaction of different types of probes according to coil layout