T.O. 33B-1-1
4-15
would also increase eddy current response to material properties immediately adjacent to the surface of an inspection
part.
4.2.3.2
Depth of Penetration.
The intensity of eddy currents decreases exponentially with depth in a material. The intensity at any given depth is
affected by the same variables that influence the surface intensity of eddy currents, although not always in the same
manner or by the same amount. To put it another way, the depth of penetration of a specific intensity of eddy currents
is affected by the variables, as indicated in Table 4-4. Generally, any parameter that increases the depth of penetration
would provide an equivalent eddy current response at a greater depth in a test part.
4.2.3.3
Standard Depth of Penetration.
Three of these variables (conductivity, relative magnetic permeability, and frequency) are used to define the standard
depth of penetration. Standard depth of penetration is the depth below the surface of the inspection article at which the
magnetic field strength, or the intensity of the induced eddy currents, is reduced to 36.8 percent of the value at the
surface. (0.368 = 1/e, where e = 2.71828, the base of natural logarithms). The standard depth of penetration is
expressed by the following formula:
d = KA / (p f m s)
½
Where:
d
=
One standard depth of penetration (inches)
p
=
3.14
f
=
frequency (Hertz)
m
=
relative magnetic permeability (1.0 for nonmagnetic material)
=
Conductivity (%IACS)
KA = 46.12
Example: Copper at 1KHz: d = 46.12 / (3.14 x 1000 Hz x 1 100)1/2= 0.008 inches.
4.2.3.3.1
Because the depth of penetration is related only to a percentage of surface field strength or surface eddy current
intensity, test variables that affect the strength of the field applied to the surface are not included. Therefore, coil
configuration, size and current, and magnetic coupling are not considered in this formula. These variables affect the
absolute magnitude of the eddy currents at a specified depth but not the standard depth of penetration.
