When electrical conductivity in %IACS and relative permeability are known.
or
Where:
d 
= 
Standard Depth of Penetration (mm or in) 
f 
= 
Test Frequency (Hz) 
m_{r} 
= 
Relative Magnetic Permeability (dimensionless) 
s 
= 
Electrical Conductivity (%IACS) 

Eddy currents are more concentrated at the surface
and decrease in intensity with distance below the surface of
the metal. This effect is known as the "skin effect." The depth at which eddy current density has decreased to 1/e,
or about 37% of the surface density, is called the standard
depth of penetration (d). Although
eddy currents penetrate deeper than one standard depth of penetration,
they decrease rapidly with depth. At two standard depths of
penetration (2d), the eddy current
density has decreased to 1/e squared or 13.5% of the surface
density. At three depths (3d), the
eddy current density is down to only 5% of the surface density.
The depth of penetration is dependent of test
drive frequency, the test material's conductivity and magnetic
permeability. The depth of penetration decreases with increasing
frequency, conductivity and permeability. It is important to
know the standard depth of penetration because for some testing
(like flaw detection) the inspection should be conducted at
a frequency that places the depth of any likely flaws at 1d or less where eddy currents are strong. When making conductivity
tests, the sample should be at least 3d
thick so that changes in the thickness of the sample do not
affect the measurements. When electrical conductivity in % IACS
and permeability in H/mm are known, the standard depth of penetration
can be calculated using the equation to the right.
Example
1
What is the standard depth of penetration when
performing an eddy current inspection on a piece of 304 Corrosion
Resistant Steel? The material has a relative permeability of
1.05 and a conductivity of 2.5% IACS. The frequency used to
drive the eddy current probe is 50 kHz.
Simply plugin the known values and solve the equation.
Example 2
Determine the frequency needed to achieve a depth
of penetration of 2mm in 7075T76 aluminum plate.
Aluminum is nonmagnetic so its relative permeability
is one. With a measurement or from the Electrical
Conductivity and Resistivity Property Tables, determine
the conductivity value for the material. The value given in the table is 38.5% IACS.
The depth of penetration equation must be rearranged
to solve for frequency (f). Start by squaring both sides to clear the square root.
Then solve for frequency.
Plug in the known values and solve.