Definitions and Data Tables for Eddy Current Testing

Ohms Law

According to Ohms Law, the voltage is the product of current and resistance.

   i.e., V = I x R

Where V = Voltage in volts, I = Current in Amps and R = Resistance in Ohms

Phase Angle and Impedance

Phase angle is expressed as follows:

   TanΦ = XL/R

Where Φ = Phase Angle in degrees, XL = Inductive Reactance in ohms and R = Resistance in ohms.

Impedance is defined as follows:

Where Z = Impedance in ohms, R = Resistance in ohms and XL = Reactance in ohms.

Magnetic Permeability and Relative Magnetic Permeability

Magnetic permeability is the ratio between magnetic flux density and magnetizing force.

   i.e., μ =B/H

Where μ = Magnetic Permeability in Henries per meter (mu), B = Magnetic Flux Density in Tesla, H = Magnetizing Force in Amps/meter.

Relative magnetic permeability is expressed as follows:

   μ r = μ/ μo

Where μr = Relative magnetic permeability (mu) and μo = Magnetic permeability of free space (Henries per meter = 1.257 * 10-6). μr = 1 for non-ferrous materials.

Conductivity and Resistivity

Conductivity and resistivity is related as follows:

   σ =1/ρ

Where σ = Conductivity (sigma) and ρ =Resistivity (rho). Conductivity can be quantified in Siemens per m (S/m) or in Aerospace NDT in % lACS (International Annealed Copper Standard). One Siemen is the inverse of an ohm. Another common unit used for conductivity measurement is Siemen per cm (S/cm).

Resistance and Conductivity

Resistance can be defined as follows:

   R = l/Aσ or R = ρl/A

Where R = the resistance of a uniform cross section conductor in ohms (Ω), l = the length of the conductor in the same linear units as the conductivity or resistivity is quantified, A=Cross Sectional area, σ = conductivity in S/m and ρ = Resistivity in Ωm.

Standard Depth of Penetration

Standard depth of penetration is given as follows:

Where δ = standard depth of penetration in m; f = frequency (Hz); μ = Magnetic Permeability (Henries per meter); and σ = conductivity in S/m.

The influence of frequency and conductivity on standard depth of penetration is illustrated in Figure 1.

Influence of frequency and conductivity on standard depth of penetration.

Figure 1. Influence of frequency and conductivity on standard depth of penetration.

Current Density Change with Depth

The change in current density with depth is expressed as follows:

Where Jx = Current Density at distance x below the surface (amps/m2); J0 = Current Density at the surface (amps/m2); e = the base of the natural logarithm (Euler's number) = 2.71828; x = Distance below the surface; and δ = standard depth of penetration in meters.

Depth of Penetration and Probe Size

Smith et al have introduced the idea of spatial frequency.

Where D = the effective diameter of the probe field in meters, limiting the depth of penetration to D/4. The probe effective diameter is considered to be infinite in the usual equation.

Phase change with Depth

Phase change with depth is expressed as follows:

   θ = 57.3x/δ

Where, θ = Phase lag (degrees); 57.3 = 1 radian expressed in degrees; x = Distance below the surface; and δ = standard depth of penetration.

The change in phase and current density with depth of penetration is depicted in Figure 2.

Phase and current density change with depth of penetration.

Figure 2. Phase and current density change with depth of penetration.

Frequency

Frequency is expressed as follows:

Where f = frequency (Hz); x= material thickness in meters; μ = Magnetic Permeability (Henries per meter); and σ = conductivity in S/m.

Conductivity of some common non-ferrous metals is listed in Table 1:

Table 1. Conductivity of Some Common Non-Ferrous Metals

Types and/or Description Conductivity % lACS Mega Siemen per m (MSm-1)
Aluminium
Pure Aluminium (A1) 65.86% 38.20
Aluminium Alloy 1100-0 59.45% 34.48
Aluminium Alloy 1100-H18 57.47% 33.33
Aluminium Alloy 2024-0 50.71% 29.41
Aluminium Alloy 2024-T4 30.25% 17.55
Nickel and Alloys
Nickel - (Pure) (Ni) 25.00% 14. 50
Nickel - (Wrought) 18.00% 10.44
Nickel - (Dura) 301 4.06% 2.35
Monel 400 3.38% 1.96
Monel K-500 2.83% 1.64
Stainless Steels
Stainless Steel Type 420 3.13% 1.82
Stainless Steel Type 403, 410 & 416 3.02% 1.75
St Steel Type 430, 430-F, 434 & & 436 2.87% 1.66
Stainless Steel Type 405 & 409 2.83% 1.64
Stainless Steel Type 442 2.69% 1.56
Copper
99.995% Min (Vacuum Cast Pure Copper) (Cu) 102.00% 59. 16
C10100 99.99% Min 101.00% 58.58
C10200 99.95% Cu+Ag 100.00% 58.00
C10400, C10500 & C10700 +0.5 to 0.15% Ag 100.00% 58.00
C10300 +0.001 to 0.005% P 98.20% 56.96
Miscellaneous
Brass 95% Cu + 5% Zn 55.00% 31.90
95.7% Cu + 4.3% Ti 10.00% 5.80
Barium (Ba) 2.76% 1.60
Beryllium (Be) 38.50% 22.33
C17300 Cu+Be+Pb 22.00% 12.76
C34000 65% Cu; 34%Zn 26.00% 15.08
C85200 (Yellow Brass) 18.00% 10.44
C86200 (Manganese Bronze) 7.50% 4.35
Cobalt (Co) (99.8% Pure) 17.80% 10.32
Cobalt (Co) (Pure) 27.60% 16.01
Lead Alloy +0.07% Ca 7.91% 4.59
Lead Alloy +0.07% Ca +0.7% Sn 7.87% 4.56
Molybodenum (Annealed) 41.00% 23.78
Molybodenum (Hard Drawn) 35.20% 20.42
Rhodium (Rh) 38.20% 22.16
Rubidium (Rb) 14.90% 8.64
Uranium (U) 5.86% 3.40
Vanadium (Va) 6.90% 4.00
Zircalloy2 2.40% 1.39
Zirconium (Zr) 4.13% 2.40

About Ether NDE

Ether NDE are a dedicated manufacturer of Eddy Current NDT equipment and accessories including a vast range of EC Weld Probes, ET Weld Probes and ECT Weld Probes and are the 'go to' company for all your Eddy Current NDT Solutions.

Ether NDE continually strives to provide innovative solutions to eddy current testing in all possible inspection conditions.

Offering a range of innovative eddy current testing instruments and probes, Ether NDE will endeavour to find the solution that best fits the clients specific needs.

This information has been sourced, reviewed and adapted from materials provided by Ether NDE.

For more information on this source, please visit Ether NDE.

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Comments

  1. Hiệu Trọng Trần Hiệu Trọng Trần Vietnam says:

    Dear Sir,
    Can detect carbon steel in stainless steel weld by using eddy current?
    Thank you very much

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