Multi-Frequency Analysis: Field Attenuation and Material Phase with Depth
Different test frequencies penetrate to different depths in conductive materials. LOW frequencies penetrate deeper but have less sensitivity to small surface defects. HIGH frequencies concentrate near the surface with excellent sensitivity to near-surface flaws.
This tool compares all three frequencies simultaneously so you can see how the same physical depth gives different normalized field attenuation and one-way material phase lag at each frequency.
TEST PARAMETERS
Low Frequency Field Model
Medium Frequency Field Model
High Frequency Field Model
Relative Field Amplitude is field/current-density magnitude normalized to its surface value (100%), not predicted defect-signal amplitude. Material Phase Lag is one-way depth-dependent phase lag in the idealized skin-effect model, not measured flaw-signal or coil-impedance phase.
PENETRATION DEPTH COMPARISON
Normalized Attenuation & Phase Visualization
Combines normalized exponential attenuation and material phase lag to visualize their relationship with depth. This is an illustrative graphic, not a calculated coil-impedance or defect-response plot.
Understanding Field Attenuation & Material Phase
Standard Depth of Penetration (SDP)
SDP = 503.3 / sqrt(f x ur x sigma), giving SDP in mm, where f is frequency in Hz, ur is relative permeability (1 for non-magnetic), and sigma is conductivity in MS/m. Input %IACS is converted using sigma = 0.58 x %IACS (100% IACS = 58 MS/m). Lower frequencies have larger SDP values, allowing detection of deeper defects.
Frequency Selection Strategy
Use LOW frequency for subsurface/deep defects. Use HIGH frequency for near-surface sensitivity. Use MEDIUM frequency for general inspection. Multi-frequency inspection provides comprehensive coverage across all depths.
Idealized Skin-Effect Model
At depth d: Relative Field Amplitude = 100% x e^(-d/SDP) and Material Phase Lag = (d/SDP) x 57.3 degrees. These describe normalized field/current-density attenuation and one-way material phase lag, not measured defect or coil responses. Each frequency has a different SDP, so the same physical depth gives different attenuation and phase.