Electromagnetic Field Penetration and Eddy Current Skin Depth Mechanics
Standard skin depth defines the exponential field decay boundary, dictating coil drive frequencies and target thickness minimums for displacement sensing.
Target Thickness Sizing
Measurement Concept
Non-destructive testing systems measure the physical thickness of metal sheets or tubes by analyzing electromagnetic impedance changes. This process is known as target thickness sizing, which relies on the relationship between the material thickness and the intensity of the induced eddy currents. When the material is thin, the eddy currents are constrained.
Material Characterization
Electrical conductivity and magnetic permeability values of the material must be known to convert the signals into thickness units. In target thickness sizing, any variation in the alloy’s composition can mimic a change in thickness, leading to incorrect readings. To prevent this, the material’s properties must be evaluated before the thickness measurement begins.
This dual-parameter analysis is typically handled by multi-frequency systems that separate material effects from thickness changes.
Sensor Calibration
Precision step blocks are used to calibrate the system across the expected range of measurements. Each step on the reference block represents a known value, allowing the software to build a calibration curve for target thickness sizing. This curve is verified before each testing run to ensure that drift has not occurred.
If the sensor has drifted, the calibration curve is adjusted to restore the specified accuracy.
Instrument Limitation
Structural geometry can interfere with the signal and cause errors near edges or welds. If the sensing coil is positioned too close to the boundary of the specimen, the electromagnetic field leaks into the air, distorting the measurements. In target thickness sizing, this edge effect can be minimized by using shielded probes or by keeping the sensor a minimum distance from any physical boundaries.
This limitation must be factored into the scan plan to avoid false thickness readings and to guarantee the reliability of the output data.