Eddy Current Testing (ECT) is a non-destructive testing (NDT) method widely used to measure the thickness of non-conductive and conductive coatings over conductive substrates. This technique is especially useful for applications in aerospace, automotive, and manufacturing industries. This method is based on Lift Off factor of Eddy currents.
Testing Procedure for Coating Thickness Measurement
Testing Procedure for Coating Thickness Measurement
This method utilizes a probe with integrated electronics and a built-in display to measure the thickness of a coating, whether conductive or non-conductive. To perform a measurement, the probe must be placed on a specific area of the material and the ‘TEST’ button pressed. Once activated, the probe determines the coating thickness and
Material Sorting Based on Metallurgical Properties
A coil carrying an alternating current (AC) generates a magnetic field, which induces eddy currents in the test material. The characteristics of these eddy currents depend on:
- Electrical Conductivity – Different metals and alloys have unique conductivity values.
- Magnetic Permeability – Ferromagnetic and non-ferromagnetic materials respond differently.
- Thickness & Density – Variations affect eddy current penetration and response.
Material sorting Testing Procedure
Principle of Operation
- Dual Absolute Coils: Two identical coils are used, placed in a differential configuration to eliminate temperature variations.
- Frequency Settings: Typically1 kHz, but it may vary based on the conductivity of the material.
- Gain Adjustment: Set between 6-40 dB to amplify signals effectively.
Procedure for Grade Sorting
- Balancing the Apparatus
Two identical (OK) samples of the same material grade are placed in the coils:
Master Coil (Reference Coil) → Contains one OK sample throughout the process.
Test Coil → second ok sample is placed for balancing.
The system is balanced using these two OK samples, setting a baseline output.
- Establishing the OK Sample Threshold
- At least 10 OK samples are tested one by one in the test coil.
- The system records their outputs in the software.
- A threshold is set around the recorded outputs to define the acceptable variation.
- Verifying the Rejected Sample Response
- A known rejected (other grade) sample is placed in the test coil.
- The output must be 5-10 mm outside the OK sample threshold to confirm differentiation.
- If differentiation is unclear, the frequency needs to be adjusted:
Higher conductive materials → Require >1 kHz frequency.
Lower conductive materials → Typically use ≤1 kHz frequency
Key Considerations
Eddy current application testing procedures
- Temperature Compensation: Differential setup removes temperature effects.
· Frequency Optimization: Adjust frequency to improve signal clarity for different materials.
· Threshold Setting: Needs at least 10 OK samples for accurate classification.
· Rejection Verification: A rejected sample must fall significantly outside the OK range.
Conductivity Measurements
An alternating current (AC) flows through a probe coil, creating a changing magnetic field. When the probe is placed near a conductive surface, eddy currents are induced in the material. The strength and penetration of eddy currents depend on the material’s electrical conductivity (σ, measured in % IACS) in International annealed copper standard. The probe detects changes in eddy current flow, which are used to determine the material’s conductivity.
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Cable Industries
Railways
Aviation Industry
Transformer Industries
Testing procedure for Conductivity Measurement
A probe with an integrated electronic unit is used for conductivity testing. The process begins by placing the probe on a test specimen made of 100% IACS copper to establish a reference balance. Once balanced, the probe is ready to test any specimen or material. The device provides a reading in percentage, which is directly proportional to the metal’s purity.