Eddy current testing is a non-destructive method used to detect surface and near-surface defects in electrically conductive materials.
For crack detection, however, simply using an eddy current instrument is not enough. The quality of the inspection depends heavily on how the test parameters are set.
Test frequency, probe track width, test speed, high-pass filtering, and low-pass filtering all influence whether a defect signal is clearly detected or lost in background noise.
Properly adjusted parameters can help improve inspection stability, increase signal-to-noise ratio, and reduce false rejects.
How Does Eddy Current Crack Testing Work?
Eddy current crack testing uses electromagnetic fields to identify changes in the surface of an electrically conductive component.
A probe generates an alternating magnetic field that induces eddy currents within the test material.
When the surface is free of defects, the eddy currents flow in a relatively consistent pattern.
A crack, pore, inclusion, or other discontinuity interrupts this flow. The resulting change in the electromagnetic response is detected by the probe and evaluated by the testing instrument.
Eddy current crack testing can be performed either dynamically or manually.
What Is Dynamic Eddy Current Crack Testing?
In dynamic crack testing, either the test probe or the component moves during the inspection.
For example, a component may rotate while a stationary probe scans its surface, or the probe may rotate around a stationary component.
The probe covers a defined track width during each pass.
If the inspection area is wider than the probe track, the probe must move across the component so that the complete test area is scanned.
Dynamic testing is particularly useful for automated production because it supports:
- Fast inspection speeds
- Repeatable probe movement
- Automated signal evaluation
- High-volume testing
- Automated OK/NOK sorting
What Is Manual Eddy Current Crack Testing?
In manual crack testing, an operator moves the probe across the test surface by hand.
This approach is often used when:
- Components cannot easily be automated
- Only specific areas need inspection
- Maintenance testing is required
- Access to the component is limited
Manual eddy current testing is common in maintenance and aerospace applications where localized surface crack detection is required.
Why Are Eddy Current Test Parameters Important?
The defect signal must be separated from normal variations in the component and from electrical or environmental noise.
If the test parameters are not set correctly, several problems can occur:
- Small defects may be missed
- Harmless variations may be classified as defects
- Signal amplitudes may become unstable
- Background noise may increase
- False rejects may increase
Optimizing the test parameters helps ensure that relevant defect signals remain visible while unwanted signals are reduced.
Three important parameters are:
- Test frequency
- High-pass filter
- Low-pass filter
1. How Do You Choose the Right Eddy Current Test Frequency?
Test frequency is one of the most important parameters in eddy current crack detection.
It influences how deeply the eddy currents penetrate the material and how sensitive the system is to defects at different depths.
The correct frequency depends on:
- Material
- Electrical conductivity
- Magnetic permeability
- Probe design
- Expected defect depth
- Required sensitivity
How Does Test Frequency Affect Penetration Depth?
As a general rule:
Higher frequencies concentrate the eddy currents closer to the material surface, while lower frequencies allow greater penetration into the material.
This means higher frequencies are generally better suited for detecting surface-breaking cracks and pores.
Lower frequencies can provide greater sensitivity to defects located slightly below the surface.
However, deeper penetration usually comes with reduced sensitivity to very small defects.
For this reason, the test frequency should be selected based on the defect that actually needs to be detected.
Why Do Conductivity and Permeability Matter?
Test frequency is not the only factor that affects eddy current penetration.
The material's electrical conductivity and magnetic permeability also influence how the eddy currents are distributed.
Two different materials tested at the same frequency may therefore produce different penetration depths and signal responses.
The appropriate frequency should always be selected based on the specific material and inspection task.
How Does Probe Distance Affect the Eddy Current Signal?
Eddy current signals are sensitive to the distance between the probe and the test surface.
This distance is commonly referred to as lift-off.
Changes in lift-off can alter the measured signal even when no defect is present.
Possible causes include:
- Surface roughness
- Component movement
- Uneven geometry
- Probe positioning variation
- Mechanical runout
Controlling probe distance or using distance compensation can help improve repeatability.
2. What Does a High-Pass Filter Do in Eddy Current Testing?
A high-pass filter removes or reduces low-frequency signal variations while allowing higher-frequency signals to remain.
In crack testing, these slower variations can come from factors such as:
- Gradual geometry changes
- Conductivity differences
- Lift-off changes
- Slow component movement
- Fill-factor variations
The objective is to suppress these unwanted signals without removing the defect signal.
How Does Test Speed Affect the High-Pass Filter?
The frequency of the defect signal depends partly on how quickly the probe moves relative to the component.
In general:
Higher test speeds produce higher-frequency signals.
Lower test speeds produce lower-frequency signals.
Probe track width also affects the signal frequency.
A smaller probe track creates a shorter signal duration as the probe passes over the defect, which results in a higher-frequency signal.
A wider probe track generally creates a lower-frequency signal.
This means:
- Higher speed + smaller probe track = higher signal frequency
- Lower speed + larger probe track = lower signal frequency
The high-pass filter must therefore be selected based on both the test speed and the probe characteristics.
What Happens If the High-Pass Filter Is Set Incorrectly?
If the high-pass filter is set too aggressively, part of the defect signal may be removed.
This can reduce signal amplitude and make defects more difficult to detect.
If the filter is set too low, unwanted slow variations may remain and increase the background noise.
The correct setting should suppress irrelevant low-frequency signals while preserving the full defect indication.
3. What Does a Low-Pass Filter Do in Eddy Current Testing?
A low-pass filter suppresses high-frequency interference while allowing lower-frequency defect signals to remain.
Typical unwanted high-frequency signals may come from:
- Electronic noise
- Electromagnetic interference
- Test equipment
- Nearby machinery
- Other electrical sources
The cut-off frequency should be selected so that this interference is reduced without affecting the relevant defect signal.
What Happens If the Low-Pass Filter Is Set Too Low?
If the low-pass cut-off frequency is too low, part of the defect signal may also be filtered out.
This can reduce the measured amplitude and make small defects harder to identify.
If the cut-off is too high, excessive electronic or electromagnetic noise may remain in the test signal.
The objective is to find a setting that removes unnecessary high-frequency noise while keeping the relevant defect signal intact.
How Do High-Pass and Low-Pass Filters Work Together?
The high-pass and low-pass filters define a frequency range in which the relevant eddy current signal is evaluated.
The high-pass filter removes slower signal changes below the desired range.
The low-pass filter removes faster interference above the desired range.
Together, these filters create a bandpass that emphasizes the signal frequencies associated with the defect being inspected.
An optimized bandpass can help make the defect signal easier to distinguish from background noise.
What Is Signal-to-Noise Ratio in Eddy Current Testing?
The signal-to-noise ratio (SNR) compares the strength of the relevant defect signal with the background noise of the inspection.
A high SNR means the defect signal is clearly distinguishable from unwanted variation.
A low SNR means the defect signal is closer to the background noise and may be more difficult to evaluate reliably.
The signal-to-noise ratio can be influenced by:
- Test frequency
- Filter settings
- Probe design
- Probe distance
- Test speed
- Surface condition
- Material properties
- Component geometry
Improving the SNR generally makes the inspection more stable.
How Can Better Filter Settings Reduce False Rejects?
A false reject occurs when an acceptable component is classified as NOK because an unwanted signal exceeds the rejection threshold.
These unwanted signals may come from:
- Surface roughness
- Geometry changes
- Lift-off
- Electrical noise
- Material variations that are not relevant to the defect
Proper filter settings help suppress these signals so that the inspection focuses more strongly on the actual defect indication.
This can help reduce unnecessary rejection while maintaining the required defect sensitivity.
How Do Probe Track Width and Rotation Speed Affect Crack Detection?
Probe track width and rotation speed directly affect the time it takes for the probe to pass over a defect.
That timing influences the frequency characteristics of the resulting signal.
For example, a small probe moving quickly over a crack creates a short signal pulse with relatively high-frequency content.
A wider probe moving more slowly creates a longer signal with lower-frequency content.
Because of this relationship, frequency and filter settings should not be selected independently from the mechanical inspection setup.
The complete system should be optimized around:
- Probe track width
- Component rotation speed
- Probe feed rate
- Expected defect size
- Expected defect orientation
How Do You Optimize Eddy Current Parameters for Crack Testing?
A practical optimization process should begin with known reference defects.
These reference defects should represent the type and size of flaw that must be detected.
The test setup can then be adjusted by reviewing:
- Signal amplitude
- Background noise
- Test frequency
- High-pass filter
- Low-pass filter
- Probe distance
- Test speed
- Probe track width
The objective is to maximize the relevant defect signal while minimizing unwanted variation.
What Should You Check When Crack Detection Results Are Unstable?
If test results are inconsistent or false rejects are increasing, consider the following questions:
- Is the correct test frequency being used?
- Has the test speed changed?
- Has the component rotation speed changed?
- Is the probe track width appropriate?
- Is the probe-to-part distance consistent?
- Is surface condition affecting the signal?
- Is the high-pass filter removing part of the defect signal?
- Is the low-pass filter allowing excessive noise?
- Have nearby electrical or mechanical conditions changed?
- Are the reference defects still representative of the inspection requirement?
Reviewing these factors can help determine whether the problem is caused by the component or the test setup.
Can Eddy Current Filter Settings Be Optimized Automatically?
Modern eddy current systems can help simplify parameter setup.
FOERSTER's STATOGRAPH CM / CM+ includes functions designed to assist with optimizing inspection parameters.
Automatic parameter optimization can help determine suitable settings for characteristics such as:
- Test frequency
- Sensitivity
- High-pass filtering
- Low-pass filtering
- Phase
- Distance compensation
This can reduce the amount of manual trial-and-error required when setting up an inspection.
How Does Distance Compensation Improve Crack Testing?
Variations in the distance between the probe and the component can create unwanted signal changes.
Distance compensation helps reduce this influence.
The system evaluates changes related to probe distance and compensates for them so that similar defects can produce more consistent signals even when small distance variations occur.
This can be particularly useful for:
- Rotating components
- Components with runout
- Slight geometry variations
- Automated inspection environments
What Are the Most Important Settings for Reliable Eddy Current Crack Detection?
Reliable crack detection depends on balancing several electrical and mechanical parameters.
The most important considerations include:
- Select a test frequency appropriate for the expected defect depth
- Match filter settings to test speed and probe characteristics
- Use a consistent probe-to-part distance
- Select the appropriate probe track width
- Maintain a repeatable rotation or feed speed
- Optimize the signal-to-noise ratio
- Verify the setup using known reference defects
There is no single frequency or filter setting that works for every application.
The correct setup depends on the material, component geometry, defect type, and inspection speed.
How Can FOERSTER Support Eddy Current Crack Testing?
FOERSTER's STATOGRAPH CM / CM+ is designed for automated and semi-automated eddy current crack detection.
Functions such as automatic parameter optimization, adjustable filter settings, and distance compensation can help operators develop a stable inspection process while reducing unnecessary signal interference.
The correct configuration should be developed around the specific component, material, required defect size, and production speed.
Having trouble finding the right frequency or filter settings for your crack detection application?
