Manufacturers need reliable ways to detect cracks and other defects before components move to the next production step or enter service. The challenge is finding these defects quickly without damaging the part being inspected.
Eddy current testing is a non-destructive testing (NDT) method used to inspect electrically conductive materials, including many metallic components. It is particularly useful for detecting surface and near-surface defects such as cracks and pores.
Because the inspection is contactless and can be integrated into automated production processes, eddy current testing is widely used for quality control where fast, repeatable inspection is required.
What Is Eddy Current Testing and How Does It Work?
Eddy current testing uses electromagnetic fields to identify changes in the material being inspected.
During crack testing, either the test part rotates while a stationary probe scans its surface, or a rotating probe scans a stationary component. The probe typically contains transmitter and receiver coils.
An alternating current is applied to the transmitter coil, creating a magnetic field. When this field interacts with an electrically conductive test part, it induces eddy currents within the material.
The receiver coil measures the resulting electromagnetic response. When the material is consistent and free of defects, the eddy currents follow a predictable path. A crack or other discontinuity disturbs this flow, changing the measured signal.
How Does Eddy Current Testing Find Cracks?
An eddy current instrument evaluates changes in the received signal compared with the expected response from the material.
When a crack interrupts the flow of eddy currents, the signal changes in characteristics such as amplitude and phase. The testing instrument evaluates these changes to determine whether an indication may represent a defect.
This makes eddy current testing particularly effective for detecting defects located at or close to the surface.
The method is commonly used when manufacturers need to:
- Detect cracks and other surface discontinuities
- Inspect components without damaging them
- Automate inspection during production
- Separate acceptable and non-acceptable components
- Improve inspection consistency
How Deep Can Eddy Current Testing Detect a Crack?
One of the most common questions when selecting an eddy current inspection method is how deeply the test can detect defects.
The penetration depth of eddy currents depends primarily on three factors:
- Electrical conductivity
- Magnetic permeability
- Test frequency
Electrical conductivity and magnetic permeability are properties of the material being inspected. Test frequency, however, can be adjusted to optimize the inspection.
How Does Test Frequency Affect Penetration Depth?
As a general rule, higher frequencies provide greater sensitivity to defects close to the surface, while lower frequencies provide greater penetration into the material.
There is a tradeoff. As the test frequency decreases and penetration depth increases, sensitivity to small defects also decreases. A deeper defect can therefore be more difficult to detect than a similar defect located directly at the surface.
For this reason, there is no single test frequency that is ideal for every application. The frequency should be selected based on the material, component geometry, expected defect location, and required sensitivity.
Why Does Surface Quality Matter in Eddy Current Crack Testing?
Surface condition can have a significant effect on the reliability of an eddy current inspection.
Eddy current testing is a contactless inspection method, meaning the probe does not need to physically touch the component. However, the probe should remain at a small and repeatable distance from the surface.
A smooth or polished surface generally allows the probe to operate closer to the component, which can improve sensitivity to small defects.
Rough or uneven surfaces create a different challenge. If the probe is positioned too close, surface irregularities can contact and potentially damage the sensor. Increasing the distance can protect the probe, but it can also reduce sensitivity.
Surface variation can also create unwanted signal responses, commonly referred to as background noise.
What Is Signal-to-Noise Ratio in Eddy Current Testing?
A defect signal must be distinguishable from the normal background response of the component. This relationship is described using the signal-to-noise ratio (SNR).
For reliable and repeatable defect detection, a commonly used target is for the defect signal to be at least three times greater than the background noise, resulting in an SNR of approximately 3:1 or better.
If background noise becomes too high, distinguishing a true defect from normal surface variation becomes more difficult.
Improving surface conditions, probe positioning, test frequency, filtering, and other test parameters can help improve the signal-to-noise ratio.
What Happens If the Probe-to-Part Distance Changes?
Consistent probe positioning is another important part of reliable crack detection.
If a component does not rotate symmetrically, the distance between the probe and the surface may change during a 360-degree rotation. These changes can affect signal amplitude even when the actual defect size remains the same.
Distance compensation helps reduce this influence.
An additional measurement channel monitors changes related to the distance between the probe and the test part. The testing system can then compensate for these variations so that similar defects produce more consistent signal amplitudes.
This can help create a more stable inspection when small variations in component positioning cannot be completely avoided.
Which Eddy Current Parameters Affect Crack Detection?
Several instrument parameters can be adjusted to improve defect detection. With eddy current instruments such as the FOERSTER STATOGRAPH CM / CM+, important parameters include:
- Test frequency
- Sensitivity
- Bandpass filtering
- Phase position
- Distance compensation
Each parameter affects how the instrument detects, separates, and evaluates signals.
What Does Sensitivity Do?
Sensitivity controls how strongly the eddy current signal is amplified.
During setup, known artificial or natural reference defects are typically used to establish the required sensitivity. The goal is to make sure the relevant defect signal can be reliably distinguished from the background signal and evaluated against a defined threshold.
If the signal exceeds the defined threshold, the component can be classified accordingly.
In automated production environments, interfaces such as PROFINET can transfer inspection results to a PLC for further processing or automatic sorting.
Why Are Filters Used in Eddy Current Testing?
Not every signal generated during an inspection represents a defect.
Changes in component geometry, vibration, surface condition, and other process influences can introduce unwanted signals.
Bandpass filtering helps suppress frequencies that are not relevant to the inspection. This makes the desired defect signal easier to identify and can reduce false indications and unnecessary part rejection.
What Is Phase Adjustment?
Different material or process conditions can generate signals with different phase characteristics.
Phase adjustment allows the operator to orient the relevant defect signal so that it can be evaluated more effectively against the defined threshold.
This helps separate relevant defect indications from signals that do not need to influence the final inspection decision.
Why Are Reference Defects Important for Eddy Current Testing?
An eddy current inspection should be set up and verified using known reference defects.
These may be artificial defects created specifically for the inspection process or suitable natural defects with known characteristics.
Reference defects help establish whether the system can reliably detect the defect size and type that matters for the application.
They are also used to determine appropriate settings for parameters such as sensitivity, frequency, filtering, phase, and evaluation thresholds.
This helps create a repeatable inspection process rather than relying only on individual signal interpretation.
How Are Eddy Current Signals Used to Determine OK and NOK Parts?
Once the inspection parameters have been established, threshold values can be used to evaluate the measured signals.
If a relevant signal remains within the defined acceptance criteria, the component can be classified as OK. If the signal exceeds the established rejection threshold, the component can be classified as NOK.
In an automated production environment, these results can be transferred directly to the production control system.
More advanced evaluation methods can also help identify changes before components consistently reach the rejection limit. If inspection signals begin trending toward the NOK threshold, manufacturers may be able to investigate the production process before a larger quality problem develops.
Eddy current testing can therefore provide information not only about individual components, but also about the stability of the manufacturing process.
How Can Automatic Parameter Optimization Simplify Eddy Current Testing?
Finding the right combination of frequency, sensitivity, filtering, phase, and other settings can be challenging, particularly when setting up a new inspection.
FOERSTER has developed the STATOGRAPH CM / CM+ to simplify this process and support repeatable crack detection in production environments.
Automatic parameter optimization can help determine suitable inspection settings based on the test task and reference defects. This reduces the amount of manual adjustment required and helps operators establish a reliable test setup more efficiently.
The STATOGRAPH CM / CM+ also supports integration into automated production environments, allowing inspection results to contribute directly to quality control and process monitoring.
Is Eddy Current Testing Right for Your Crack Detection Application?
Eddy current testing can be an effective solution when you need to inspect conductive components for surface or near-surface defects without damaging the part.
However, reliable crack detection depends on more than simply selecting an eddy current instrument. Material properties, surface condition, probe distance, test frequency, sensitivity, filtering, phase settings, and reference defects all influence the final result.
Understanding these factors makes it easier to develop an inspection process that provides repeatable results while reducing false indications and unnecessary rejects.
