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Continuous eddy-current monitoring of fatigue degradation in an unnotched steel sample

#characterization #Digital instrumentation and the factory of the future #modeling
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Metal fatigue, which can result in the gradual degradation of a structure’s mechanical properties, is a critical phenomenon in a wide range of industrial settings. To ensure the safety and durability of metal structures, the ability to monitor degradation is vital. Electromagnetic monitoring methods, with their sensitivity to microstructural changes, are particularly well-suited to this task.

We studied a C22E/C22R-grade structural steel sample with a minimum cross-section of 7.6 mm and a gauge of 12 mm. We applied a cyclical load of 230 MPa at a frequency of up to 5 Hz until failure. To monitor variations
in magnetic permeability, eddy-current measurements were taken at different excitation frequencies (50 kHz, 100 kHz, 250 kHz, and 500 kHz).

A semi-analytical model was used to solve the direct problem: the calculation of impedance as a function of magnetic permeability. The experimental results were compared to simulations to validate the model. The material’s relative permeability, μ, was estimated in the range 150-180, with permeability decreasing as a function of time over the course of the experiment.

 


Figure 1 : Sensor assembly.


 
The results showed a clear difference between the low-frequency (below 3,000 fatigue cycles) and high-frequency portions of the measurements. This difference can be attributed to different microstructural mechanisms occurring at different times during the experiment. The high-frequency portion of the measurements revealed almost linear behavior, confirmed by an analysis of the actual voltage values.

We also took magnetic Barkhausen noise (MBN) measurements to round out the eddy-current measurements. Thermographic data was used for correlation. We also factored in temperature variations to correct for any drift caused by the sample heating up.


 

Figure 2 : Time evolution of signatures in the complex plane.

 

According to our results, eddy-current measurements are suitable for monitoring fatigue-induced degradation in metal. Variations in magnetic permeability can be correlated with microstructural changes, enabling the non-destructive monitoring of the material’s condition.

Now that eddy-current measurements have been demonstrated on the monitoring of fatigue-induced degradation, the door is open to new industrial solutions for real-time monitoring to help improve the safety and durability of metal structures.

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Use cases, applications, technology transfer

  • The continuous monitoring of metal structures for fatigue-induced degradation is crucial to ensuring safety and durability. Eddy-current measurements used in conjunction with semi-analytical models offer an effective method for assessing microstructural changes and monitoring the condition of the material. These results confirm that our approach can be used for real-time monitoring, creating new possibilities for industrial solutions.

Major flagship

  • « Continuous eddy current monitoring of fatigue degradation in an unnotched steel specimen ». Skarlatos A, Boller C, Lyamkin V, et al. International Journal of Applied Electromagnetics and Mechanics. 2025; https://doi.org/10.1177/13835416251345615

Contributor to this article

  • Anastassios Skarlatos, Research Engineer and Senior Expert, CEA-List.