The primary limitation of Eddy Current Testing in detecting subsurface defects is:

Eddy Current Testing (ET) is a non-destructive testing method that primarily relies on the electromagnetic principles to detect surface and near-surface defects in conductive materials. One of the significant limitations of ET is its inability to penetrate thick materials effectively. The eddy currents generated are primarily localized to the material's surface and decaying exponentially with depth. This means that for thicker materials, the eddy currents may not reach subsurface defects, making it challenging to detect issues that are deeper than a few millimeters.

This limitation is crucial in various fields, especially in aerospace and structural applications where components may have substantial thickness. Therefore, while ET is invaluable for surface crack detection and other near-surface anomalies, its effectiveness diminishes significantly as the thickness of the material increases. In scenarios where deeper evaluation is necessary, alternative testing methods may be required.

Understanding this limitation is vital for practitioners to select the appropriate Non-Destructive Testing (NDT) method based on the material thickness and the expected depth of defects.