Which of the following cannot be tested by the eddy current testing method?

Eddy current testing is an effective non-destructive testing method primarily used for detecting surface and near-surface discontinuities in conductive materials. The technique relies on inducing eddy currents in the material and measuring the change in these currents as they encounter flaws.

When considering the thickness of materials, the capability of eddy current testing diminishes with increased thickness. A 102 mm (4 in.) thick plate is too thick for this method to effectively detect small discontinuities. The penetration depth of eddy currents is limited, often being a fraction of a millimeter to a few millimeters, but can be significantly less in very thick materials. This makes it impractical for identifying small flaws deep within such a substantial thickness.

In contrast, eddy current testing is highly suitable for detecting surface cracks in tubing, assessing laps and seams in rods, and evaluating variations in outside diameter of tubing, as these conditions primarily involve surface and near-surface anomalies which are within the accessible range of this testing technique.