What effect does increasing the coil lift-off distance in eddy current testing have on the results?

Increasing the coil lift-off distance in eddy current testing typically results in reduced resolution. The lift-off distance refers to the spacing between the coil and the surface of the test material. When this distance is increased, the electromagnetic field generated by the coil becomes weaker at the surface of the material. As a consequence, the sensitivity to small defects or variations in the material diminishes.

Resolution in eddy current testing relates to the ability to distinguish between small changes or defects in the material being tested. With a greater distance from the coil to the material surface, the interaction between the eddy currents induced in the material and the magnetic field from the coil is less effective. This results in a broader detection pattern and ultimately lowers the ability to differentiate closely spaced flaws or features within the material, leading to decreased resolution.

In the context of the other options, while it might be tempting to associate increased distance with enhanced sensitivity or improved signal-to-noise ratios, both of these scenarios usually occur at shorter coil-to-surface distances where the eddy currents are more concentrated and clearly detected. Therefore, as lift-off distance increases, the resolution worsens, making it harder to identify and differentiate defects.