Can ACFM detect sub-surface defects or porosity?
Summary Generally not on ferrous metals, but sub-surface defects can be detected in non-ferrous metals if they are near the surface. Detail The a.c. skin effect means that the currents induced in the ACFM technique are confined to a thin layer at the surface of the material. ACFM can only detect defects lying within this layer where they perturb the current and so can be detected. The thickness of this layer (the “skin-depth”) is inversely proportional to the square root of the magnetic permeability of the metal so is much smaller for ferritic steel than for non-ferrous metals. The skin depth in ferritic steel is too small to allow detection of sub-surface defects, but the larger skin depth in non-ferrous metals makes it possible to detect defects that do not break the top surface. The signal from a sub-surface is much less “sharp” than that from a surface breaking defect. To be detected sub-surface defects must disturb a significant portion of the current flow. To achieve the required
