  Accurate characterisation of small defects remains a challenge in
non-destructive testing (NDT). In this paper, a principle-component
parametric-manifold mapping approach is applied to single-frequency
eddy-current defect characterisation problems for surface breaking defects in a
planar half-space. A broad 1-8 MHz frequency-range FE-circuit model &
calibration approach is developed & validated to simulate eddy-current scans of
surface-breaking notch defects. This model is used to generate parametric
defect databases for surface breaking defects in an aluminium planar half-space
and defect characterisation of experimental measurements performed.
Parametric-manifold mapping was conducted in N-dimensional principle component
space, reducing the dimensionality of the characterisation problem. In a study
characterising slot depth, the model & characterisation approach is shown to
accurately invert the depth with greater accuracy than a simple amplitude
inversion method with normalised percentage characterisation errors of 38% and
17% respectively measured at 2.0 MHz across 5 slot depths between 0.26 - 2.15
mm. The approach is used to characterise the depth of a sloped slot
demonstrating good accuracy up to ~2.0 mm in depth over a broad range of
sub-resonance frequencies, indicating applications in geometric feature
inversion. Finally the technique is applied to finite rectangular notch defects
of surface extents smaller than the diameter of the inspection coil
(sub-aperture) over a range of frequencies. The results highlight the
limitations in characterising these defects and indicate how the inherent
instabilities in resonance can severely limit characterisation at these
frequencies.
