The major limitation\index{model, limitations} of the \amms\ arise from the way volumetric defects are subdivided. Rectangular slots are modelled exactly; however, defects that cannot be modelled as a collection of rectangular elements, like cracks, holes, elliptical slots, etc., have to be approximated using rectangular cells. This can be done because each element has its own conductivity value.
Approximating complex defect shapes is done by using the volume fraction of defect in each cell. The volume fraction is the ratio of defect volume in an element to total element volume. If the element is entirely inside the defect the volume fraction is 1, if it is entirely outside the volume fraction is zero and so on. The defect volume fractions can be used to introduce more complicated features into defect models, like bridging, corrosion products, inclusions and so on.
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