Incompatible mode solid elements add internal, non-nodal displacement modes to a solid element to enrich its deformation field and reduce locking or overly stiff behavior.
How It Works
The source introduces extra internal degrees of freedom for selected 6-node wedge and 8-node hexahedral elements. The standard displacement interpolation is augmented by additional mode functions, producing an expanded strain-displacement matrix:
B_tot = [ B B_inc ]
The resulting stiffness is assembled from the expanded matrix. Because the incompatible mode degrees of freedom are internal to the element and are not global nodal unknowns, they are eliminated by static condensation before the global solve.
Why It Matters
Low-order displacement-based solid elements can be too stiff in deformation states their interpolation cannot represent well. Incompatible modes are a local enrichment strategy: they improve element flexibility without changing the global mesh topology or adding nodal unknowns.
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