### 3.7.7. Known Limitations

Awareness of the limitation of solid modeling can help you build a better solid model. It is worth spending time thinking about aspects of your model such as mesh sizing, extrusion methods, and geometry operations. For example, it can often be more robust to extrude from the inside out.

This section gives an overview of the limitations of these advanced modeling features:

Mesh Extrusion

  The solid model relies on the extrusion of a shell geometry mesh. As the extrusion directions and operations increase the solid model generation reaches a limit of how heavily a shell mesh can be distorted. If the topology of the structure is complex then the extrusion operations can result in ill-formed elements which are subsequently deleted in the element check.

Drop-Off Elements

  There are transition regions where an edge of a solid model extrusion is reduced to a series of drop-off elements. This reduction in thickness will result in a local reduction in stiffness that should not be overlooked.

Cut-off Elements

  Cut-off elements are decomposed into homogenous tetrahedral and prism elements. These elements can cause abrupt changes in the structural stiffness which may lead to higher stresses and IRFs. The results in the proximity of cut-off elements should be evaluated with care.

Connect Butt-Jointed Plies

  The ability to connect adjacent plies is currently restricted to plies that appear sequentially in the same modeling group. Consequently, there a certain arrangement where a ply drop-off cannot be evaded.

Solid Model Extrusion Offset

  The solid model extrusion starts from a reference shell and the layup definition. An extrusion with an offset to the reference geometry is not possible.

Sampling Points, Section Cuts, and Sensors

  Sampling points, Section Cuts, and Sensors give information about the lay-up as it was defined in the Modeling Ply Group. The changes as a result of geometric operations in the solid model are not reflected in these features. While cut-off plies still appear in the sampling point, these plies show zero stress and strain during postprocessing.

Recomputation of ISS

  The recomputation of interlaminar shear stresses only corrects the stresses of layered elements. Homogenous elements (drop-off and cut-off elements) retain the interlaminar shear stresses of the .RST file.

Cutting Operations Effect on Interface Layers

  Interface elements intersecting with the cut-off geometry are deleted and not updated according to the new shapes of the adjacent bottom and top elements.

Node Merge Operations on Composite Solid Model Assemblies

  The use of node merge operations in Mechanical on composite solid models can lead to incorrect deformation results in ACP-Post deformation plots. When node merge operations are used, the Read Strains and Stresses option in the Solution Properties should be active. Appropriate warnings are issued.

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