### 3.7.1. When to Use a Solid Model

From an analysis point of view, the choice between a shell or solid element analysis models largely depends on the structure and the type of structural investigation.

Solid models are inherently larger and computationally more expensive than shell models. It is therefore wise to start an analysis with a shell model before moving on to a solid model. It provides a basis for comparison but it is also a good check on whether the model is solvable.

Typically, a solid model describes the behavior of a structure more precisely when its out-of-plane response becomes significant. ACP has the unique feature of representing the 3-D stress state for a shell model. Shell model stress behavior can therefore be taken as a first indication of the 3-D stress state. If the out-of-plane stresses are significant then it may be worthwhile analyzing the structure as a solid element model.

The following list shows cases where are a solid element model can be used. The solid modeling feature is not limited to these examples.

- Analysis of thick structures

- Investigation into 3-D stresses (high element resolution necessary)

- Investigation into debonding

- Investigation into edge effects

- Buckling analysis of sandwich structures

There are no hard rules on this matter. It remains entirely the choice of the designer when to use a solid model in addition to or instead of a shell model.

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