Conventional shell versus continuum shell

Shell elements are used to model structures in which one dimension, the thickness, is significantly smaller than the other dimensions. Conventional shell elements use this condition to discretize a body by defining the geometry at a reference surface. In this case the thickness is defined through the section property definition. Conventional shell elements have displacement and rotational degrees of freedom.( 使用殼單元常常來模擬結構，其中一個維度，即厚度，明顯小于其他維度。傳統的殼單元利用這個條件，通過在參考面上定義幾何形狀來離散一個物體。在這種情況下，厚度是通過截面屬性定義的。傳統的殼體有位移和轉動自由度。)

In contrast, continuum shell elements discretize an entire three-dimensional body. The thickness is determined from the element nodal geometry. Continuum shell elements have only displacement degrees of freedom. From a modeling point of view continuum shell elements look like three-dimensional continuum solids, but their kinematic and constitutive behavior is similar to conventional shell elements.( 相反，連續殼單元離散整個三維體。厚度由單元節點幾何形狀確定。連續殼單元只有位移自由度。從建模的角度看，連續殼單元類似于三維連續體，但其運動學和本構特性與常規殼單元相似。)

The following picture illustrates the differences between a conventional shell and a continuum shell element. 下圖說明了傳統殼單元和連續殼單元之間的區別。

What calls for special attention is that In large-deformation (geometrically nonlinear) analysis these local directions rotate with the average rotation of the surface at that point.( 需要特別注意的是，在大變形(幾何非線性)分析中，這些局部方向隨該點表面的平均旋轉而旋轉)