Learning Forum

[Fy Sh]

I am programming a finite element program on layered shell elements myself and using ansys apdl as my programming reference. I'm only interested in the relationship between displacement and strain.

In a simple rectangular shell model consisting of SHELL181, one side is fixed and a pressure load is applied to form a cantilevered like structure. In 2 cases, the shell is modeled by a homogeneous material and a layered material (similar to a sandwich structure). The thickness of both (the thickness of the homogeneous shell, the total thickness of the sandwich shell) is the same, and the load is changed to achieve the same deflection (Uz) for both. The strains (epsi_x, epsi_y, tau_xy) of the top (or bottom) surface of the two shells were extracted respectively, and the results showed that they were quite different. This puzzled me, because in both cases, the displacement of the shell nodes and the thickness of the shell are the same, but the strain is different. If the first-order shear deformation theory is used in the SHELL181 element, I would like to ask what is the reason behind for such difference in the layered or homogeneous cases?

[Avnish Pandey]

Hi,

Hope by homogenous, you mean a single layer plate. And by sandwich you mean there are at least 3 layers comprising different materials.

If that is the case, then the strains will differ. The sandwich plate, with its layers, behaves differently under load due to the variation in material properties through the thickness and higher stiffness. For a single layer plate the bending and shear stiffnesses are coming from one layer. Whereas for a layered structure, the stiffnesses are summed for the number of layers (A,B,D matrices you might have heard of, check any composites textbook). These factors combined lead to distinct strain patterns despite both plates being bent by the same amount.

One thing you can test out. Try using the same material for the homogenous plate and the layers of the sandwich. You may observe the two deflecting by the same amount for the same loading and same thickness.

Best