Learning Forum

[manuel.uruena]

Dear all,

I am seeking for a more clear explanation about when to use the option KEYOPT(5)=1 and KEYOPT(5)=2 in SHELL181 apart from the explanation in ANSYS Mechanical APDL Element Reference.

Thanks

[John Doyle]

Perhaps you have seen this already, but if not, below is a paragraph I copied from the SHELL181 Chapter.This is located in the notes just below Figure 181.5: Typical Bending Applications.The practical advice I would add to these notes is that if you are dealing with a curved surface meshed with shells, start with the default, KEYOPT(5) = 0. If you are suspicious of the quality of your results, try KEYOPT(5) = 1. If the answers are different and the mesh is sufficient, your KEYOPT(5) = 1 is probably needed for better accuracy based on the description below. If you run into convergence trouble with KEYOPT(5) = 1, then try KEYOPT(5) = 2. If all three options produce the same answer, then, you are probably ok with the default (KEYOPT(5) = 0).
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When KEYOPT(5) = 1, the element uses an advanced formulation that incorporates initial curvature effects. The calculation for effective shell curvature change accounts for both shell-membrane and thickness strains. The formulation generally offers improved accuracy in curved shell structure simulations, especially when thickness strain is significant or the material anisotropy in the thickness direction cannot be ignored, or in thick shell structures with unbalanced laminate construction or with shell offsets. The initial curvature of each element is calculated from the nodal shell normals. The shell normal at each node is obtained by averaging the shell normals from the surroundingSHELL181elements. A coarse or highly distorted shell mesh can lead to significant error in the recovered element curvature; therefore, this option should be used with a smooth, adequately refined mesh only. To ensure proper representation of the original mesh, a nodal normal is replaced by the element shell normal in the curvature calculation if the subtended angle between these two is greater than 25 degrees.
When KEYOPT(5) = 2, a simplified curved-shell formulation is adopted. Unlike the advanced curved-shell formulation (KEYOPT(5) =1), the curvature effects in the shell offset handling are ignored. The simplified formulation generally leads to more robust nonlinear convergence.
The curved-shell formulation is automatically disabled for excessively thick and curved structures with an r / t ratio below 5 / 6, where r is the radius of curvature measured at shell mid-plane and t is the total shell thickness.

[manuel.uruena]

Dear jjdoyle Thanks for the answer. I am aware of the ANSYS chapter you quoted. I tried both KEYOPT(5)=0 and KEYOPT(5)=1 for my model with curved shell elements which is r/t= 8 to r/t=13 and the results slightly differ.

[John Doyle]

As the doc implies, keyo(5)=1 "...generally offers improved accuracy in curved shell structure simulations". So I would go with that, all other things being equal.