Shell locations

Stress and strain results for shell elements are interpolated across the elements' top and bottom surfaces between shell stress recovery points. Shell stress recovery points, such as node locations or element centroids, vary depending on your solver, element formulation, and output requests.

When post processing models that include shell elements, you can specify the shell location for stresses and strains.

On the Result tab of the Post View dialog box, select Stress – Element-Nodal or Strain – Element-Nodal, and from the Shell list, select one of the following:

• Top — Returns results for the shell surface in the element's positive normal direction.

• Bottom — Returns results for the shell surface in the element's negative normal direction.

• Mid — Returns results for the shell midsurface.

• Minimum — Compares the absolute value of the top, mid, and bottom surfaces, and returns the smaller value.

• Maximum — Compares the absolute value of the top, mid, and bottom surfaces, and returns the larger value.

• Average — Returns the average of the signed values for the top and bottom shell surface.

• Top and Bottom — Returns results for the element’s positive and negative normal directions.

• Bending — Isolates the absolute bending stress (or strain), without including the other components, such as the membrane stress and shear stress, that have the same stress value at the top and bottom of the element. The absolute bending results are calculated from the top and bottom stress results as:σB = |1/2 (σTS – σBS)|where σTS is the selected stress component from the top surface of the element and σBS is the selected stress component from the bottom surface.

The default location is Shell Top.

Note:

• How the software calculates the shell location values depends on the order of precedence. For more information, see Order of precedence for calculating shell location values.

• It is important that your shell meshes have consistent element normals in order to provide meaningful results at the specified location. For information about checking and reversing element normals, see Checking and orienting the normals of 2D elements.

Working with top and bottom results

You can display the stress or strain results on both the top and bottom of 2D elements simultaneously using the Top and Bottom shell location option. The shell top corresponds to the positive element normal. The shell bottom corresponds to the negative element normal.

Note:

This option uses the Backface Culling visualization preference to display the result correctly. When you request Top and Bottom stress or strain results, you are prompted to turn on Backface Culling.

When you use the Graph, Identify, and Marker commands on Top and Bottom results, note the following:

• When you graph results on a top-and-bottom display, results are extracted from only the top of the elements.

• When you use the Identify command, both top and bottom results are listed.

• When you use the Annotation command, the annotations list both the top and bottom results at specified locations. The contour plot legend always displays the minimum and maximum of the entire model, regardless of the top or bottom face.

Bending example

In the following example, the model is loaded in the axial and vertical directions. The difference between the top and bottom stress result and the bending stress result is relatively small, which indicates that most of the stress is due to bending.

Top and Bottom stress result, 9970 psi

Bending stress result, 9074.22 psi

Order of precedence for calculating shell location values

When you specify a shell location for stress or strain, the software uses the following order of precedence to determine the displayed value:

1. Calculates stress tensor results for each shell locationIf nodal averaging is turned on, the software calculates the averaged primary tensor value for each node. The software calculates the value using the unaveraged primary values of connected elements. Averaging occurs on primary values and not on invariant quantities (derived results). Averaging also occurs on the top and bottom surfaces separately.

2. Combines nodesThe software determines the invariant quantities (Von-Mises) from the averaged or unaveraged tensor for the top and bottom surfaces.

3. Performs shell operationThe software then performs the shell operation (minimum, maximum, average, or bending) from the top and bottom derived results.

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