© 2011 ANSYS, Inc. February 21, 2013 1

Using Command Script to Obtain Surface Solution on Selected Solid Faces

Jay Shi & Sheldon Imaoka

© 2011 ANSYS, Inc. February 21, 2013 2

• Stresses and strains are calculated at integration points and are extrapolated or copied to surface nodes, which reduces their accuracy.

• Surface solution is critical for some post-analysis calculation, e.g., fatigue life assessment.

• There is an option to output surface solution for certain legacy solid elements (// Feature Archive // I. Legacy Features // 5. Surface Solution) with limitations:

Legacy types are not used by default in Workbench Mechanical;

It cannot be used with nonlinear material models, nor large-strain effects, either.

To address the above issues, one can use current-technology shell elements’ “stress/strain evaluation only” option to obtain accurate stresses & strains on surfaces.

Problem Description

© 2011 ANSYS, Inc. February 21, 2013 3

• Use APDL command objects with ESURF to overlay a shell skin on selected solid face(s) using Named Selections; then, use ETABLE to collect stress/strain quantities on the shell skin.

• The procedure can be applied to linear or quadratic solid elements with quad or triangular surface mesh. It can be used with nonlinear materials and large deflection.

• The example also demonstrates how to:

obtain element areas of the selected face;

pass geometry/solution quantities to user-defined arrays; and

calculate area-weighted average of stress over the selected face.

Solution

© 2011 ANSYS, Inc. February 21, 2013 4

• The model features a solid, cantilever beam.

• One section from the top surface of the beam is selected and named “area” in the Named Selections.

1. Define a surface Named Selection

© 2011 ANSYS, Inc. February 21, 2013 5

Develop a command script with following procedure:

1. Select elements attached to the faces’ nodes

2. Determine the proper element/section/material type IDs for the shell skin. Note that the element type ID will be stored in variable MY_MAX_ETYP for later reference in the Solution command object. The material used should be the same as the underlying solid

3. Choose the correct shell element based on the solid element type

4. Use Keyoption (1) = 2 for shells in stress/strain evaluation only (no contribution to structural stiffness). Note that thickness will be one by default

5. Use ESURF command to generate the shell skin.

2. Overlay a shell skin on faces

© 2011 ANSYS, Inc. February 21, 2013 6

Insert a command object that implements the procedure described in the previous slide:

2. Over-lay a shell skin on “area”

© 2011 ANSYS, Inc. February 21, 2013 7

Develop an APDL command script for post-processing:

1. Select shell elements based on element type ID

2. Use ETABLE to fill element Mises stresses and volumes. Note the element area equals to volume in value, due to unitary thickness.

3. Pass element table values to properly dimensioned array parameters: V_S, V_A

4. Calculate weight average of stresses:

5. Return result in parameter: MY_S_AVG.

3. Obtain surface quantities from the overlaid shell skin

IIShells

IIIShells

A

A

:

:

© 2011 ANSYS, Inc. February 21, 2013 8

Insert a command object in the Solution branch for procedure in the previous slide:

3. Obtain surface quantities from the overlaid shell skin

© 2011 ANSYS, Inc. February 21, 2013 9

• Note that the result MY_S_AVG is returned automatically into the Details view, due to its prefix “my_”.

• The created Shells are not available in Mechanical but can be processed and visualized using commands such as PLESOL.

4. Results