An expression is composed of one or more existing field output variable tags and, optionally, one or more operators, transformations, and scalars. See Using operations on field output, for information on supported operators. Expressions are evaluated as Python input; entries containing syntax errors will generate non-specific Python exceptions.
The Create Field Output dialog box lists field output by the variable name and by a unique tag created by Abaqus/CAE. The tag for a given field output variable is composed of the step and frame numbers prepended to the output variable name; for example, s1f1_RF is the reaction force at Step 1, Frame 1. This tag enables you to combine output from different steps and frames in one expression.
Field output variables can consist of different types of data that may not be compatible. The Create Field Output dialog box lists the type of each field output variable as one of three general types: scalar, vector, and tensor. The tensor types are further subdivided into five subtypes that describe the dimensionality of the tensor and the components available. The following rules apply:
Only like data object types and subtypes can appear in an expression.
Operations on complex data use only the real component.
The multiplication and division operations are not supported between two vector objects or between two tensor objects.
Operations on tensors are performed on the tensor component data that Abaqus/CAE reads from the output database. Subsequently, the display of the resulting field output variable may give unexpected values for extrapolated components or for computed invariants. For example, if the components of a stress tensor are negative, applying the absolute value operation to the stress tensor will produce positive values for the stress components, but the values of pressure—an invariant computed after the absolute value operation has been applied—may be negative. Similarly, applying the sine operation to a stress tensor will produce component values within the range {−1, 1}, but the extrapolation used to compute contour values of such components may produce values beyond this range.
Operations on fields associated with different regions are not supported.
The following examples demonstrate valid field output expressions:
- Example 1
To create a field output variable by finding the difference in the stress fields for two increments, type:
s1f2_S - s1f1_S
in the expression text field of the Create Field Output dialog box.
s1f2_S and s1f1_S are field output variables representing stress at two different increments of a particular step. The result of this equation is a field output variable representing the difference in the stress fields for the two increments.
- Example 2
To create a field output variable by expressing pressure in decibels as a function of acoustic pressure, type:
20.0 * log10(s1f1_POR/Pref)
in the expression text field of the Create Field Output dialog box.
Pref is the reference pressure. The result of this equation is a field output variable representing pressure in decibels as a function of the analysis variable POR.
- Example 3
To create a field output variable by transforming a tensor result to a user-specified coordinate system:
Select Transformation from the Function list on the right side of the Create Field Output dialog box.
Select a tensor result from the list of field output variables; for example, s1f10_S.
Select a coordinate system transformation to apply; for example, a fixed coordinate system named CSYS-1.
The expression s1f10_S.getTransformedField(datumCsys=o_CSYS_1)appears in the expression text field.
The result of this equation is a field output variable representing the transformation of the stress tensor s1f10_S to the CSYS-1 coordinate system.