Extracting a single value from the set of results (margin of safety)

In the Load Extraction dialog box, the combination of your selections from the Result Type and Component lists, and your selection of nodes or elements determines the set of results for the margin of safety calculation. The options in the Operations group determine which single value from the set of results the software uses in the calculation. The software applies these options sequentially as follows:

1. Filters the set of results.

2. Takes the absolute value of the filtered results.

3. Applies the aggregation option.

Filtering options

To exclude values in the results set from the calculation, use the options in the Filtering list.

• To exclude values outside the range you specify, select Range.

• To exclude negative values, select Only Positive Values (> = 0).

• To exclude positive values, select Only Negative Values (< = 0).

For example, suppose your selections produce the following set of results: (3.4, 5.1, -2.7, 7.5, -4.6, -3.9, 2.0, 9.1, -3.3, -8.4)

If you select Only Negative Values (< = 0), the software excludes all the positive values. All other values remain under consideration for use in the calculation. Thus, the set of results becomes: (-2.7, -4.6, -3.9, -3.3, -8.4)

Absolute value option

To make all the values in the set of results that are left after filtering positive, from the Absolute Values list, select Yes.

Continuing the example, if you select Yes, the set of results becomes: (2.7, 4.6, 3.9, 3.3, 8.4)

Aggregation options

The options in the Aggregation list specify how the software obtains the single value from the remaining set of results.

• To pick the algebraic maximum value, select Maximum.Continuing the example, if you select Maximum, the software uses 8.4 in the calculation.

• To pick the algebraic minimum value, select Minimum.Continuing the example, if you select Minimum, the software uses 2.7 in the calculation.

• To use the arithmetic mean of the values remaining in the set of results, select Average.Continuing the example, if you select Average, the software uses 4.58 in the calculation.

• To use the arithmetic sum of the values remaining in the set of results, select Sum.Continuing the example, if you select Sum, the software uses 22.9 in the calculation.

Note:

The Aggregation list is not available for methods that employ method aggregation.

For more information on method aggregation, see Custom methods.

Conversion box

You use the Conversion box to correct for incompatibility between the result type that you select for the working load, stress, deflection, and so on, and the result type the method requires.

The Conversion box is available when the software detects an incompatibility between the result type that you select and the result type the method requires. When the Conversion box is unavailable, you must use the Factor box to correct for any incompatibility.

When the software detects an incompatibility, it notifies you of the data that it needs to correct for the incompatibility. Consider the following examples:

• In the Calculation - Method Selection dialog box, suppose that you select Tresca plane stress as the method. The method requires stresses. If in the Load Extraction dialog box, from the Result Type list, you select a force result, the Conversion box appears as Conversion (Force to Stress - divide by) because the software needs the area over which the force is distributed. To correct for the incompatibility, the software divides the force result by the value for the area that you specify in the Conversion (Force to Stress - divide by) box.

• In the Calculation - Method Selection dialog box, suppose that you select Bolt shear as the method. The method requires a force. If in the Load Extraction dialog box, from the Result Type list, you select a stress result, the Conversion box appears as Conversion (Stress to Force - multiply by) because the software needs the area over which the stress acts. To correct for the incompatibility, the software multiplies the stress result by the value for the area that you specify in the Conversion (Stress to Force - multiply by) box.

• In the Calculation - Method Selection dialog box, suppose that you select Column Euler as the method. The method requires a stress. If in the Load Extraction dialog box, from the Result Type list, you select a force result, the Conversion box appears as Conversion (Force to Stress - divide by) because the software needs the area over which the force is distributed. To correct for the incompatibility, the software divides the force result by the value for the area that you specify in the Conversion (Force to Stress - divide by) box.

You have a variety of options for specifying the value that the software uses to correct for incompatibility between the result type that you select for the working load, stress, deflection, and so on, and the result type the method requires. For example, you can:

• Type the value directly into the Conversion box.

• Extract the value from a measurement, beam section, or laminate definition.

• Define the value with an expression or field.

The following table shows the incompatibilities that the software detects and the correction that is required.

Dimensions of result type required by method	Dimensions of result type you selected
Stress	Force	Force per unit length	Strain	Length	Moment
Stress	Compatible	Divide by area	Divide by length	Multiply by stress	Not detected	Divide by volume
Force	Multiply by area	Compatible	Multiply by length	Not detected	Not detected	Divide by length
Force per unit length	Multiply by length	Divide by length	Compatible	Not detected	Not detected	Divide by area
Strain	Divide by stress	Not detected	Not detected	Compatible	Divide by length	Not detected
Length	Not detected	Not detected	Not detected	Multiply by length	Compatible	Not detected

Factor box

You use the Factor box to specify a multiplier that scales the result that the software uses in the margin of safety calculation as the working load, stress, deflection, and so on.

When the Conversion box is unavailable, you can also use the Factor box to do the following:

• Specify a multiplier that corrects for an incompatibility between the result type that you select as the working load, stress, deflection, and so on, and the result type that the method requires.

• Specify a multiplier that converts the units of the result that the software uses in the margin of safety calculation as the working load, stress, deflection, and so on, to the units of the failure load, stress, deflection, and so on.

Note:

To assist you in determining the correct multiplier when the Conversion box is unavailable, the Unit Information group appears.

For example, suppose that in the Calculation - Method Selection dialog box, you select Generic allowable as the method. Because the software does not know how to interpret the dimensions and units for the numerical value that you enter in the Allowable box on the Calculation Creation dialog box, the Conversion box is unavailable. Thus, you must use the Factor box to correct for any dimensional and unit incompatibilities, and, if desired, specify a multiplier that scales the result that the software uses in the margin of safety calculation.

You can, however, use an allowable method with units to create a margin of safety calculation. Consider the following scenario for a model where you use the Stress allowable method to implement the von Mises criterion. Assume that the units for the model are kilogram, millimeter, and second. Thus, the units for a stress result that the software uses in the margin of safety calculation is kPa.

Note:

These units may differ from the default units that the software uses to display stress results in post-processing.

In post-processing, suppose that you use the Identify Results command to determine that the von Mises elemental stress for an element is 3.67906 MPa. If you enter 280 in the Allowable box and change the units to MPa on the Calculation Creation dialog box, the correct margin of safety is:

MS = \left( {\frac{{280{\rm{ MPa}}}}{{3.67906{\rm{ MPa}}}}} \right) - 1 = 75.106

However, when you use the Load Extraction dialog box to select the von Mises elemental stress, and you enter 1 in the Factor box, the software calculates the margin of safety to be -0.923894. The incorrect result occurs because of a unit discrepancy. The software uses 3679.06 kPa in the margin of safety calculation as follows:

MS = \left( {\frac{{280{\rm{ MPa}}}}{{3679.06{\rm{ kPa}}}}} \right) - 1 = - 0.923894

To correct for the unit discrepancy, enter 0.001 in the Factor box. This value is the conversion factor between MPa and kPa. Then the software obtains the correct result from the following calculation:

MS = \left( {\frac{{280{\rm{ MPa}}}}{{3.679.06{\rm{ kPa}}\left( {0.001{\rm{ MPa/kPa}}} \right)}}} \right) - 1 = 75.106

Previewing values for the margin of safety calculation

To view the values that the software will use in the margin of safety calculation prior to solving for the calculation, in the Preview group, click Preview to Info Window. To also view the unaggregated values expressed in the preferred data entry units, select the Do Extended Preview check box prior to clicking Preview to Info Window.

Note:

Use the Units Manager command to select the preferred data entry units.

After solving for the calculation, you can view the values that the software used in the margin of safety calculation by opening the calculation log. For instructions on how to open a calculation log, see Margin of safety results.

Where do I find it?

Application	Pre/Post
Prerequisite	An active margin of safety solution
Simulation Navigator	Calculations node→right-click a calculation→Edit
Location in dialog box	Inputs group→Inputs tab→Load Extraction

How do I

Create a margin of safety calculation

Import values from a spreadsheet

Extract the area from a beam cross section

Extract a projected length

Create a margin of safety calculation using an allowable method

Bolt shear method example

Column Euler method example

Nodal force report extraction example

Learn more

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Parent/child behavior in margin of safety calculations

Manual load extraction

Nodal force report extraction

Extracting shell element plane stress components relative to a common CSYS

Property extraction

Beam cross section property extraction

Laminate extraction

Laminate property extraction

Sources manager (margin of safety)

Displaying result sources for margin of safety calculations

Look up more details

Methods for buckling of orthotropic laminates

Quick links

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Pre/Post video examples

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Margin of Safety methods documentation

Extracting a single value from the set of results (margin of safety), Simcenter 3D 2021.1 Series

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