Safety factor is not correctly reconciled in Nastran

Autodesk Support

Nov 28, 2024

Products and versions covered

Issue:

There is a difference in computation from the graphical data provided and hand calculations with regards to safety factor. (This issue applies to Nastran In-CAD and Inventor Nastran.)

Solution:

Two methods exist to calculate the safety factor:

First, calculate the average stress from all elements attached to the node. Then calculate the safety factor.
First, calculate the safety factor from all elements attached to the node. Then calculate the average safety factor.

Nastran uses the second method.

Example:

In an analysis, each node has multiple stress values: one stress value for each element attached to the node. The stress that is shown at the node in a contour plot is an average. For example, if four elements at node N have stress values of 100, 110, 130, and 135, the average stress that is shown is 118.75 (= (100+110+130+135)/4).

Un-averaged stress results at a node

Averaged stress result at a node

Figure 1: Average stress is shown at the nodes.

Nastran calculates the safety factors using the separate nodal stresses, and then displays the average safety factor.

If the allowable is 200 in the example, the safety factor that is shown is 1.709 (= (200/100+200/110+200/130+200/135)/4).
This result is different than the safety factor based on the average stress which would be 1.684 (=200/118.75).

Un-averaged safety factors at a node

Averaged safety factor at a node

Figure 2: Is the safety factor at a node based on the averaged stress (resulting in a safety factor of 1.684) or the average of the safety factors (1.709)? Nastran displays the average of the safety factors (1.709).

Note: The stress values used in the safety factor calculation may depend on the element type and mesh element order. For example, the stress values may be from the element corner or element centroid.

Products:

Inventor Nastran;

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