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Tips & Tricks - Inplane effects and limitations of free FEA software

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The example below highlights how compressive and tensile loads effect the capacity of a part to resist lateral bending loads. A cantilevered round tube is subject to a 100N bending load and a 2kN axial load, and the resultant displacement calculated comparing inplane effects, large displacement formulation and a full non-linear solution.

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Compressive Load

Investigate the effect of a compressive axial load on a beam under bending:

Inplane Effects Excluded	Inplane Effects Included	Large Displacement	Non Linear
4.86 mm	5.6 mm	5.6 mm	5.6 mm

Tensile Load

Investigate the effect of a tensile axial load on a beam under bending:

Inplane Effects Excluded	Inplane Effects Included	Large Displacement	Non Linear
4.85 mm	4.29 mm	4.29 mm	4.29 mm

The compressive load caused an increase in lateral displacement, caused by stress softening, while the tensile load caused a decrease in lateral displacement, caused by stress stiffening.

A buckling analysis should also be run to check how close the beam is to collapsing under an infinitesimal increase in load (often at stresses much lower than yield). In this case, the Buckling Load Factor is 7.0, inferring that axial and bending loads 7 times greater than the applied loads are needed to cause buckling.

So watch out for these loading situations when using free limited versions of FEA software, as the advanced analysis options aren't always available to the analyst. The solutions run with no inplane effects showed virtually no change in displacement, despite the change in load direction!

In this case however, the inplane, large displacement and nonlinear solutions gave the SAME results. In our next Tips & Tricks section, we will show you a example where this is NOT the case, and some more things to watch out for.