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Engineering | Finite Element Analysis | Stress

Value Design

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Problems With Units.

In many cases, problems with units are easy to detect. However, there are many that can go undetected until it is too late. So, to save you from embarrassment:

1)Review the deformed model shape to ensure it is roughly what you expected.

2) As a matter of course, check that the model is defined in the units you intended.

3) A back of an envelope check should always be completed before any results are available or reviewed. This will help debug the model & identify problems at an early stage.

Incorrect Loading and Boundary Conditions.

There are two main ways of loading the model incorrectly, namely errors with units and oversights in the application of loads and boundary conditions. The first is common with the event of pushbutton analysis packages, this type of error is usually easy to detect. Loading oversights are much more subtle and often impossible to detect. For example, if a bending moment is required and is formed by applying a force at the end of the moment arm, then a direct force is created as well as the bending moment. This is also a common mistake in torsion problems.

Incorrectly applied boundary conditions are common because it is very easy to over or under constrain FEA models and these may give inaccurate results. Another common oversight is to apply a static steady state load value where there will be much higher loads experienced due to impulse. For example, if large boulders are loaded onto a truck trailer, they probably will not be lowered in gently. Therefore, the impulse loads experienced by the trailer chassis will far exceed the static load of the boulders, especially when the trailer is almost full.

Element Polynomial Order.

Finite elements used in meshing solid parts may be either linear or quadratic (parabolic). Linear elements have vertex (corner) nodes only and their edges are straight (for example 4 noded pyramid or 8 noded brick). Parabolic elements have, in addition to corner nodes, a mid-side node along each side creating quadtratic type edges, which can be a quadratic curve rather than a straight line (e.g. 10 noded pyramid or 20 noded brick). Linear elements are also called first, or lower order elements. Parabolic elements are sometimes referred to as second or higher order elements.