Choosing the wrong solution type is one of the most common sources of misleading FEA results. This Book Byte gives you the questions to answer, in the form of simple decision tables, to choose between a static or dynamic solution, a linear or nonlinear solution, a buckling solution and a normal mode solution.
Define the type of analysis required to capture the real effects
The first two questions to answer are:
- Should you run a static, buckling, normal mode or dynamic solution?
- Should you run a linear or a nonlinear solution?
Should you run a static or a dynamic solution?
| Question | Static | Dynamic |
|---|---|---|
| Is the loading constant over a relatively long period of time? | Yes | No |
| Are the inertial and damping effects negligible? | Yes | No |
| Are the loads applied gradually? | Yes | No |
| If the structure is subjected to vibrations, is the excitation frequency less than one third of the structure's lowest natural frequency? | Yes | No |
Should you run a linear or a nonlinear solution?
| Question | Linear | Nonlinear |
|---|---|---|
| Does the stiffness of the structure change when the loading is applied? | No | Yes |
| Are the displacements (translations and rotations) large? | No | Yes |
| Are the loads and boundary conditions constant in magnitude and direction during the analysis? | Yes | No |
| Is the behavior of the structure independent of load history? | Yes | No |
| Do the stresses in the model exceed the proportional limit? | No | Yes |
| Does the structure exhibit stress stiffening (tension-bending coupling, where the bending stiffness increases because tension is carried by the membrane)? | No | Yes |
Should you run a buckling solution?
| Question | Static | Buckling |
|---|---|---|
| Do you need to check the linear stability of the structure? | No | Yes (linear) |
| Do you need to compute the load factor at which the structure becomes unstable (reserve factor before buckling)? | No | Yes (linear) |
| Do you need to check the stability of the structure under large displacements or beyond yield? | No | Yes (nonlinear) |
| Do you need to know the deflections and/or stresses of the structure at the instability point? | No | Yes (nonlinear) |
| Do you need to know what happens to the structure after buckling (post-buckling)? | No | Yes (nonlinear) |
Should you run a normal mode solution?
| Question | Static | Buckling | Modal |
|---|---|---|---|
| Do you need to determine the natural frequencies at which a structure tends to vibrate, to avoid resonance? | No | No | Yes |
| Do you need to compute the deformed shape of the structure at a specific natural frequency? | No | No | Yes |
| Do you need to understand the behavior of a structure under dynamic loads before running a dynamic analysis? | No | Yes | Yes |
| Do you need to identify possible modeling mistakes (missing connections between components, incorrectly defined constraints)? | Yes | Yes | Yes |
Go further
This Book Byte summarizes guidance found throughout Practical Finite Element Analysis for Mechanical Engineers by Dominique Madier. The topic is covered in depth, with worked examples, in the on-demand course Practical Basic FEA.
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