If the effects of inertia, damping, and transient loading are significant, then a nonlinear transient analysis should be used. Additionally, “quasi-static” models that undergo buckling or other instable loading conditions will often converge better in a nonlinear transient analysis due to the inertia effects keeping the model stable.
A nonlinear transient analysis requires both dynamic and nonlinear setup steps. Autodesk Inventor Nastran solves both analyses essentially simultaneously, making it one of the most complex yet exciting solution types in FEA.
An important element to having a stable nonlinear transient (NLT) solution is to provide damping in the model. There are two types of damping that can be applied in NLT solutions:
The increased flexibility of material based damping (i.e., different damping values can be applied to different areas/materials of the model) makes it the logical choice for NLT analysis.
A note of caution when using damping in a NLT solution is that for models where the velocity/inertia is the main driver of the analysis such as in an impact solution, damping can have a significant effect on the acceleration/velocity/displacement of the model. This is because the solver cannot make a distinction between rigid body motion/velocity, and flexible motion/velocity, so the damping is applied to any part of the structure that has a velocity. For impact analysis it is recommended to use no damping or a small “stability” damping value (i.e., 1.E-6). The Solve Impact Analysis topic in the User’s Guide contains additional information about impact analysis.
There are a few guidelines to follow when performing an impact analysis that will have a large effect on solution time and quality of the results.
This is a very important and often overlooked stage. You need to know the linear response characteristics of the structure to get some idea of what the actual nonlinear frequencies and mode shapes are going to be. It can never be an exact representation, but it gets you in the right ball park for several key input parameters:
Constrain (fully fixed) the area of the model that you expect to make contact with the ground (or other impactor) and run a normal modes solution with ~20 modes. Look at the mode shapes and find the mode you would consider to be the “dominate” response of the structure during/after impact. A look at the modal effective mass table in the *.OUT file may also help determine the critical mode. The frequency of the mode can be used to calculate the key input parameters above:
In most situations it is best to perform a hand calculation to find the velocity at impact and then start the two models near each other. This approach will net shorter analysis times, and better fidelity than starting the two bodies at a physical distance (i.e., as in a drop test). A good method for calculating the small separation distance is to use the equation:
d = v * (2*dt)
where:
d = separation distance
v = velocity
dt = time increment
This separation distance will allow for the solution of 2 time steps before impact.
When pre/post-impact behavior is desired, using multiple subcases is a good way to fine-tune the analysis such that detailed time stepping can be used during impact, and a much coarser time-step can be used after impact.
Autodesk Inventor Nastran features an automated impact analysis solution type that automatically does the steps mentioned in the above Impact Analysis section. The AIA solution type is activated via the IMPACTGENERATE Case Control card (see the Autodesk Nastran Reference Manual located in the Inventor Nastran install directory for more info on the IMPACTGENERATE card). The solver will automatically execute the following steps to perform the AIA analysis:
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