9.6 Checklist for Dynamic FE Analysis

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1. Study the physical system and decide on the type of problem, i.e., beam, 2-D, shell, or 3-D, and the

type of elements to be used. Typical questions to be addressed are:

* Can the geometry be idealized as 2-D or shell or do we have to use 3-D analysis? (Section 9.1.2 to

Section 9.1.6).

* Can the loading and constraints be idealized as 2-D?

* Are we concerned about responses that are not compatible with the loading type?

* Are we interested in displacements, stresses, or natural frequencies?

* Can we have a simple spring/mass model?

* What type of element might be used in the analysis, i.e., linear, quadratic, or cubic (Section

9.1.7)? Generally, linear and quadratic elements are used for the majority of analyses. The choice

between linear and quadratic elements is rather arbitrary and may depend on certain advanced

analysis aspects. For linear analysis, there is little difference between the response of the two

elements if the overall model has the same or similar total DoF.

2. Study all types of loading to identify the required analysis type. Important questions to be

addressed are:

* Is the load steady, cyclic, transient, or random (Section 9.2)?

* What is the frequency content of the load? A simple Fourier analysis may be required to define

the main harmonics of a transient load input.

* What are the first few natural frequencies of the structure? This question will be answered in

detail by performing a modal analysis. It is important, however, to have an approximate idea of

the range of frequencies of the structure and to compare this to the loading frequency in order to

identify the proper type of analysis (Section 9.2). In order to answer this question adequately, a

very simplified mass/spring model and quick hand calculations may be considered.

* What kind of data is required from the analysis? Data required may be in terms of

displacements, velocities, accelerations, stresses, strains, etc. Also, identify if peak values or time

histories are required. This decision may have an impact on what analysis type is required.

* What is the type of analysis required (Section 9.2)?

3. Identify other data required to perform the analysis. This may include:

* Material properties.

* Damping consideration, modeling, and values (Section 9.4.4).

* Identifying all load cases and all constraints of the system. Special attention must be paid to the

modeling of constraints and nonstructural masses (Section 9.3).

* If modal superposition is going to be used, how many modes are required for the analysis?

4. Start the actual modeling process, testing and verification of the model. Important issues to be

considered may include:

* A decision should be made of whether or not a simple or coarse model is required. In general,

dynamic analysis of large models is quite costly. Dynamic analysis results (such as natural

frequencies, mode shapes, and to some extent displacements, velocities, and accelerations) may

be accurately obtained from a coarse model or even a mass/spring model. The main concern

would be in obtaining stress and strain responses. In many cases, it may be advisable to have two

different models — one for dynamic analysis and the other for stress analysis. The output from

the dynamic model may be used as input to the stress model. In any case, if the model is large, it is

advisable to have a coarser model for testing and basic understanding of the behavior of the

structure (Section 9.3).

* Choose a program to perform the analysis. The key issue in choosing a program would be the

availability of the required analysis type. Other issues that may be considered include ease of

model generation, ease of postprocessing, and cost effectiveness. It is always advisable to have the

same program for all phases of the analysis, i.e., modeling, solution, and postprocessing.

Finite Element Applications in Dynamics 9-41

© 2005 by Taylor & Francis Group, LLC

* Build the model geometry (Section 9.1.1). In 2-D problems, the model geometry may be

imported from a CAD program. In 3-D and shell problems, however, this process entails many

problems and it may be faster for the analyst to build the model directly in a FE database.

* Build the finite element model and apply load cases and constraints (Section 9.2).

* Test the model. Most FE programs provide extensive testing capabilities that may be invoked to

test the quality of the mesh and the continuity of the model. It is important to note that

additional testing is required. The response of the system to simple load cases should be

considered. Testing should involve load cases that may be easily verified. This may include

computing structure response due to unit impulse, step input, or sudden release of an initial

displacement. Also, simple static analysis runs can be very useful in this regard and may provide

initial conditions for the final dynamic run.

5. Analyze the model for the actual load cases. The following important issues should be considered:

* Identify solution parameters. This includes mass and damping formulation, damping

parameters (Section 9.4.4), time integration, and time stepping parameters, as well as

parameters to control output and postprocessing options. If modal analysis is considered,

identify the modal extraction method (Section 9.5).

* Check and apply restart capabilities in the program. Large size dynamic analyses should, in

general, be done in steps and the restart option should be invoked.

* If modal analysis is performed, a careful check of mode shapes will provide important

information about the validity of the model and the appropriateness of solution steps to follow.

* Identify solution control parameters that invoke the storage of specific data required for

postprocessing.

6. Postprocess the results. This is the final stage of the analysis and may involve:

* Graphical and contour plots of various response quantities. This may be done in the form of time

history plots, envelopes of maximum responses, or contour plots of stress maxima. The amount

of output data associated with large dynamic analysis runs is vast and such graphical display of

results is essential.

* Most programs have extensive capabilities in results animation, search facilities, preparing

output summaries, and some can also assist in report writing. Such capabilities may become

important in processing the results of large problems.

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