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a6-dynamics-kw

By Kim Austin,2014-10-05 07:13
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a6-dynamics-kw

    Answers 6

    Dynamics

    Keywords Version

    Question W61: Are there modes of the physical system that cannot be

    captured by your model because of limitations in element type

    or mesh? (Remember that the elements are planar and the

    mesh is somewhat coarse).

    Answer: Because the model is two-dimensional, it cannot capture the

    modes that occur out of the plane of the model, including

    torsional modes.

    The mesh is too coarse to capture modes other than the first

    five. Use more elements (at least 10) to look at all 10

    requested modes.

    Question W62: Do any of the mode shapes for your model look nonphysical? Answer: No.

    Question W63: How does this compare with the frequency calculated in the

    eigenvalue analysis?

    Answer: The frequency calculated from the history plot of the tip

    displacement is approximately 5.8, which agrees very closely

    with the frequency calculated in the eigenvalue analysis.

    Question W64: Why is the NLGEOM parameter set to NO in this analysis? Answer: By default, Abaqus/Explicit invokes a large-displacement

    formulation. Since the Abaqus/Standard results to which we

    wish to compare were obtained using small-displacement

    theory (which is the default formulation used in

    ? Dassault Systèmes, 2008 Introduction to Abaqus/Standard and Abaqus/Explicit

    WA6.2

    Abaqus/Standard), we must set the NLGEOM parameter to NO in the Abaqus/Explicit analysis.

    Question W65: How do the results compare with one another? What factors contribute to the discrepancies in the solutions?

    Answer: The results for the first step are in close agreement: the peak vertical displacement in the Abaqus/Standard analysis is 30.66 units (downward), while the peak vertical displacement in the Abaqus/Explicit analysis is 31.31 units (downward). Closer agreement can be obtained by reducing the loading rate in the first step in the Abaqus/Explicit analysis. This will further reduce the effect of inertia in this step, thereby improving the quasi-static nature of the loading. Reducing the loading rate will lead to increased computation time, however. You must be careful to balance accuracy and efficiency in this case. The results for the second step show some discrepancies between the two analysis methods. In particular, the peak displacements and the frequency of vibration predicted by the two analysis codes differ slightly. The differences are due to the fact that the fixed time increment that was used to integrate the equations of motion in the Abaqus/Standard analysis was too large to yield the same level of accuracy that Abaqus/Explicit provides. Reducing the time increment used in the Abaqus/Standard analysis yields much closer agreement with the Abaqus/Explicit solution, as illustrated in Figure WA61.

    ? Dassault Systèmes, 2008 Introduction to Abaqus/Standard and Abaqus/Explicit

    WA6.3

    Figure WA61. Comparison of the tip node displacement

    history: reduced implicit time increment

    Question W66: Was the loading rate small enough to ensure a quasi-static

    response in the first step?

    Answer: A comparison of the internal and kinetic energies of the model

    reveals that the kinetic energy is very small relative to the

    internal energy in the first step. This indicates that inertia

    effects are minimal, and the response in this step can be

    considered quasi-static.

    ? Dassault Systèmes, 2008 Introduction to Abaqus/Standard and Abaqus/Explicit

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