Solving a random base excitation event

The Response Dynamics solution process for the random base excitation (RBE) analysis references a Simcenter Nastran SOL 103 Response Dynamics solution from which it reads the following data:

• Modal data: eigenvalues, eigenvectors, and modal participation factors

• Residual vectors

• Failure data for homogeneous materials: ultimate strengths or yield strengths

• Failure data for laminates: ply failure theories and material stress and strain limits

Generated RBE results

When you solve an RBE event, the software uses the RBE solver, which can return the following results:

• Nodal peak and the number of positive zero crossings responses for acceleration, displacement, velocity, multipoint constraint (MPC) force, single-point constraint (SPC) force, and grid point force. For grid point force, you can only request peak contours. The software automatically computes phase-consistent translational and rotational magnitudes for acceleration, displacement, and velocity.

• Elemental peak and number of positive zero crossings responses for stress, strain, and force. The software automatically computes Von Mises stress and strain for both homogenous and laminate elements. You can also specify how the RBE solver computes the principal stress and strain.

• Failure indices, strength ratios, and margins of safety. You can only request strength ratio for laminates.

• Power spectral density XY functions for acceleration, displacement, velocity, MPC force, SPC force, stress, and strain.

• Peak results that account for the user-defined confidence level, which are typically 99.73% or 3-sigma. You are not required to post-process the results, as you do for a heritage random event, by multiplying the root mean squared (RMS) results by 3. The solver uses advanced methods to accurately determine the peak value for results that do not follow a Gaussian distribution, such as Von Mises stresses or Tsai-Wu failure indices.

For more information, see Random base excitation results and Random Output Requests dialog box.

RBE solver

The RBE solver uses advanced integration algorithms to produce the specified output in a single RBE analysis and implements multithreading to enhance performance. It automatically determines the required excitation frequencies on a result by result basis, so you are not responsible for tracking the number of spectral lines you define. The solver does not force an excitation frequency at each input function definition point and, uses even and uneven excitation functions in exactly the same way.

To compute homogeneous and laminate element failure metrics, and to process any residual vectors, the RBE analysis requires that, for the SOL 103 Response Dynamics solution, you select the Random Base Excitation Event check box in the Solution dialog box. When you select this check box, the software also verifies that:

• The version of Simcenter Nastran is 2021.1 or higher.

• You defined at least one translational enforced motion location, but not more than three, and all of them are defined at the same node.

• No rotational enforced motion locations exists.

• All translational enforced motion DOFs are defined, when enforced motion CSYS is not BASIC.

While you can still solve an RBE event when the Random Base Excitation Event check box is cleared and if you validate that previously listed items adhere to the software requirements. However, the preferred workflow is to select this check box so that the software performs the validation. Selecting this check box has no effect on any other Response Dynamics events you may define.

Ideally, the FEM is constrained (SPC) at the same node on which enforced motions are applied. This is not verified by the software.

The RBE solver manages SOL 103 Response Dynamics subcases as follows:

• Static offsets and dynamic loads are excluded.

• Differential stiffness and residual vectors are supported.

The RBE analysis consumes all activated normal modes from the solved modal model. It queries the viscous, hysteretic, and physical damping. The software converts the hysteretic and physical damping to viscous damping and summed with viscous damping. You can modify the viscous and hysteretic damping values for normal modes, and elect to exclude the physical damping.

The RBE solver excludes attachment modes, sensors, and strain gages.

Specifying the PSD excitation function

Before solving the RBE event, you must define a single enforced motion Translational Nodal Excitation. The software automatically selects the node where the translational enforced motion is located. You specify the base excitation power spectral density (PSD) function.

The PSD function can be even or uneven, and must have acceleration as the ordinate measure. You can choose a general or response dynamics type function.

Running from a command line

You can run the RBE solver to solve an RBE event from a command line, leveraging other computers at your disposal, including Linux machines. For more information, see Running a random base excitation event from a command line.

Where do I find it?

Application	Pre/Post
Prerequisite	An active, up to date RBE event
Command Finder	Solve Event
Simulation Navigator	Right-click the random base excitation node→Solve→Solve Event

How do I

Create and solve a random base excitation

Learn more

Random Base Excitation

Look up more details

Right-click commands for Random Base Excitation nodes

Random base excitation results

Random base excitation output files

Running a random base excitation event from a command line

Random Base Excitation dialog box

Random Output Requests dialog box

Random Output Requests Manager dialog box

Quick links

Command reference

Pre/Post video examples

Bulk Entry Descriptions

Simcenter 3D tutorials

Browse Simcenter 3D help by product area

Solving a random base excitation event, Simcenter 3D 2021.1 Series

© 2020 Siemens

window.mainLanguage="en_US"

window.delivId=""

window.projectId=""

MathJax.Hub.Config({ TeX: { extensions: ["autoload-all.js"] }, tex2jax: { displayMath: [ ] }, "SVG": { scale: 125 } });