Dynamic Time Analysis Overview
You can use a dynamic time analysis to examine the system’s transient response to time-varying loads. The load input takes the form of a time history.
In a dynamic time analysis, Creo Simulate calculates displacements, velocities, accelerations, and stresses in your model at different times in response to a time varying load. Use dynamic time analyses if you are interested in transient or non-steady forced response.
For base excitation only, you can direct Creo Simulate to calculate the modal mass participation factors to enable you to better assess the accuracy of your results. Creo Simulate also calculates all valid measures for dynamic time analyses that you have defined for the model.
You can direct Creo Simulate to report full results at specified time intervals. For dynamic time analysis with full results, you can animate results of a fringed display for each time interval of your analysis.
Return to Dynamic Time Analysis.
Base Excitation for a Dynamic Time Analysis
Base excitation is only available if your dynamic analysis references one constrained modal analysis.
If no loads are defined for your model, you must define the dynamic analysis using base excitation. In this case the structure is excited by a prescribed shaking of the supports that hold it.
Base Acceleration Time Dependence—Specifies the direction of motion of the supports and the base acceleration as a function of time when you select Base excitation as the loading type. This area displays the following items:
• Excitation type—Specifies the type of excitation of the supports from this drop-down list. Items displayed on the dialog box change based on the selected type of excitation.
◦ Uni-directional translation—Allows the supports to have only translational motion in one direction. Specify the direction in terms of its X-, Y- and Z-components in the WCS or a selected Cartesian coordinate system and the base acceleration along that direction.
 |
For Uni-directional translation, the magnitude of the direction vector is ignored, and only its directional information affects the acceleration.
|
◦ Translations & rotations—Allows the supports to have both translational and rotational motion. Specify X-,Y-, and Z- components for translational and rotational acceleration of the supports along each axis of the WCS or a selected coordinate system. Rotations are about the origin of the referenced coordinate system.
◦ Translations at 3 points—Specifies the motion based on six translational directions at 3 point supports. To use this option, the referenced modal analysis must be constrained with 3 point constraints. All translations must be constrained at the first point, 2 translations constrained at the second point and 1 translation constrained at the third point in such a way so that only rigid body motion is prohibited.
|
|
2D plane stress and 2D plane strain models can have Uni-directional translation or Translations & rotations as the excitation type. For 2D axi-symmetric models you can have only Uni-directional translation as the excitation type.
|
• Coordinate system—For Uni-directional translation select a coordinate system to specify the direction of movement of the supports. WCS is the default. You can also select a Cartesian user coordinate system.
For Translations & rotations select a coordinate system to specify the translational and rotational components of acceleration.
For Translations at 3 points you define the coordinate system when you define the point constraints for the three point supports.
For a selected excitation type you need to specify the following:
• Value—Specify a value for base acceleration. Select units of acceleration or angular acceleration from the adjacent drop-down list. Value acts as a multiplier. The time dependence function is multiplied by value to give the base acceleration of the support for each component. When the excitation type is Translations & rotations or Translations at 3 points you need to specify individual values for each component.
• Time Dependence—Create or select a function for time dependence, or use the default function. The time dependence function is a function without units. The value of the function varies as a function of time.
You can use the default unit impulse function or click 
to open the Functions dialog box and create a new function. You can create symbolic or table functions for time dependence.
to open the Functions dialog box and create a new function. You can create symbolic or table functions for time dependence.• When the excitation type is Translations & rotations or Translations at 3 points, you must select the check box for each direction to activate the Value, and Time Dependence fields.
| | For models created in Wildfire 5.0 and earlier releases, excitation type is selected as Uni-directional. Value is specified as 1 and the default function is selected for time dependence. |
Return to Dynamic Time Analysis.
Translations at Three Points
For a dynamic time, frequency, or random analysis, to specify the motion of the base in terms of translations at three point supports you need to constrain your model in a specific way. In the referenced modal analysis you must define point constraints for 3 non-collinear points in your model.
The point constraints must satisfy the criteria illustrated in the following example:
• Consider three points as shown in the figure. The first point, must have all translational degrees of freedom (DOFs) fixed, the second point must be constrained along 2 axes and the third point must be constrained along only one axis. With these three point constraints only the rigid body motion of the model is constrained.
• The constraint set for the three point constraints must be in terms of the WCS or a common Cartesian UCS.
• The first point must have all translational DOFs fixed. In the example Point 1 must have translations constrained along the X-, Y- and Z- directions.
• The line joining Point 1 to Point 2 must be parallel to one of the axes of the coordinate system. Point 2 must be free along this axis. It should be constrained along the other two axes. In the example the line joining Point 1 and Point 2 is parallel to Y-axis. Point 2 should have translations in the X- and Z- directions constrained.
• Point 3 must be in the plane through line joining Point1 and Point2 and parallel to one of the coordinate planes. Point 3 must be constrained in the direction normal to the plane defined by the three points. In the example the three points define the X-Y plane. Point 3 must have translation in the Z- direction constrained.
Return to Base Excitation for a Dynamic Time Analysis.or Base Excitation for a Dynamic Frequency Analyses or Base Excitation for a Dynamic Random Analysis
Output for a Dynamic Time Analysis
Use the following items on the Output tab of the Dynamic Time Analysis dialog box:
• Output Steps—Specifies the number of steps in the time range at which you want Creo Simulate to report results.
Return to Dynamic Time Analysis.
Calculate Quantities for a Dynamic Time Analysis
Use the Calculate area on the Output tab to specify the type of results to be calculated for a dynamic time analysis.
Select one of the following check boxes if you want Creo Simulate to calculate full results:
• —Calculates stresses. If you do not need stress results, clear this check box to save disk space and drastically reduce analysis time.
• —Calculates the rotational degrees of freedom of beams and shells over the entire model.
Rotations are not calculated if your model consists only of 3D solid, 2D solid, or 2D plate elements, even if this check box is selected. Rotations are always zero for these element types.
• Ply Stresses—Calculates the ply-by-ply stress results. This check box is only for models that have laminate shell properties defined.
The following options are available on the dynamic time analysis dialog box only if loading is by base excitation:
• Mass participation factors—Calculates the modal mass participation factors. This can result in greatly increased computational time.
Mass participation factors are important for determining if sufficient vibration modes obtained from modal analysis are included to ensure accurate results.
• Displacements, velocities, accelerations relative to—Calculates results for displacement, velocity, and acceleration results with respect to ground or supports.
You cannot access full results for quantities you do not select here. Creo Simulate calculates all measures valid for an analysis, regardless of your selections here. You can access results for measures through the summary report or by graphing the measure in a results window.
| | If you select Time/Frequency Eval when you define a dynamic measure, you can access results for this measure only through a results window. |
Return to Output for a Dynamic Time Analysis
Mode Options for Dynamic Analyses
These items appear on the Modes tab on the dialog boxes for dynamic time, dynamic frequency, and dynamic time analyses:
• Modes Included—Specify which modes from the modal analysis you want included in the dynamic analysis.
• Damping Coefficient (%)—Assign damping coefficients to the modes in the analysis.