UG刚柔耦合仿真分析

1.In this lesson

After completing this lesson, you will be able to:

?Create an SEMODES 103 – Flexible Body solution in Advanced Simulation. ?Connect the flexible body finite element model to the degrees of freedom in the motion mechanism.

?Solve the finite element model and generate the RecurDyn Rflex input file.?Define the flexible body in Motion Simulation and solve the motion mechanism.

?Animate the motion mechanism and observe the flexible body deformation.

??2. Overview

Typical motion simulations represent mechanisms using rigid bodies that move in prescribed degrees of freedom according to constraints. These rigid-body motion simulations cannot represent certain dynamic characteristics, especially those resulting from conditions such as sharp impacts, sudden changes in motion, or when the component is flexible enough to affect the motion of the mechanism. For these situations, you can use a flexible body analysis to combine both elastic deformation and rigid body motion.

This type of analysis requires NX Motion Simulation with the RecurDyn solver and NX Advanced Simulation with the NX Nastran solver.

T o set up a flexible body analysis for a component in your mechanism, you create a finite element model on the component and define stiffness at the points where it is connected to the mechanism (typically at joint locations). The NX Nastran SEMODES 103 – Flexible Body solution reduces the dynamic behavior of the flexible body to a set of mode shapes, which are stored in an output file. After you solve this modal solution, you associate the flexible body output file (.rfi file) with the link on which the component is defined in the motion simulation. When you solve the motion simulation, the RecurDyn solver communicates with NX Nastran and recovers the FE results. When you animate the mechanism, the contour plot for the flexible component is animated along with the rigid body animation.

3. Workflow

Advanced Simulation steps

1.Create a finite element model and NX Nastran SEMODES 103 – Flexible

Body solution.

2.Create a finite element mesh on the flexible component and assign

material properties.

https://www.360docs.net/doc/eb9151619.html,e a 1D Connection (spider element) or other constraint elements to

define the component's connection points to the mechanism.

4.Add Fixed Boundary Degrees of Freedom constraints to define

connection degrees of freedom.

5.Add Free Boundary Degrees of Freedom constraints to define load

degrees of freedom.

6.Solve the modal solution. A RecurDyn Rflex input (.rfi) file is generated

for the flexible link.

7.Repeat steps 1–6 for each additional flexible component in the

mechanism.

Motion Simulation steps

1.Create a Flexible Body Dynamics motion simulation.

2.Create flexible links on the flexible components. Associate each .rfi file

with each flexible link.

3.Create a Flexible Body solution and add the flexible links to the solution.

4.Solve the mechanism.

5.Run Animation to view the combined rigid body and flexible body motion.

6.Plot the modal degrees of freedom vs. time to determine the contribution

of selected modes to the flexible body results.

You can select one or more modes of the flexible body and then plot the modal displacement, acceleration, or velocity.

4. Activity: Flexible body analysis — Introduction

Estimated time to complete: 12–16 minutes

You will learn how to:

Define the flexible body in Motion Simulation and solve the

motion mechanism.

Animate the motion mechanism and observe the flexible body

deformation.

Note T o complete this activity, you must have the nx_motion_rflex license, in addition to Motion Simulation, Advanced Simulation, and NX Nastran.

Launch the Flexible body analysis — Introduction activity.

5. About the flexible body modal solution

You will use the Advanced Simulation application to create a finite element model of the flexible body and perform a modal analysis. A modal analysis solves for the natural frequencies and mode shapes of flexible bodies.

The NX Nastran SEMODES 103 – Flexible Body modal solution reduces the flexible body's mass and stiffness to modal space to represent its dynamic behavior. These reduced matrices are saved in the RecurDyn Rflex input file (.rfi).

Then, in Motion Simulation, you associate the .rfi file with a link to create the flexible link.

Example mode shapes

6. About the finite element mesh

The finite element model of a given body consists of a finite element mesh, material properties, and constraints.

When you create a mesh on the body, the software divides the body into discrete regions called elements that are joined together at points called nodes. A group of elements is called a mesh.

The mesh follows the shape of the body. When the model is solved, as the nodes in the mesh are displaced due to the analysis conditions such as loads, the behavior of each element is described by mathematical equations. The software finds the analysis solution by summing the individual element solutions.

Example mesh of tetrahedral elements

In Advanced Simulation, you create the mesh in the FEM file.

When you use a smaller element size (resulting in more elements in a mesh), the accuracy of the solution is improved. However, as you refine the mesh, the solution time and computer resources needed to solve are also increased.

7. Assigning a material to the mesh

In Motion Simulation, material properties for a component are inherited from the master model on which you create the simulation. In Advanced Simulation, you must assign the same material to the flexible body finite element mesh as the material used for the flexible link in Motion Simulation.

T o assign a material to the mesh, edit the mesh collector that contains the mesh. Then, modify the physical property table for the collector and open the Material List dialog box, where you can select the material to use.

8. Connecting the flexible body FEM to the mechanism

In Advanced Simulation, in the FEM file, you must define the points where the flexible body is connected to the motion mechanism. These connection points must be defined at the origin point of every joint, bushing, force, torque, spring, or damper motion object on the flexible body.

Although you can use any element type to define these connection points, typically you will use a 1D Connection (spider element).

For accuracy in your flexible body solution, a single independent node (such as the core node of the spider element) in this connection element should be coincident with the motion object's origin point. You will define stiffness at this connection node using special constraints described in the next section.

Spider elements defined at revolute joint locations

Make sure the connection node is at a location that creates balanced loading. For example, suppose you are connecting the flexible body to a revolute joint that is defined on a hole. Define the connection node in the bore center of the hole, rather than at the edge of the hole.

Example of pinned connection (Revolute joint defined at bore center of hole)

Connection node improperly defined at edge of hole

Connection node properly defined at bore center of hole

9. Defining connection and load degrees of freedom In Advanced Simulation, in the Simulation file, you must add special constraints to define the connection and load degrees of freedom for the flexible body.

?At each connection node where the flexible body will be connected to the mechanism through a joint or bushing in Motion Simulation, create a Fixed Boundary Degrees of Freedom constraint to define the connection degrees of freedom.

?At each connection node where a force, torque, spring, or damper will be applied to the flexible body in Motion Simulation, create a Free Boundary Degrees of Freedom constraint to define the load degrees of freedom.

10. Solving the NX Nastran modal solution

In the Solution dialog box for the SEMODES 103 – Flexible Body solution, you must specify the number of modes for which to solve and the result types to recover. By default, stress and displacement results are selected.

After you have defined the finite element mesh, connection and load degrees of freedom, and solution attributes (such as the number of desired modes), you can solve the model.

The solve produces several results files that you must not delete, move, or rename:

?.dat — Nastran input file, needed for later results recovery.

?.op2 — Contains the model geometry and component modes resulting from the modal analysis.

?_0.op2 — Needed for later results recovery; contains the component mode definitions, modal mass, and modal stiffness.

?.rfi — RecurDyn Rflex input file, needed for representing the flexible body in the RecurDyn solve.

Remember the location of the .rfi file because you will need to point to it when creating the flexible link in Motion Simulation.

11. Review

Question

In the following example, suppose you are defining this excavator bucket as

a flexible body in Advanced Simulation. In the motion mechanism, a single revolute joint is defined between the two holes (1 in the graphic below) at the top of the bucket. A force load is applied to one of the teeth (2).

In Advanced Simulation, which constraints must you apply before solving the SEMODES 103 – Flexible Body solution? 2

A Fixed Translation constraint and a Pinned constraint

A Fixed Boundary Degrees of Freedom constraint and a Free Boundary Degrees of Freedom constraint

A User Defined constraint and a Pinned constraint

T wo Fixed constraints

After solving the SEMODES 103 – Flexible Body solution, when you create the flexible link in Motion Simulation, what file must you associate with the link to represent the dynamic behavior of the flexible body in the

mechanism?

The .RFI file

The .DAT file (Nastran input deck)

The .OP2 file

The .MDF file

?Show feedback

12. Activity: Flexible body analysis — Advanced Simulation tasks

Estimated time to complete: 12–16 minutes

You will learn how to:

Create an SEMODES 103 – Flexible Body solution in Advanced

Simulation and specify a body as flexible.

Connect the flexible body finite element model to the degrees

of freedom in the motion mechanism.

Solve the finite element model and generate the RecurDyn

Rflex input file.

Note T o complete this activity, you must have the nx_motion_rflex license, in addition to Motion Simulation, Advanced Simulation, and NX Nastran.

Launch the Flexible body analysis — Advanced Simulation tasks

activity.

13. Defining the flexible link in the mechanism

In Motion Simulation, you must create a Flexible Link that is associated with the component or body that you defined as flexible in Advanced Simulation.

In the Flexible Link dialog box, you associate the link with the RecurDyn input file (.rfi file) generated by the NX Nastran modal solution. The Flexible Link Preview window helps you determine the correct method for positioning the flexible body on the link.

In the graphics window, the flexible link is displayed with its finite element mesh.

Meshed flexible link defined in the mechanism (and Preview view)

14. Choosing modes to include in the analysis

By default, all modes from the NX Nastran modal solution are included in the motion simulation solve except the first six modes, which are considered

rigid-body modes.

Including all modes ensures a more accurate representation of the structure. However, for better solve performance, you can reduce the modal model by removing insignificant modes.

T o determine which modes to remove, you can view and animate the mode shapes using the Post Processing Navigator and Post Processing toolbar. Make sure to include all modes that resemble the behavior you are analyzing.

Example mode shape

??15. Solving the model and animating the mechanism

In Motion Simulation, you create a Kinematics/Dynamics solution of type Flexible Body.

When you solve the Flexible Body solution, the RecurDyn solver calculates the physical deformations at each point where the flexible body is connected to the rest of the mechanism, for each configuration of the motion mechanism. It saves these deformations in a modal deformation file (.mdf).

Using the .mdf file, the motion solution process calls the NX Nastran solver to recover the deformation, displacement, stress, and other results on the original, unreduced flexible body. These transient results are then returned to Motion Simulation, where you can view them in an animation of the mechanism.

By default, when you animate a flexible body solution, translational deformation results (nodal displacements) are displayed for the flexible body. You can also display other results, such as Displacement – Nodal, Stress – Element – Nodal and Strain – Element – Nodal, depending on the results you requested in the output request for the SEMODES 103 – Flexible Body solution in Advanced Simulation.

16. Activity: Flexible body analysis — Motion Simulation tasks

Estimated time to complete: 12–16 minutes

You will learn how to:

Define the flexible body in Motion Simulation and solve the

motion mechanism.

Animate the motion mechanism and observe the flexible body

deformation and stress.

Run an interference check between the flexible link and a body

panel.

Note T o complete this activity, you must have the nx_motion_rflex license, in addition to Motion Simulation, Advanced Simulation, and NX Nastran.

Launch the Flexible body analysis — Motion Simulation tasks

activity.

Flexible body analysis — Introduction

1. Open the Motion Simulation

Open (Standard toolbar)

?

?

?

?

OK

?

The motion simulation opens in the Motion Simulation application.

2. Reset the dialog box settings

The options you select in NX dialog boxes are preserved for the next time you open the same dialog box within an NX session. To ensure that the dialog boxes are in the expected initial state for each step-by-step activity, you should restore the default settings.

Preferences→User Interface

?

Reset Dialog Box Settings

?

OK

3. Enable the Flexible Body Dynamics environment The Flexible Body Dynamics environment is available only with the Dynamics analysis type.

Environment (Motion toolbar)

?

Flexible Body Dynamics

?

OK

??4. Define the flexible link

Flexible Link (Motion toolbar)

?

?

Flexible Model

?Browse

?intro_flex_body/fbcam_assy_sim2-solution_1_0.rfi

This pre-generated file contains a modal reduction of the flexible body's mass and stiffness, which represen ts the body’s dynamic behavior. The NX Nastran solver generates these reduced matrices and saves them in a RecurDyn Rflex input (RFI) file. In a later activity, you will solve an NX Nastran solution and generate an RFI file.

?

OK RFI File dialog box

Leave the Flexible Link dialog box open for the next step.

5. Finish defining the flexible link

The Flexible Link dialog box is still open from the previous step.

?The Flexible Link Preview window shows you the meshed flexible body and its orientation relative to the absolute coordinate system.

?

?Because the flexible body is in the same orientation relative to the absolute coordinate system as the corresponding link in the mechanism, Absolute Origin is the appropriate selection.

?

OK Flexible Link dialog box

6. Create a Flexible Body solution

Motion Navigator

?motion_1

?New Solution

?

?

?

?

?

OK

7. Add the flexible link to the solution

Motion Navigator

?Flexible Links

?L002–fbcam_assy_sim2-solution_1

?Add to Solution

8. Solve the motion simulation

Motion Navigator

?Load Container

?G002

?Add to Solution

For illustration purposes, this vector force simulates a load on the clevis in the X direction, to exaggerate the flexible behavior.

?Solution_2

?Solve

The solve process may take several minutes to complete. After the RecurDyn solve finishes, NX Nastran is called automatically to run the SEMODES 103 –Flexible Body solve. Nastran uses the RecurDyn results file and the original NX Nastran input file to recover the deformation, displacement, stress and other results for the flexible link.

Caution Do NOT close any solve windows until all NX Nastran command prompt windows have closed and the message ―Nastran results

recovery completed‖ appears in the NX status line.

9. Animate the mechanism