Introduction
In this exercise, we will simulate a ball dropping from 12 inches onto a steel plate and rebounding back up into the air. This is an example of a nonlinear transient analysis. It requires both dynamic and nonlinear setup steps.
Autodesk Inventor Nastran solves both domains essentially simultaneously.
Even though we are dropping the ball from 12 inches, we will position the ball in the model so that it is almost touching the plate. This is done for solution efficiency. We will assign an initial velocity to the ball of 96.3 in/s, which is the velocity the ball would reach after dropping 12 inches.
V = SQRT(2 x G x d) = SQRT(2 x 386.4 x 12) = 96.3 in/s
Note that we are assuming that the difference in velocity from the initial position to the actual contact is negligible.
The other consideration is to determine roughly how long it will take the ball to traverse the distance in the model (0.06 inches) at a velocity of 96.3 in/s.
T = d/V = 0.06 / 96.3 = 0.0006 s
This is necessary to determine a time step size that is small enough to adequately resolve the impact. After the impact, we can increase the time step in a second subcase.
1. Open the Model and Start the
Autodesk Inventor Nastran Environment
Start Autodesk Inventor, and open
Ball Impact2.iam from the
Section 17 - Ball Impact sub-folder of your training exercises folder.
From the
Environments ribbon tab, click
Autodesk Inventor Nastran. In the Model tree, expand
Idealizations. If any Idealizations are already defined, like Solids 1 and 2 below, right-click
, to ensure that no unwanted materials can participate in the analysis.
You should see this:
2. Change the Analysis Type
- From the Analysis tree, right-click
.
- On the Analysis dialog, set the
Type to
Nonlinear Transient Response.
- Enable both
Velocity and
Acceleration Nodal Output Control types.
- Click
OK.
3. Define the Property and Material
- Click
Idealizations from the ribbon.
- Click the
New Material icon.
- On the Material dialog, click
Select Material. Expand the
Inventor Material Library, and select
Alloy Steel. Click
OK and
OK to close both dialogs.
- In the
Idealizations dialog, check the
Associated Geometry box.
- Select both parts.
- Click
OK.
4. Constrain the Plate
- Click
Constraints from the ribbon.
- Make sure all degrees of freedom are constrained (checked).
- Select the end of the plate.
- Click
OK.
5. Constrain the Ball
This constraint ensures the ball does not bounce sideways as a result of element face angles. Although not required for the analysis, it ensures that we get the response in the expected direction.
- Click
Constraints from the ribbon.
- Uncheck all degrees of freedom except
Tz.
- Select the two edges on the lower half of the ball that run parallel to the long dimension of the plate.
- Click
OK.
6. Define the Initial Velocity
This value defines the velocity of the ball at the beginning of the analysis. This is the value we computed in the introduction, and is the ball's velocity after having been dropped from 12 inches above the plate.
- Click
Loads from the ribbon.
- Set the
Type to
Initial Condition.
- Set the
Sub Type to
Velocity.
- Select the solid ball. An easy way to do this is to hover over one of the surfaces of ball (but don't click it). When the Select Other menu appears, expand it, and select
Solid1.
- Enter
-96.3 in/s for
Vy.
- Click
OK.
7. Enable Gravity
- Click
Loads from the ribbon.
- Set the
Type to
Gravity.
- Enter
-386.4 in/s2 for
Fy.
- Click
OK.
Note that this is the only load that persists throughout the analysis. The initial condition only applies at the beginning of the analysis.
8. Assign Mesh Control to the Contacting Faces
- From the Analysis tree, right-click
.
- Enter an
Element Size of
0.1 for the Face Data, and click in the Selected Faces field to activate it.
- Select the four bottom surfaces of the ball and the split face on the end of the beam.
- Click
OK.
9. Define Contact
- Click
Manual from the
Contacts panel in the ribbon.
- Click in the
Primary Entity field, and select the surface on the plate.
- Click in the
Secondary Entity field, and select the four bottom surfaces of the ball.
- Enter
0.1 as the
Max Activation Distance.
- Click
OK.
10. Define Damping
For this model, we will choose the first bending mode as the Dominant Frequency.
- Right-click on
Damping 1 from the Analysis tree, and click
Edit.
- Enter
4 as the
Damping Value, G. (This value represents 2% damping entered as 2X.)
- Enter
247 as the
Dominant Frequency, W3.
- Click
OK.
11. Define the First Subcase
We will use two subcases to control the time step size. In the first subcase, we will assign a very small time step to capture the descent and impact of the ball. In the next step, we will create a second subcase with a larger time step to capture the motion of the ball as it rebounds off of the plate.
- Right-click on
Subcase 1, and click
Rename. Enter the name
SmallSteps.
- Right-click on
Nonlinear Setup 1, and click
Edit.
- On the Nonlinear Transient Parameters dialog, select
AUTO as the
Stiffness update method. Click
OK.
- Right-click on
Dynamics Setup 1, and click
Edit.
- Enter
0.0005 as the
Time Step.
- Enter
16 as the
Number of Timesteps.
- Click
OK.
12. Define the Second Subcase
We create the second subcase by copying the first subcase and modifying the time step parameters.
- Right-click on the
SmallSteps subcase, and click
Duplicate.
- Click on the new subcase,
SmallSteps - Copy, and rename it to
BigSteps.
- Right-click on the initial condition load,
Load 1 - Copy, and click
Remove.
- Right-click on
Dynamics Setup 2, and click
Edit. Change the
Time Step to
0.01, and the
Number of Timesteps to
20. Click
OK.
13. Mesh the Model and Run
- Right-click on
Mesh Model, and click
Edit.
- Change the
Element Size to
0.2.
- Set the
Element Order to
Linear.
- Uncheck
Continuous Meshing.
- Click
OK to close the Mesh Settings dialog box and generate the mesh.
- Run the analysis by clicking
Run from the ribbon.
Note: The analysis will take up to 30 minutes to run.
14. Review the Time-History of Displacement
- From the
XY Plot
branch, right-click on
Maximum Displacement Versus Time, and click
Show XY Plot.
You should see this:
The area circled in red is the initial bounce, and was computed with the smaller time step in the first subcase.
15. View a Multiset Animation
- Right-click on
Results, and select
Multiset Animation Settings.
- Click the
Deform Options tab. Set the
Vector to
Displacement, and the
Type to
Total. Change the
Deformation Scale to
Actual, and specify a
Value of
1.0.
- Set the
Start Set to
STEP 1, and the
End Set to
STEP 22.
- Click the
Animation Options tab, and change the
Mode to
Half.
- Click
Animate and then
OK to close the dialog box.
- To stop the animation, right-click
Results and uncheck
Multiset Animate.
Summary
In this exercise, we combined both dynamics and nonlinear responses to produce a nonlinear transient analysis. These types of analyses can be complex, but they are some of the most exciting solution types in FEA.