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Bracket — Rigid Connector Analysis
Introduction
Rigid connectors provide an alternative way for modeling geometrical parts that are of little interest in the analysis and have a negligible deformation. Such parts can be replaced with virtual rigid bodies with appropriate boundary conditions. This saves computational time and memory.
In this example, you study the stress in a bracket connected to a pin where a load is applied. The pin is modeled using rigid connector.
It is recommended that you review the Introduction to the Structural Mechanics Module, which includes background information and discusses the bracket_basic.mph model relevant to this example.
Model Definition
This model is an extension of the example described in the section “The Fundamentals: A Static Linear Analysis” in the Introduction to the Structural Mechanics Module.
The model geometry is represented in Figure 1.
Figure 1: Bracket geometry.
A pin is connected between the bracket hole where a load is applied. The pin is assumed to be perfectly rigid and is modeled with a rigid connector. Thus, the pin is not represented in the model geometry.
The loading consists of two parts:
•
The pin is forced to rotate 0.05° around the y-axis. The center of rotation is a point located at (0, −0.4, −0.1) m.
•
A force of 10 kN is applied on the negative y direction at the center of rotation.
The center of rotation is constrained against translation and rotation in all other degrees of freedom.
Figure 2 below shows the boundaries connected to the rigid connector and the position of its center of rotation.
Figure 2: Center of rotation of the rigid connector applied to the bracket geometry.
Results and Discussion
Figure 3 shows the von Mises stress in a deformed geometry. You can see the effect that the pin’s rotation and applied force has on the bracket arms.
Figure 3: Von Mises stress distribution.
Notes About the COMSOL Implementation
The rigid connector has the translational and rotational degrees of freedom of a rigid body. Options for this feature include applying an external load or moment to the rigid body, or defining mass and moments of inertia in dynamic analyses. The rigid connector can also be used to apply loads and constraints at points so that these are distributed and thereby preventing singularities.
To visualize the position of the center of rotation you need to enable the physics symbols. This is done in the settings for the physics interface.
Application Library path: Structural_Mechanics_Module/Tutorials/bracket_rigid_connector
Modeling Instructions
Application Libraries
1
From the File menu, choose Application Libraries.
2
In the Application Libraries window, select Structural Mechanics Module > Tutorials > bracket_basic in the tree.
3
Click  Open.
Solid Mechanics (solid)
Rigid Connector 1
1
In the Physics toolbar, click  Boundaries and choose Rigid Connector.
Add a rigid connector that connects the holes in the bracket arms to simulate the presence of the pin.
2
In the Settings window for Rigid Connector, locate the Boundary Selection section.
3
From the Selection list, choose Pin Holes.
By default, the location of the center of rotation is computed automatically. You can also manually specify its location.
4
Locate the Center of Rotation section. From the list, choose User defined.
5
Specify the Xc vector as
0
x
-0.40
y
-0.10
z
To visualize its position you need to enable the physics symbols.
6
In the Model Builder window, click Solid Mechanics (solid).
7
In the Settings window for Solid Mechanics, locate the Physics Symbols section.
8
Select the Enable physics symbols checkbox.
The displacements of the rigid connector are constrained in the x − and z − directions at its center of rotation.
9
In the Model Builder window, click Rigid Connector 1.
10
In the Settings window for Rigid Connector, locate the Prescribed Displacement at Center of Rotation section.
11
Select the Prescribed in x direction checkbox.
12
Select the Prescribed in z direction checkbox.
13
Locate the Prescribed Rotation section. From the By list, choose Prescribed rotation.
Apply a prescribed rotation of the rigid body of 0.05 degrees around the y-axis.
14
Specify the Ω vector as
0
x
1
y
0
z
15
In the ϕ0 text field, type 0.05[deg].
Applied Force 1
1
In the Physics toolbar, click  Attributes and choose Applied Force.
Apply an external load of 10 kN at the center of rotation of the rigid body.
2
In the Settings window for Applied Force, locate the Applied Force section.
3
Specify the F vector as
0
x
-10[kN]
y
0
z
Add Study
1
In the Home toolbar, click  Add Study to open the Add Study window.
2
Go to the Add Study window.
3
Find the Studies subsection. In the Select Study tree, select General Studies > Stationary.
4
Click the Add Study button in the window toolbar.
5
In the Home toolbar, click  Add Study to close the Add Study window.
Study 1
In the Study toolbar, click  Compute.
Results
Stress (solid)
1
Click the  Zoom Extents button in the Graphics toolbar.
The default plot shows the von Mises stress on a deformed geometry.