Objectives Upon successful completion of this lesson, you will be able to:
I Perform structural analyses of simple assemblies.
I Apply and define contact conditions.
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Contact Analysis
When we analyze an assembly, we must understand how the
components interact with each other so that our mathematical model correctly computes the stress and deformation.
Different conditions must be considered where the parts can pull apart or penetrate each other and whether or not the surfaces can slide over each other.
Case Study:
Pliers with Global Contact
In this lesson, we analyze a simple hand tool. It consists of four components: two identical arms, a hinge pin, and a piece of flat stock squeezed by pliers.
We are not interested in the contact stresses that develop between the arms and the piece of flat stock.
Therefore, we can simplify the model by suppressing the flat stock and replacing it with the appropriate fixture.
Project Description
Calculate the stresses that develop in the arms when a 225 N [50.6 lbf]
“squeezing” force is applied to the end of each arm. The design strength is set at 138 MPa [20,016 psi], approximately 22% of the material yield strength.
Stages in the Process
Some steps in analyzing an assembly.
I Apply materials
Materials can be applied to all components together or individually.
I Add fixtures
Fixtures are added in the same way they are done in parts to restrain the motion of the model.
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131 I Apply component contact conditions
Component contact conditions dictate how parts or sub-assemblies in contact or close proximity interact in the absence of local overrides.
I Apply local contact conditions
Local contact conditions override the component contact.
I Apply mesh control
The mesh can be refined in areas of stress concentrations or rapidly changing geometry.
I Mesh the model
I Run the analysis
I Analyze the results
Determine if the results are accurate enough or further refinement of the analysis is needed.
Procedure To begin this case study:
1 Open an assembly file.
Open pliers from Lesson03\Case Studies folder.
2 Suppress flat.
Suppress the part flat in the SolidWorks FeatureManager design tree.
3 Create study.
Create a static study named pliers. 4 Examine the Simulation Study Tree.
There is now a Parts folder with three components because there are three parts in the assembly to be analyzed.
Applying Materials to Assemblies
You can apply the same material to all components of an assembly or to each component individually.
To apply material to the components:
I To apply the same material to all components, right-click Parts and select Apply Material to All.
I To apply different material to each component, right-click a part and select Apply/Edit Material.
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5 Apply materials to components.
Apply Plain Carbon Steel material properties to all components including the pin.
6 Apply fixed restraints.
Define Fixed Geometry fixture on both jaws.
The applied restraints simulate the suppressed piece of squeezed flat stock.
This condition assumes that the flat is not sliding when held by the jaws.
7 Apply force to handles.
Apply a 225 N [50.6 lbf] force to both handles. The outer face of each arm has a split face so that the load is only applied to part of the face.
In the Force/Torque PropertyManager, select Normal.
Component Contact
Whenever we create a study of an assembly, a new folder named Connections is added to the
Simulation Study tree. We use this folder to define how the assembly components interact with each other.
We have defined the fixtures and external loads, but we are not yet ready to mesh this assembly. We have to account for the contact between the two arms.
The Component Contact options defines the way components interact with each other. You can override the component conditions by
defining different conditions locally for selected pairs of features.
Local contact conditions are discussed later in this lesson.
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Component Contact: Options
The available options for the component contact are: Bonded, Allow penetration and No penetration. These options are explained in the following figure and table.
I In the Simulation Study tree, right-click Connections and select Component Contact.
I Select Contact/Gap in the Simulation menu and then click Define Contact for Components.
I Select the Simulation tab in the CommandManager, then select Component Contact from the Connections Advisor pull-down list.
Component Contact Types
Bonded This is the default choice. Select this option when all touching faces are bonded and the assembly behaves as one part. The only difference between a part and an assembly with bonded parts is that in an assembly we can assign different material properties to individual components.
Allow penetration Select this option when the assembly is a series of unattached components with no structural connection between them.
No penetration Select this option when touching components can come apart, but cannot penetrate each other.
The coefficient of friction can be specified in the component contact property manager.
No penetration
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Component Contact: Default setting
Default component contact setting is bonded contact between all touching faces for the top level assembly.
Editing default component contact, Global Contact, shows that it is applied to the top level assembly.
Component
Contact: Hierarchy and Conflicts
It is possible to delete and re-define the top level assembly contact condition. However, multiple top level component contact conditions would result in a conflict and are not permitted.
Any additional component contact between parts and subassemblies must not be in conflict and will override the top assembly level component contact. If the conflict is detected a warning message will be displayed.
8 Check for existing interferences.
Click Tools, Interference Detection. In the Options dialog, select Treat coincidence as interference and click Calculate.
Three sets of faces in the assembly are touching.
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Note In the parts of this assembly, the manufacturing clearance between the pin and the arms is ignored. That is why the coincident contact of cylindrical shape between the pin and the arms was detected.
9 Change top level component contact option.
In order to allow the relative movement of the arms while the model deforms under the load, change the default component contact (Global Contact) condition to No Penetration.
Expand the Connections folder, edit the Global Contact item and change it to No Penetration. Click OK.
10 Mesh the model.
Select Curvature based mesh under Mesh Parameters.
Mesh the assembly with Draft quality elements and the slider all the way to the
right. This should produce the Maximum element size of 4.912mm, Minimum element size of 0.982mm, Number of elements in a circle as 8, and Ratio of 1.6.
Important! Meshing must always be performed after the contact conditions are defined.
11 Run the analysis.
12 Switch to exploded view.
Switch to the exploded view.
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13 Plot von Mises stresses.
Display the von Mises stress plot by double-clicking on the Stress1 plot icon.
We want to see if the von Mises stresses in any portion of the model exceed 138 MPa [20,016 psi], which is our design stress. To determine whether the von Mises stresses exceed the maximum we can change the plot options.
14 Change the plot.
While the plot is displayed, right-click Stress1 and select Chart options.
Under Display options, select Defined, and then enter the minimum stress as 0 and the maximum stress as 138,000,000.
Click OK.
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15 Change Plot Settings.
Right-click Stress1 and select Settings. Under Fringeoptions, select Discrete.
Click OK.
Viewing Assembly Results
Areas with stresses higher than 138 MPa would appear in red.
Note that an exploded view offers a very convenient way of examining the analysis results of an assembly, whereas, in normal viewing, components may obstruct the view.
Another way of reviewing results of an assembly is to hide some assembly components.
16 Isolate the arm.
Isolate arm<1>.
17 Define stress plot of one arm.
In the Simulation Study tree, right-click the Results folder, and select Define Stress Plot.
Click OK.
Note You can also use the existing plot Stress1 after hiding an assembly component arm.
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18 Max/Min annotations.
In Chart Options, select Show max. annotation and Show min.
annotation.
The maximum stress locations and their magnitudes are indicated for the displayed arm.
Conclusion The maximum von Mises stress of approximately 93.0 MPa is produced by normal operation of the pliers when a 225 N force is applied to the handles. This load can be (perhaps with some difficulty) applied by hand and 93.0 MPa can easily be tolerated by the pliers’
material (which has a yield stress of almost 220 MPa).
Before concluding that our design is safe, re-meshing the model and looking for stress convergence would be required.
Handle Contact We wish to determine the maximum stress that the pliers undergo when squeezing a 5 mm stock plate. The maximum stress corresponds to the situation where the handles are blocked.
19 Show the hidden arm and pin.
20 Collapse the assembly.
21 Create UY: Y displacement plot.
To determine the force that brings the ends of the two handles together, we need to create a displacement plot showing the y component of the displacements.
Double-click the Displacement1 plot icon to make the plot active.
Right-click the Displacement1 plot icon and select Edit definition.
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Select UY as the Displacement Component, and select mm as the Units.
Under Deformed shape, select True scale. This option plots the deformation in 1:1 scale.
Click OK.
Required Force We see that under the 225 N force, the end of each handle travels 0.393 mm. Consequently, the distance between the two ends decreases by twice that amount, 0.786 mm.
Since the original distance is 15.24 mm, the force magnitude must be increased by a factor of:
15.24 mm / 0.786 mm = 19.39
Therefore, the force required to bring both arms in contact is equal to 19.39 x 225 = 4362 N. This is based on fundamental assumptions of linear analysis where the structural response is assumed to be proportional to the applied load.
Pliers with Local Contact
We will now load the pliers with a force that significantly exceeds 4265 N to ensure that both arms come in contact. The appropriate definition of the contact will ensure that the handles can come together, but cannot penetrate each other.
1 Create new study.
Duplicate the study pliers and name the new study pliers with local contact.
2 Edit force.
Edit the force magnitude to 4,500 N. This is an arbitrary magnitude based on our “rough” estimation of forces that will definitely bring the two arms together.
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Local Contact The top level component contact condition remains the same (No Penetration) as in the previous study. However, now that the force is considerably larger in order to bring the two arms together, we need to specify a local contact condition that prevents their penetration (No penetration top level assembly component contact applies to initially touching faces only).
This local contact condition has precedence over the component contact. In general, the hierarchy of the contact conditions can be explained by the pyramid shown in the following figure.
Top assembly level contact (only one definition is permitted) is
overridden by other user defined component conditions. All component contacts are then overridden by local conditions.
The local contact conditions can be defined by right-clicking on the Connections folder and selecting Contact Set.
High precedence
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Local Contact Types
In addition to Bonded, No Penetration, and Allow penetration, the local contact features two more contact types: Virtual wall and Shrink fit.
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The table briefly describes the local contact condition types.
Note Each local contact type features various options described at various locations throughout the manual.
3 Define contact set.
To define a contact zone between the ends of the two handles, we use the two small split faces on the inside of the handles to define a contact pair.
Right-click Connections and select Contact Set.
Local Contact Types
No
penetration
The features (both initially touching and separated by a gap) may move away from each other but preserve the physical requirement that they may not penetrate each other. Friction coefficient and initial geometrical offset can be specified in the contact options.
Bonded
The selected features will become bonded, similarly to the component level contact types. While
component bonded contact only applies to touching faces, local condition is capable of bonding features separated by a gap.
Shrink fit
The program creates a shrink fit condition between the selected faces. The faces may or may not be cylindrical. This condition requires that the two parts exhibit a finite volume interference.
Allow penetration
The selected pair of features is free to move in any direction. Free features can penetrate into each other, a physical impossibility. You should use this option only when you are absolutely sure that the specified loading will not cause the features to penetrate.
Virtual wall
This provides a sliding support in a way similar to Roller/Sliding restraint, except that a friction coefficient and wall elasticity can be specified.
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In the Contact Set PropertyManager, select No Penetration.
Click one face to define it as the Set 1, and then click the other face to define it as Set 2.
Click OK.
Note The selection of the faces for Set 1 and Set 2 is arbitrary.
No Penetration Local Contact Options
The above image shows two properties of the two local condition, Friction and Gap (clearance).
I Friction: Any value of the friction coefficient is permitted.
I GAP (clearance): In many applications, two entities cannot come into full contact due to the manufacturing limitations and the modeling approaches that we use. This feature restricts such two entities from coming closer than the initial geometrical offset. For a more detailed explanation, please refer to Lesson 7.
No Penetration Local Contact:
Advanced Options
The No Penetration contact also features advanced options accessible through the simulation study options. These options, generally not required, are discussed in Lesson 7.
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No Penetration Local Contact:
Accuracy
The default algorithm for the local contact is fast and suitable to most contact solutions. However, if contact stresses are of primary
importance, or if the areas in contact are large and/or the default solution for contact stresses is spotty or discontinuous, Improve accuracy for contacting surfaces feature should be activated.
Introducing:
Improve Accuracy for contacting surfaces
Improve accuracy for contacting surfaces feature employs advanced solution algorithm resulting in improved results. While such contact solution is more accurate it may take significantly more time.
Where to Find It Shortcut Menu: Right-click <study name>, Properties and under the Options tab, click the Improve accuracy for contacting surfaces check box.
No Penetration Local Contact:
Remarks
Edges and vertices in local No Penetration contact condition can onlybe selected in the first field (Set 1), while the second field (Set 2) accepts faces only.
Note Because the friction forces are small and no initial geometrical offset exists in this case, neither the Friction nor the Gap (clearance)
properties will be used. Since the contact stresses are of no interest in this simulation Improve accuracy for contacting surfaces option will not be utilized.
Because the contact conditions have changed, the warning signs indicate that remeshing and re-calculation of the results are necessary.
4 Mesh the model.
Select Curvature based mesh under Mesh Parameters.
Mesh the model with High quality elements and the same element size as before.
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6 Large/Small displacement.
While the study is running, the following message will appear:
Excessive displacements were calculated in this model. If your system is properly restrained, consider using the Large Displacement option to improve the accuracy of the
calculations. Otherwise, continue with the current settings and review the causes of these displacements.
Click No to complete the analysis as linear.
Note The large displacement dialog box warns us that the large
displacements of some parts in the assembly were detected. The large displacement computations are the subject of Lesson 14. At this point, we will ignore this fact.
7 Plot von Mises stresses.
After the analysis is complete, create a von Mises stress plot, with discrete fringes, the mesh showing, and the stresses scaled from 0 to 220 MPa [89,925 psi].
The region in red indicates the yielding material. We can observe that the maximum reported von Mises stress is approximately 1,806 MPa.
This value is, of course, unrealistic. Yielding of the material indicates that a linear analysis is no longer valid and that a nonlinear analysis would be required.
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Contact Stresses After the handles are blocked, any further increase in force magnitude has little effect except for increasing the contact stresses where the handles touch.
Question: Can we analyze the contact stresses in the current study?
Answer: No, the element size in the contact area is much too large in comparison to the size of the contact area. This
comparison is best seen in a side view.
The two handles touch only along the edge. For accurate modeling of contact stresses, we need several elements along the length and width of the contact zone.
8 Save and Close the file.
Summary In this lesson we analyzed the simple assembly model of pliers with various contact conditions. To simplify the geometry, the flat was suppressed and its presence was simulated with the help of the fixed geometry fixtures on the jaws. When the analysis was run, we saw a maximum von Mises stress of 93 MPa. This stress is below our
specified design strength of 138 MPa. To be sure of our stress results, a more refined mesh should be run to insure that the stress is converging.
Additionally, we saw that the maximum displacement produced was 0.402mm. We used this result to change our loading application to investigate what happens if the load was so large that the handles touch.
The contact conditions can be grouped into two distinct categories:
component and local. Both categories were introduces and practiced in this lesson.
The local contacts take precedence over all of the component
conditions, while all user defined component contacts take precedence over the top assembly level component condition (essentially serving as the global contact condition for the entire assembly). While the component contacts apply to initially touching faces of parts or
assemblies, the local conditions may feature gaps and initial separation.
Various types (namely Bonded, Allow Penetration, No Penetration, Shrink Fit and Virtual Wall), properties and options of the contacts
Various types (namely Bonded, Allow Penetration, No Penetration, Shrink Fit and Virtual Wall), properties and options of the contacts