Fundamentals of SolidWorks Simulation

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Bolt Fracture

Case Study: Bolt Fracture 

Hardware such as bolts, nuts and washers are generally used for connections and fixtures.  They form an integral part of the component structural integrity.  Bolts are governed by standards such as ASTM and are typically stamped with an identifying marking on the head during the manufacturing process.  The bolts selected for any application typically take into account the manner in which it is applied while taking into account the most prevalent mode of failure.  It follows that it would be in the designer’s best interest to understand how bolts fail and to avoid those situations.

Consider a simple bolt modeled in SolidWorks:

Roundhead Bolt 12mm DIA x 152MM LONG ASTM A307

It is worth noting that the tensile strength for that grade of bolt is approximately 60,000 PSI.  Therefore, using plain carbon steel as a template, the tensile strength was adjusted accordingly.

Next, we can set up a test scenario using SolidWorks Simulation where the bolt is fixed at the top with a base tensile load of 10,000 PSI applied at the bottom

We can now mesh the bolt

After running the Static analysis we can do a section plot to understand how the stress a distributed in the cross section of the bolt.  We find that the highest point of stress concentration occurs at the neck as expected with a value of 28,067 PSI.  

Note that this value is fairly close to yield strength of the material 31,994 PSI.  If a stress value approaches the yield strength, the general assumption of linear behavior of the material begins to breakdown.  Therefore it is in our best interest to run a non-linear analysis as well.

Creating a new non-linear study and running a similar analysis we find that the actual stress is approximately 27,197 PSI. This value is less than the linear analysis but it is still close to yield. 

Given these results it would be in the designer’s best interest to either reduce the load, increase the bolt diameter or chose a higher grade bolt.

Sometimes bolt failure can occur due to other circumstances as well.  Specifications such as how much a bolt must be tensioned at installation can make a difference.  In other cases large batches can yield bolts with manufacturing defects which may fail below the yield stress.  

Difference between Direct Transfer and Rigid Connection

In SolidWorks Simulation you may define external loads known as remote loads.  Upon creation of a remote load you have the option of setting it to be either Direct Transfer or Rigid Connection.  What is the difference?

Consider a simply supported cantilever composed of two parts: front and back: 

We may create a simulation study that reflects the cantilever beam loading conditions.  It is fixed on one end and has a load applied on the other end.

We can simulate the results of this study by excluding half of the beam and applying a remote load in place.  

When creating a remote load, the user must specify whether it is a direct transfer or rigid connection

Direct Transfer: Assumes that the component between the remote force and the connection point is adequately flexible, allowing the structure to deflect.  The program decomposes the remote force into a shear force and coupling moment acting on the connection face.  This allows the connection face to distort.  Displacements must be within the small displacement assumption.

Rigid Connection:   Applies rigid links/ connections from the remote load to the connecting face.  As a result when the face deflects, the face shape is maintained.  High stresses can develop near faces with rigid connections.

The results of the remote load direct transfer is shown below:

The results of the remote load rigid connection are shown below:

At first glance the results are not easily distinguishable; however upon closer inspection along the vertical axis of the connection face reveals that the stress distribution from the direct transfer and rigid connection options are different.

Choosing whether the remote load will be applied as a direct transfer or rigid connection will affect the simulation results.

Symmetry in SOLIDWORKS Simulation - Guide to Applying Loads & Restraints Appropriately

We are frequently asked here in techsupport what value of a load to apply when using symmetry to cut down on the size of the overall problem that needs to be solved.  As you will see, it is different for how you handle Force and Pressure loads in Simulation FEA.  We can even look into comparing this to the mass and volume flow rates and velocity or pressure conditions in Flow Simulation.   You can use this blog post as a guide to help you in modeling up your own problems when you want to use the benefits of symmetry.

First off, let’s talk a little about when you can use symmetry.  Note that the true determination of whether you can use symmetry or not depends on the final results, but that seems a bit contradictory because your looking to solve the problem and don’t have any results yet.  But there are 3 hints that can clue you into whether symmetry might be feasible: 1) Obviously the geometry has to be symmetrical, but even if it is not absolutely symmetrical, such as some details that don’t affect the overall results and you can assumptively ignore those details .  A typical example I recall is the screw for a cap on the top of a bottle.  2) The restraints and loads are symmetric.  You should look at this from the perspective of a free-body diagram (FBD), and the example I use below will help to explain this better.  3) The material is symmetric; a rare case when considering symmetry is where the materials are different, but it could be an odd case when working with assemblies.

Again these 3 clues are not fail safe, and the ultimate determination is in the final results.  Using symmetry is a modeling assumption, and for all analyses, you need to take note and manage your assumptions.  If you have some experience with your model and how it will behave, then this can also help to lead you to a decision if symmetry is OK to use.

Let’s take the above example of a flat plate with a hole in the center.  It has a fixed restraint on the left-hand side and a uniform Force applied on the right-hand side face.  When we check the geometry using a SOLIDWORKS tool: Tools > Symmetry Check, you can see that the part is symmetric about all three directions showing that we can choose to keep a 1/8^th section of the original.  Now when considering if the loads and restraints are left-right symmetric, it initially doesn’t seem so since we have a Force on one end but a Fixed restraint on the other.  But from a FBD perspective, you will know that the restraint will apply an equal and opposite reaction force, so it actually is a symmetric loading case.  And it is the same material throughout the part, so no problem there.

Note that the face where the load is applied is cut into four parts, so the question is: Do we need to change the load?  If we think about it, it makes sense that if the same force were applied to only a quarter of the model, then the results would be larger… exactly 4 times larger, in this linear test case.  So the conclusion can be made that we should divide the original force by 4, or F/4.  If the original load magnitude was 100, it should now be 100/4 or 25.

Then, what happens in the case where we have a Pressure load applied?  Pressure is defined as a force over unit area.  If the area is decreased by 4 times by the symmetry cuts, then the resulting force that the Pressure exerts is automatically 4 times less.  Thus, when we apply a Pressure in the context of using symmetry, the magnitude of the load does not need to be adjusted.

There is a special case of symmetry that I need to point out where both the Pressure and Force are unchanged.  The special case is when we use the 2D Simplification tool available in Simulation.  The full load, whether force or pressure, is applied to the edge of the 2D geometry as if it were to be applied to the entire model thickness (in the case of a plane strain or plane stress problem) or the entire 360^o revolve (in the case of an axisymmetric problem).

In SOLIDWORKS Flow Simulation, currently you can do a half or quarter symmetry solution by only changing the Computational Domain options, i.e. you don’t actually cut the model but instead change the bounding box where the calculation is performed.  The values for Mass and Volume Flow Rate boundary conditions are absolute (akin to a Force in FEA), So you will need to reduce the flow rate by ½ or ¼ if you are using a half or quarter domain, respectively.  Pressure is the same as pressure in a structural calculation, so no adjustment needed there.  What about velocity?  Velocity does not need to be changed either; if you think about it from a flow rate perspective, where the rate needs to be reduced because the amount of fluid moving through that opening changes, but the velocity of the fluid will always be the same.

Important details not to forget when using symmetry:

 Make sure you apply the appropriate symmetry conditions on all the faces that have been cut.  There is a restraint type called Symmetry (found under the Advanced restraint types), but this has the limitation that it can only be applied to faces that are orthogonal (90^o) to one another, hence will not work on a pie sliver type of cut, for example.  So it’s best to know that actual definition of a symmetry restraint, in case that you need to apply manually using the Use Reference geometry restraint type, is that the face can only translate on the plane and cannot rotate out of plane.  In other words, the Normal translation and the other two directional Rotational degrees-of-freedom are held to zero.  (Aside: Did you know that you can apply an Anti-Symmetric restraint by applying just the opposite conditions as described above?)

Symmetry can be used for the following study types: linear Static, Thermal and Nonlinear.  It SHOULD NOT BE USED in a Frequency, Buckling, Drop Test or Linear Dynamic study.  The results from all of these will most definitely have non-symmetric responses.  If you use symmetry in a Frequency study, for example, you will only be able to extract the resonant frequencies which are symmetric, and you would miss all of the non-symmetric shapes.

For a Thermal study, when a face has no condition set on it, it is defined as adiabatic, that is no heat enters or leaves through this face, hence the symmetric condition is set by not defining a condition to it.  A Heat Power load (in Watts) is absolute, so like a Force, has to be divided.  A Heat Flux (in W/m²) is an integrated over an area, so like a pressure does not need to be changed.  Temperature is temperature, like a prescribed value, so no need to change as well.


Final Stress Results from the plate with a hole using a proper symmetry loading conditions.

What's new in SolidWorks Simulation 2014

With every year's major release in SolidWorks, the enhancements that is made in all modules of  SolidWorks Simulation 2014 (SolidWorks Simulation Professional, SolidWorks Simulation Premium and SolidWorks Flow Simulation) is truly outstanding.

The SolidWorks Simulation 2014 release focus on four important theme:

Boost Productivity with Intelligent Tools

• Automatically convert Toolbox Fasteners to bolt connectors
• New Bolt Connectors Symbol

Benefits :

• Easier Simulation setup time
• Leverage CAD Simulation
• Improve productivity of Product Engineers

Perform comprehensive simulations across consistent solution

Leverage ECAD thermal properties

• Import thermal properties from Circuit Works in SolidWorks Flow Simulation
• Import PCB layers definition from Circuit Works in SolidWorks Flow Simulation

Benefits:

• Streamline product development process
• Enhance the user experience with SolidWorks Solutions.

Import Residual Stress from SolidWorks Plastics

• Import In-Molding Residual Stress (the stress generated before ejection) from SolidWorks Plastics.
• Perform Structural Analysis taking in account molding stresses.

Benefits:

• Provide complete performance test capabilities.
• Consistency of the global simulation tools.

Acheive greater insight with Simulation results

Mirror results about planes of symmetry

• Allows user to mirror results about planes of symmetry (planar,cyclic)

Benefits :

• Ease post processing of Complex symmetrical models
• Facilitate the sharing of simulation results with people unfamiliar with Simulation.

 

Results Comparison Across Configurations

Allow the user to compare simulation results across different configurations.

Benefits :

•  Ease comparison of design scenarios 
• Enable user to focus on his research to best design

Better eDrawings support of CFD results

Save multiple SolidWorks Flow Simulation output charts in a single edrawing.

Benefits:

• Ease Communication with CFD results 
• offer consistency with solidworks simulation

Performance

SolidWorks Simulation Performance Enhancements to :

• Faster file open with Simulation Active
• Large problem with Large problem direct sparse solver - Multi -Core Support
• Direct Sparse solver - upto 40% seep up for in-core problems
• Large speed up in the Interactive Solver (FFE +) - Multi-Processor, Contact convergence