Published on:	July 15, 2024
With:	Eric Miller & Jim Peters
Description:	In this episode your host and Co-Founder of PADT, Eric Miller is joined by PADT’s Chief Simulation Engineer Jim Peters to discuss the breadth and depth of meshing capabilities in Ansys LS-DYNA. If you have any questions, comments, or would like to suggest a topic for the next episode, shoot us an email at podcast@padtinc.com we would love to hear from you!
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@ANSYS #ANSYS

Easily handle the complexity of the varied design environments you may face. 

With a wide range of applications and product integrations, Ansys structural analysis helps you solve your toughest product challenges. For 2024 R1, the structures product line introduces new features, enabling users to solve more complex problems efficiently. 

Join PADT’s Chief Simulation Engineer, Jim Peters for a deep dive into the latest updates available for structural simulation in Ansys 2024 R1.

This presentation focuses on updates regarding the following:

• Fracture

• Linear & Nonlinear Dynamics

• LS-DYNA

• And much more

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For those familiar with implicit ANSYS Mechanical, a similar element is referred to as a solid-shell element (SOLSH190).  This element type is very popular in implicit ANSYS structural applications for modeling thin volumes that are too thick to be modeled with traditional thin shell formulations and computationally expensive to mesh with standard hexagonal or tetrahedral elements to capture accurate bending behavior. 

The appeal does not stop there. The SOLSH190/Thick Shell uses a low-order hexagonal element topology having 4 nodes on the top and bottom face of the element with three degrees of freedom at each node.   The element formulation is based on logarithmic strain and true stress measures and allows loads and boundary conditions to be uniquely applied to the top and bottom surfaces of the elements.

In Ansys LS-DYNA, this element has several formulations:

Per user manual remarks “Thick shell element formulations 1, 2 and 6 are extruded thin shell elements and use thin shell material models and have an uncoupled stiffness in the z-direction.  Thick shell element formulations 3, 5, and 7 are layered brick elements that use 3D brick material models. Thick shell formulations 3 and 5, and 6 are distortion sensitive and should not be used in situations where the elements are badly shaped. A single thick shell element through the thickness will capture bending response, but with thick shell formulation 3, at least two elements through the thickness are recommended to avoid excessive softness. “ 

In the Ansys Mechnical/Ansys LS-Dyna implementation, the element formulation is set to option 5 (assumed strain reduced integration with brick materials).  Thick shell elements of all formulations can be used to model layered composites, but element formulations 5 and 6 use assumed strain to capture the complex Poisson’s effects and through thickness stress distribution in layered composites.

Exposure of this element type allows for some applications to significantly reduce the analysis time needed to evaluate a given design.  To demonstrate how to access this new mesh option, we will be comparing and contrasting a double wall aluminum cylinder dropped onto concrete from a height of 12 inches using both thick shells and traditional full integration S/R solids.  

This demo model uses the drop test wizard to set up the orientation of the cylinder for a side drop, calculate the equivalent impact velocity based on the drop height provided and create the impacting floor at the onset of contact, assigned to rigid concrete in this example.

Ansys LS-Dyna Demo Model Definition:

• Aluminum 6061 T6 double wall cylinder – 8mm diameter, 50mm long and wall thickness of 0.25mm
• Concrete Floor – 30mm x 30mm x 0.50 mm thick (modeled as rigid part)
• 304.8mm (12 inches) drop height.

Figure 1:  Demo Model Geometry Reference (1/2 section)

The exposure of the thick shell in Ansys Mechanical/Ansys LS-Dyna uses the same mesh method as implicit ANSYS to mesh the SOLSH190.  The “MultiZone” method when set to Decomposition Type– thin sweep and the Element Option – Solid Shell creates *ELEMENT_TSHELL elements in Ansys LS-Dyna keyword format.

In the 2023 R2 release the Source Scoping Method was hardwired to be Program Controlled which did not always work when the mesher could not determine the thickness or the source and target surfaces for certain volumes.   In release 2024 R1, the option to define both Manual Source(s) and Target(s) was added and has greatly enhanced the robustness of getting a successful mesh.

V2023 R2 MultiZone Details Menu

V2024 R1 MultiZone Details Menu

The default number of through thickness integration points set for this element type is 2 unless the user inserts a Section object to specify up to 9 integration points.

There is one more 2024 R1 feature related to meshing that was used on this topology for the thick shell version of the model.  When meshing circular volumes, it is often necessary to use a pave option for the surface mesh to prevent small elements from being generated.  The thick shell is more sensitive to mesh shape quality as well, so a new feature was used.  In release 2024 R1 the Face Meshing object has a new beta feature added.  This option produces a high-quality mesh transition without the need for manual decomposition.

The mesh and element type for the concrete floor was exactly the same, so the only difference in the two configurations was element type and mesh density for the double wall cylinder. 

For the thick shell version of the cylinder, the mesh size was set to 0.50 mm and produced 11808 nodes and 5954 thick shell elements.  The run time was 6m and 38s running on 15 SMP cores with a starting integration time step of 2.37e-8 seconds, consuming a maximum of 37GB of RAM.  This model run on 1 SMP core took 12m and 8s.  All versions of the model were run using Ansys LS-DYNA executable version 14.1-205-geb5348f751 (Version delivered with Ansys Mechanical/Ansys LS-DYNA V2024 R1)

Figure 2: Thick Shell Mesh Images

For the solid element version of the cylinder, the mesh size was set to 0.1mm and produced 461,895 nodes and 333825 solid elements.  The run time was 3hrs 8m running on 15 SMP cores with a starting integration time step of 4.98e-9 seconds consuming a maximum of 77GB of RAM.

Figure 3:  Solid Element Mesh Images

Let’s look at some result comparisons starting with energy summary quantities.  The most noticeable difference is in the contact energy, the thick shell version has a higher contact energy likely due to a coarser mesh density relative to the solid element version in representing the cylindrical shape.  The thick shell uses a reduced integration formulation so some hourglassing energy is expected.  The solid element model uses a full integration formulation so no hourglass energy is produced.

Figure 4: Thick Shell Energy Quantity Summary

Figure 5: Solid Element Energy Quantity Summary

Next, if we look at the peak resultant displacement (which includes both elastic and rigid body displacement) we see a good comparison.  With the cylinder impacting on the side a hertzian contact zone is developed.

Figure 6: Resultant Displacement at Initial Impact

Looking at the peak Von Mises Stress from each model, we see a slight delay with reduced magnitude in the thick shell model (again likely attributable to mesh discretization) as compared to the solid element model.

The difference in peak stress also translates to a difference in equivalent plastic strain levels. However, the focus of this blog is not to discuss the nuance differences between solids and thick shells, but rather the time savings of using thick shells over solids in obtaining a design evaluation in minutes versus multiple hours of run time for topologies that are appropriate for thick shells. 

One aspect of the post-processing that is worth discussing between the solid and thick shell versions of the models is the integration point result setting for derived quantities.  In the Ansys Mechanical/Ansys LS-Dyna environment, the default setting for derived quantities like stress and strain is to not average integration point results.  

For a model constructed of solid elements of sufficient density, the difference between averaged and non-averaged will ideally be zero, which is the case with the solid element version of this model.  For the thick shell version of the model, aspect ratio limitations will prevent convergence of averaged and unaveraged integration results and therefore the default Unaveraged display option should be used to obtain the true peak.  Although using the Averaged display option produces a visually more continuous contour, it does not report an accurate peak for the thick shells as shown below.

                                       Figure 7: Comparison of Unaveraged vs Averaged IP Results for Thick Shell          

In summary, exposure of the thick shell element formulation in Ansys Mechanical/Ansys LS-Dyna provides for a very fast and convenient way to mesh moderatelly thick volumes with thick shells and saves significant computation time for topologies appropiate for this element type.

As Bronislav continues to learn Ansys LS-Dyna, he is creating tutorials as he goes. You can find his first tutorial here. One of the more powerfull aspects of the tool is that you can edit the input file directly, whithout opening the Ansys LS-Prepost tool.

In this second installment, he shows how to configure Notepad++ to colorcode your color-code and fold/unfold keywords, and display column markers for ease of reading/writing. All critical to efficiently editing your Ansys LS-Dyna *.k file.

Please find his instructions in the PDF and some files you will need in the ZIP file below.

Ansys LS-Dyna Tutorial: Using Keywords and Editing the LS-Dyna Input File Directly

The files needed for this LS-Dyna tutorial are here:

If you need any help simulating an event with Ansys LS-Dyna, would like to bring Ansys LS-Dyna in house, or if you need custom training, PADT is here to help. Please reach out and one of our engineers will set up a time to talk.

Several of PADT’s engineers are expanding their skills by learning new tools. Bronislav is currently spinning up his Ansys LS-Dyna skills. As part of that process, he ceated an Ansys LS-Dyna Tutorial for other users so they can get up to speed even faster.

Please find his isntructions in this PDF and some files you will need in a ZIP below.

Ansys LS-Dyna Tutorial: Impacting a Plate with a Ball

The files needed for this tutorial are here:

If you need any help simulating an event with Ansys LS-Dyna, would like to bring Ansys LS-Dyna in house, or if you need custom training, PADT is here to help. Please reach out and one of our engineers will set up a time to talk.

Ansys LS-DYNA is the industry-leading explicit simulation software used for applications like drop tests, impact and penetration, smashes and crashes, occupant safety, and more. It is capable of simulating the response of materials to short periods of severe loading. Its many elements, contact formulations, material models and other controls can be used to simulate complex models with control over all the details of the problem. 

Join PADT’s Chief Simulation Engineer, Jim Peters for a look at what’s new for LS-DYNA users in Ansys 2023 R1.

This presentation will focus on updates made to the following:

• ​Connections & Contact
• Rigid Bodies
• Materials
• And much more

View the Recording

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LS-DYNA is the industry-leading software for simulation applications like drop tests, impact and penetration, smashes and crashes, occupant safety, and more; and with the release of Ansys 2022 R2 comes updates and enhancements that further solidify the value of this innovative multiphysics solver.

Join PADT’s Chief Simulation Engineer and LS-DYNA expert, Jim Peters for a closer look at the specific components that make this new release so successful.

This presentation will include enhancements made for: 

• Materials
• Multiphysics
• ALE
• SPH
• And Much More

Register Here

If this is your first time registering for one of our Bright Talk webinars, simply click the link and fill out the attached form. We promise that the information you provide will only be shared with those promoting the event (PADT).

You will only have to do this once! For all future webinars, you can simply click the link, add the reminder to your calendar and you’re good to go!

LS-DYNA is the industry-leading software for simulation applications like drop tests, impact and penetration, smashes and crashes, occupant safety, and more; and with the release of Ansys 2022 R1 comes updates and enhancements that further solidify the value of this innovative multiphysics solver. 

Join PADT’s Chief Simulation Engineer and LS-DYNA expert, Jim Peters for a closer look at the specific components that make this new release so successful, including updates made to: 

• ​​​Workbench

• Motion Integration

• Explicit Dynamics

• And much more

Register Here

If this is your first time registering for one of our Bright Talk webinars, simply click the link and fill out the attached form. We promise that the information you provide will only be shared with those promoting the event (PADT).

You will only have to do this once! For all future webinars, you can simply click the link, add the reminder to your calendar and you’re good to go!

What is Non-Linear Dynamics Simulation, and what makes it different?

When materials are deformed so fast that the rate of strain changes material properties, we refer to that as non-linear dynamics. In non-linear structural simulation, the material may be distorting in a non-linear way (usually plasticity), but the non-linear properties are dynamic. Because of this, time gets involved in the equation, as do non-linear material properties.  Think car crash, metal forming, drop, bird impact on windshields and jet engine blades, and bullets going through stuff. 

There are various software tools in the Ansys family that can do the non-linear dynamics, but our preferred program is Ansys LS-Dyna.  It is an explicit dynamics solver that solves structural, fluid, thermal, and other physics.  It is an amazing program that does many things. Still, for the class of problem we are talking about here, we only care about time-dependent material non-linearity for structural deformation.

Setting expectations

Before beginning a project of any type, it is important to establish goals.  Non-linear dynamics is no different. What is different is that you have to be realistic about what goals you can achieve.  The events you are modeling are, by their very nature are very, well, non-linear. The answers you calculate can change drastically with mesh, loads, material properties, and solver parameters.

If you need high accuracy, then you need to set the expectation that it will take longer to solve, and you have to be more careful with your model. If you don’t need accuracy and you may be looking for relative improvements, like seeing if one geometry option makes things better or worse in your design, then you can back off and be less detailed.  This difference can have a large impact on your schedule and overall cost.

So before you plan, before you start gathering information, decide what you expect to get out of your model.

Planning for the Job

Once you have set your expectations and goals, it’s time to map out the project. It is not that different from most structural or vibration jobs. You still have to get geometry, create a mesh, define loads and constraints, apply material models, run, and post processes.

However, each of those steps can be different for non-linear dynamics.  Here are some critical issues to be aware of when producing a schedule:

• Geometry
  If you are working with thin, especially sheet metal, parts, you probably want to use shells. They are more efficient and can be more accurate in many situations. You need to not just have a CAD model, but also a model that has the mid-plane surface defined as well as thicknesses.
  
  You also want to look at removing tiny features that don’t impact the solution.  The run time in an explicit dynamic solver is driven by the smallest element size. If you have tiny features relative to your overall geometry, capturing them can drive up your run times.  So set aside time to remove or simplify them.

• Meshing
  As mentioned above, small elements can drive up run time. Also, distorted elements or elements that become distorted can cause your solutions to diverge and fail. You may (probably) need to create a hexahedral (brick) mesh.  All of these things require more time to create the mesh, and from a project standpoint, you need to plan for that.
  
• Contact
  Ansys LS-Dyan rocks at contact.  It is pretty much automatic in most cases. So here, you don’t have to set aside time to define and tweak your contacts to get convergence. But there are many options, including erosion and other fancy options. Understand your contact needs and track and manage them.

• Loads
  Everything in LS-DYNA is time-dependent, and loads are no exception.  If you are lucky, your load or loads are constant over time. But if not, you need to set aside time to characterize those loads and get them specified in the right format.  In addition, loads can be calculated, say the results of an explosion or an airbag deployment. These use Equation of State models to calculate forces on the fly and are a major advantage of the tool.

• Solving
  From a project management standpoint, it is very important to plan for relatively long solves, restarts, and if possible, solving several jobs at the same time.  Non-linear dynamics is computationally intense. Do some trade studies on computer resources vs. schedule time.  Is it worth investing in more cores to solve faster or just let it chug away on a smaller computer? Also, don’t assume a single run to get the answer you want. Often you need to run the model multiple times before you understand what is really going on.
  
  
• Post-processing
  We are solving highly non-linear events, and understanding what the model is telling us is the whole point of the exercise.  Budget time for processing massive amounts of data over time and reducing it into something useful. Also, time is needed to create animations.  The analyst may also find themselves buried in the weeds at the post-processing stage, and project management should take on the role of reviewing the results from a big picture perspective and drive what tables, graphs, plots, and animations are created.

Keeping the project on the rails when things are literally blowing up and crashing

The dynamic nature of both the events being modeled and the process of creating and running the models make for a less predictable progression for the project.  A project manager needs to pay close attention to what is going on at all times and pull the engineers doing the work back up for air to find out where things are going. 

Here are some things to watch out for:

• Building a model that is more complex than needed
• Making sure that the situation being simulated in the model is what the customer needs simulated
• Too much time being spent to fit the model on a limited computer. Get a bigger computer.
• The simulation engineer is fixated on details that don’t impact the solution much
• Oversimplification of components, connections, and loads.
• Science project mode – spending time trying to learn basic information or trying to get something new to work, and not solving a specific problem.

One of, if not the most important roles for the project manager is communication.  Constantly interacting with the engineers (without nagging) and with the customer (the person who will consume the results) is critical.  This is not a throw-it-over-the-wall type of project.  And the more you accomplish, often the more you have new questions.  It may take two weeks or nine months. But either way, the PM needs to be constantly talking to everyone involved.

And yes, here at PADT, we have actually modeled a train car going off the rails. The project, though, stayed on track because we kept a close watch and stayed focus on the specifications. Things did surprise us, and we had to change some of the model when we got the first results, but we planned for that, communicated with the customer, and kept our changes to what was needed to answer the customer’s questions.

Our cars have incredible crash safety, very few planes fail because of bird ingestion, and we create amazing components out of formed sheet metal because of this type of non-linear dynamic simulation, and in most cases, Ansys LS-Dyna. Proper project management that recognizes the challenges and differences for this type of project can make a massive variety of products even better.

With Ansys 2021 R1, electronics reliability became much easier to manager with advanced capabilities for design democratization, workflow automation, and robust reliability predictions. Along with these updated components, users can better access integrated workflows between Ansys Sherlock, Icepak, Mechanical, LS-Dyna, and more to provide the results necessary to optimize product designs and ensure unparalleled reliability in the field. 

Join PADT’s Systems Application & Support Engineer Josh Stout for a presentation covering updates to existing features and the introduction of new tools available in this latest release. Learn how users can:

• Extract detailed geometries from any ECAD file

     • Predict time to failure before prototyping

     • Perform complex multiphysics analyses

     • Implement automation and optimization 

     • And much more

Register Here

If this is your first time registering for one of our Bright Talk webinars, simply click the link and fill out the attached form. We promise that the information you provide will only be shared with those promoting the event (PADT).

You will only have to do this once! For all future webinars, you can simply click the link, add the reminder to your calendar and you’re good to go!