Category "Simulation"

“What you buy makes a difference but from whom you buy makes a bigger difference”

Most often, I talk about greatness of our product offerings in my blog articles. Such kind of blogs assist prospective customers in choosing the right product. But the same product can be procured in multiple ways, either directly from the developer or through a value-added reseller also called as VAR. In this blog article, I would emphasize on how prospective customer should select the right VAR while purchasing a Dassault Systemes or Siemens simulation product.

The first thing a customer needs to verify is whether VAR is supplying just the product or the complete solution. The difference between the two is the “value added services” associated with product usage.

Without value added services, it’s not possible for a reseller to become a value-added reseller.” Please identify if you are doing business with just a reseller or a value-added reseller. Remember, simulation tools are not easy to use. There is a learning curve associated with these tools that can greatly impact the ROI and break-even timeline. The productivity of the user can be substantially enhanced if he is associated with a reseller who can provide whole bunch of services to shorten the learning curve and achieve break-even faster. Now let’s look at what type of services makes a difference in simulation space.

We are talking about software sales as well as consulting, training and support. Our software partners, Dassault Systemes, Siemens and Autodesk offer a bunch of certifications around these four components to distinguish between just “resellers” and “value added resellers.” Being certified means reseller has enough resources and knowledge to execute a given task of sales or service. Let’s talk about each component with respect to Simulation:

Software: To sell any DS SIMULIA product, the associated VAR should have “SIMULIA V6 design sight” certification as a minimum. There are further brand certifications available such as Mid-Market Articulate for product highlight and Mid-Market Demonstrate for product technical demonstration. To sell FEMAP product from Siemens, the VAR must have “FEMAP technical certification” as a minimum. All these certifications are associated with timed examinations.

Training: Training should be an integral part of simulation software sales. It gives users enough knowledge to use the software product in production environment. To offer technical training on any SIMULIA product, the VAR should have “finite element analysis with Abaqus specialist” certification as a minimum.

Support: Once users are in production environment, technical support is required on continuous basis. While many answers related to product usage are in documentation, it’s not a full source of information. Many queries are model specific that require attention of a dedicated support engineer. To offer technical support on any SIMULIA product, the VAR should have at-least one engineer who has “SIMULIA technical support specialist” certification.  This certification should be renewed every two years. It is associated with a lengthy and “hard to pass” support certification examination across all products of SIMULIA brand.

Consulting: Consulting service plays a big role when customer either does not have enough time or resources to execute projects in house in-spite of having software product. It happens during certain burst phases of demand. While there are no certification criteria for VAR’s related to consulting in simulation space, a dedicated consulting and delivery team is needed to offer the service when demand arises.

The above information should help you in ranking your VAR. Do you need to know our rank? Please contact us.

 

Perhaps one of the biggest surprises for Abaqus user community in 2018 is that the two most popular licensing schemes of Abaqus would gradually go away for new customers. These schemes are Abaqus analysis pack and Abaqus portfolio pack. It’s worth mentioning that many of our Abaqus customers are still using either of these two licensing schemes. While our current customers who have perpetual or lease licenses may be able to continue with these schemes, our future customers will have to migrate to something that is available as a replacement. Instead of putting this news as a surprise to each customer individually, I thought a common piece of information well in advance through a blog article would keep the anxiety under control.

THE MIGRATION PATH

The migration path eventually leads to a token configuration that has been available since couple of years now. It is called the extended tokens configuration. While many of our customers have already migrated to this licensing scheme by choice, others are still using one of the traditional licensing schemes. Let’s look at the logic behind this high-level decision. If we look at the history of acquisitions that Dassault Systemes has made in past few years, it looks like this:

 

The inception of extended tokens is related to acquisition of three companies in above chart: FE-Design, Safe Tech and Engineous. The product offerings from these companies, if coupled with Abaqus can greatly enhance its simulation portfolio. Following acquisition, these products were offered as point tools for a long time with their individual licensing and pricing schemes. As a result, existing Abaqus customers who wished to use either one or more of these products had to go through a complicated purchase and IT process. Dassault Systemes has been looking for a consolidated licensing scheme that would enable users to procure these products along with Abaqus in a single license file that works on a single token scheme and on a single license server. This token scheme is now called the extended tokens. At this point of time Dassault Syetemes believes it makes sense to migrate all existing Abaqus users to extended tokens through a migration path that would enhance the simulation portfolio of users in a cost-effective way.

           COMPARISON BETWEEN DIFFERENT TOKEN SCHEMES

MESHING CAPABILITIES IN ABAQUS CAE

It is a well-known fact in the CAE community that the efficiency and accuracy of finite element models are directly dependent on the quality of the underlying meshes in the model. The various quality parameters associated with elements are element size, aspect ratio, skew angle, jacobian, warp, and many more. Yet another parameter of concern is element topology, which means triangular/quadrilateral elements in case of shell meshes and tetrahedral/hexahedral elements in case of solid meshes. Each of these element topologies has its own advantages and disadvantages; for example, tetrahedral elements are easy to create on complex geometries but they have slower convergence, while hexahedral elements are very much desired in computational expensive simulations such as crash due to better convergence and accuracy but cannot be created easily.

Due to specific meshing requirements arising from the increasing complexity of part geometries, meshing techniques are becoming more important across all industry verticals. Transportation & mobility is primarily concerned with hexahedral meshes of pre-defined quality for very complex geometries. This industry has more focus on using Hypermesh and Ansa as a dedicated meshing tools. However these tools are primarily known for good meshing capabilities only. When there is a need to create input decks for advanced non-linear simulations such as with Nastran solution sequences 600/700 or for Abaqus multiphysics or acoustics, many of the solver features are not supported by Hypermesh or Ansa and have to be entered manually into the deck. Aerospace industry has almost always a requirement for composites modeling. They prefer a user interface that can either create or import composite plies and layups. The need for high quality meshes on complex geometries is rather rare. Due to these reasons Aerospace industry has been relying on MSC Patran since many years due to its composites modeling capabilities. However industry is now looking at alternate tools as Patran is losing its competitive edge on CAD import, CAD repair as well as meshing techniques. The CAD repair features are very minimal, there is no CAD associative interface to propagate design changes on FE side and meshing techniques offered are still at very basic level as well.

The objective of this blog is to highlight the meshing techniques in Abaqus CAE that makes CAE a tool of choice in situations where a decent quality mesh, tight integration with multiple CAD platforms as well as tight integration with Abaqus solver are topics of concern for the analyst. It’s worth mentioning for Aerospace industry audience that Abaqus CAE has basic composite modeling capabilities. For advanced composite modeling and visualization capabilities, there is an add on module called composite modeler for Abaqus CAE and there is tight integration between CATIA composites workbench and Abaqus CAE for transfer of FE meshes as well as ply layup information.

THE MESHING TECHNIQUES

 There are primarily four meshing techniques available in Abaqus CAE, both for solid meshes as well as for shell meshes.

Free meshing: This is the easiest of all the techniques as it almost always works with a single click. It primarily generates quadrilateral or triangular elements on surfaces and tetrahedral elements on solids, even on very complex geometries. The downside is that user has very minimal control on elements quality except controlling the mesh density using global and local seeding options.

Sweep meshing: This technique is useful when hexahedral elements are needed on solids with minimal geometry editing though this technique is applicable on surfaces as well. The meshing algorithm automatically identifies a source side and a target side on the geometry, it creates a quadrilateral shell mesh on the source side and sweeps those elements to the target side thereby converting them to hexahedral or bricks. The underlying shell mesh is automatically deleted. The downside is some geometry restrictions with respect to source and target side.

Structured meshing: This meshing techniques is useful when high quality hexahedral or near to perfect shell elements are required on solids or surfaces. This technique offers a better mesh control to the user compared to sweep meshing technique. It works by partitioning the complex solids into smaller six or eight sided parametric solids that can be brick meshed. The nodes at the boundaries are automatically fused to ensure connectivity.

Bottom’s up meshing: This is the last approach when all the other meshing techniques fails. It works on the concept of divide and rule. To some extent it resembles sweep meshing but the underlying geometry restrictions are removed.

THE COLOR CODING FEATURE

This is one feature that sets Abaqus CAE apart from other meshing tools available in the market. While doing meshing, user can see either entire part or regions associated with part (in case of partitions) in pre-defined colors. These colors helps in determining which region of the part would be meshed with which meshing technique if the mesh algorithm is executed. The color cold is as follows:

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The process is quite interactive. The orange color is most undesirable as these regions are non-meshable and require further partitions. Once the region is correctly partitioned and subdivided regions become meshable, the color code is updated instantly. What the user needs to see is the combination of greens, yellows and pinks with peach at certain times before executing meshing operation. During meshing, user has option to either mesh one region at a time or the entire part having multiple regions. In case of interfaces having different element topologies on each side such as green with pink or yellow with pink, tie constraints are automatically created at the boundary to ensure mesh connectivity.

Below is an example of a part that has been partitioned to create certain sweep meshable yellow regions where brick elements are needed. The other region is pink with tetrahedral elements associated to it.

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EFFECTIVE PARTITIONING

Transition of orange region to either yellow, green or peach requires intelligent partitioning of surfaces or solids. While there are many such partitioning tools available, achieving desired results with minimum partitions requires some practice in using these tools. Let’s highlight few of these partitioning methods:

Solid partitions:  Six options are available.

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VIRTUAL TOPOLOGY

This is an optional process that may be needed before partitioning and prior to meshing. This is a way to fix bad CAD data. Many times CAD data has more details than needed by the meshing algorithm. This includes very short edges and very small surfaces. Virtual topology offers certain tools to combine such small faces and edges. There is also an option to suppress these small features so that meshing algorithm does not recognize them.

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The Abaqus user community knows that computational fluid dynamics module was deprecated in 2017 release of Abaqus CAE. It means that within Abaqus CAE or through standalone Abaqus, it is not possible to perform CFD simulations beyond 2016 release. This has been a subject of criticism among few users. However, its worth mentioning that CFD is still available in Abaqus through 3D Experience platform fluid mechanics analyst (FLA) role. Dassault Systemes has decided to migrate the functionality from standalone products to the platform but it is still in existence. The FLA role is available both on premise as well as on cloud. So what are the value adds of performing CFD through 3DExperience platform!!

No need to create fluid domain 😊

This is a BIG BONANZA because every analyst knows how tough it is to create fluid domain for complex 3D Models. The 3DExperience platform offers a technique called hybrid meshing that has two main advantages. Firstly, it does not require a fluid CAD. The user needs to provide minimum information in terms of geometric features such as faces, planes, face normal etc. so that application can well predict fluid location and boundaries in given part of assembly geometry. Once it is will predicted, it gets well meshed also. Take a simple example: flow between two intersecting pipes. User just need to provide surface normal to two pipes in correct direction as well as three planes for inlet and outlet. With this information FLA user interface can create a bounded region internally on its own. The fluid domain tool helps in selecting respective geometries for parts, regions, openings and boundaries. Just this much information is good enough to proceed with the meshing operation.

 

Good quality hexahedral meshes with perfect boundary layer control 😉

FLA offers hex dominant meshing technique that operates on fluid domain created above. It has two outstanding offerings. First it gives maximum number of hexahedral elements. Yes, hexahedral elements and that too without any partitioning. Second, it is possible to define (and achieve) boundary layer as per user specified criteria. User can define number of layers as well as thickness of layers. Even in transition regions such as location where pipes intersect and geometry abruptly changes, the boundary layer specification is well respected.

 

Lastly, FLA role is the center of attraction of SIMULIA R&D for further enhancements. Dassault Systemes recently acquired two CFD companies named XFlow and Exa. The CFD solver offerings from these two companies work on Lattice Boltzmann principles while the traditional Abaqus CFD solver in 3DExperience work on Navier-Stokes principles. The Lattice Boltzmann based solver is suitable for external and unbounded, high speed transient and compressible flows that has many applications in aerodynamic computations in T&M and A&D industry verticals. These solvers will be integrated in future releases of FLA role or will be available as a new role in 3DExperience platform.

Users of Dassault Systemes must have seen the following image multiple times in last few years. In this article, I am talking about what this image means from simulation perspective.

 

The image above is the brand logo of Dassault Systemes next generation innovation called 3D Experience platform. This image is called 3D Compass and its four quadrants symbolizes four major offerings  available in an integrated fashion with the platform. The north quadrant symbolizes collaboration, the east quadrant symbolizes 6W Tags, the west quadrant symbolizes designer applications and the south quadrant symbolizes our favorite simulation applications. The idea behind its introduction is very clear from simulation environment.

“BREAK THE SILOS”

The word silos may seem unpleasant to hear at times but that’s exactly how FEA or MBD simulation community has been since many years. These are small departments of very specialized people in large organizations that mostly work in isolation. The reason of working in isolation has been simple and justified till now. The complexity of products these specialists use has been an overwhelming task for other departments such as design, production, marketing, procurement, IT etc. that are a part of overall product lifecycle management. Moreover simulation forms a process of product lifecycle only when a new concept design is launched that has to be virtually tested either to save cost, reduce time or meet certain requirements imposed by regulatory boards such as NHTSA, FAA, FDA etc. This is not always the case.

The 3D Experience platform integrates and brings together all the departments involved in product lifecycle including simulation experts. The integration happens at base level by a data management and collaboration server called ENOVIA that allows users with different roles to create, modify, share, manage or propagate data from one person to other without using share drives, emails or any sort of data migration. In terms of T&M and A&D companies these roles might be product designers, design engineers, manufacturing engineers, FEA specialists, material experts, method developers etc. The platform offers roles for users. Each role is a collection of apps just like we see in our mobile phones. However, in case of 3D Experience platform, these roles are divided in four different categories based on four corners of the 3D Compass. These are 3D Modeling apps, Social and Collaborative apps, information intelligence apps and simulation apps. Few of these apps serve as pre-requisites to any user who wish to be a part of platform. But others such as simulation apps are assigned only to those users who wish to perform simulation.

 

With the 3D Experience platform, a simulation expert works on the same data that is created by a designer to perform simulation without any manual data transfer. No more specialized product formats such as Abaqus CAE, Tosca, fe-safe that have their own file architecture not compatible with designer products such as CATIA, Creo, Solidworks etc. Once the simulation is complete, results are stored in the same database that can be instantly viewed by engineering managers, product designers, R&D head or any user who is not actively using simulation. So no more specialized output file formats such as odb, conf, ldf, stlx etc. A simulation role armored with simulation apps allows a specialist to get into the platform, access the data created by designer, create simulation model in same environment, perform the analysis on user machine or on cluster and publish and share the results. Simple it is! Once the silos are broken, there is yet another aspect of 3D Experience platform that makes it unique in the way it works.

“ELIMINATE FILE BASED FOLDERS FROM THE SYSTEM”

Everything created or imported in the platform is saved within the ENOVIA collaboration server. The data can reside either on-premise or on-cloud based on chosen architecture. The user can search or access the data using advanced 6W Tags search criteria based on questions such as who, what, when, where, which etc. In case of cloud based architecture, this data can be assessed and modified from anywhere using devices such as smart phones or ipads. Can your smart phone read Abaqus odb files! The answer is perhaps no but yet it is possible to access Abaqus output data on smart phones using 3D experience platform.

Any complete FEA solution has at-least three mandatory components: Pre-Processor, solver and post-processor. If you compare it with an automobile, solver is the engine that has all the steps/solution sequences to solve the discretized model. It can be regarded as the main power source of a CAE system. The pre-processor is a graphical user interface that allows user to define all the inputs into the model such as geometry, material, loads and boundary scenarios etc. In our automobile analogy, pre-processor can be regarded as the ignition key without which it is not possible to utilize the engine (solver) efficiently. The post-processor is a visualization tool to make certain conclusion from requested output: either text or binary. A good CAE workflow is regarded as one that offers closed loop CAD to CAD data transfer.

The above workflow is not closed so there is no scope of model update. Any changes in design requires all the rework. This has been the traditional workflow in organizations that have completely disconnected design and analysis departments. Designers send the CAD data to analysts who perform FEA in specialized tools and submit the product virtual performance report back to designers. If a change is mandatory, FEA is performed manually all over again. Let’s look at a better workflow.

In this workflow, if the initial design does not meet the design requirements, it is updated and sent to the solver, not to the pre-processor. It means that all the pre-processing steps are mapped from old design to new design without any manual intervention. This is an effort to bridge the gap between design and analysis departments that has been embraced by the industry so far. The extent to which the GAP can be bridged depends on the chosen workflow but to some extent, almost every CAE company has taken an initiative to introduce products that bridge this GAP. Let’s discuss in context of Dassault Systemes and Siemens.

Dassault Systemes: After acquiring Abaqus Inc in 2005, Dassault Systemes rebranded it as SIMULIA with the objective of giving users access to simulation capabilities without requiring the steep learning curve of disparate, traditional simulation tools. They have been introducing new tools to meet this objective.

  • The first one in series was Associative interfaces for CATIA, Pro-E and Solidworks which is a plug-in to Abaqus CAE. With this plug-in it is possible to automatically transfer the updated data from above mentioned CAD platforms to Abaqus CAE with a single click. All the CAE parameters in Abaqus CAE are mapped from old design to updated design. It’s a nice way to reduce re-work but design and simulation teams are still separate in this workflow.
  • Next initiative was SIMULIA V5 in which Abaqus was introduced in CATIA V5 as a separate workbench. This workbench includes additional toolbars to define Abaqus model and generate Abaqus input file from within CATIA. Introduce Knowledge ware, and user has all the nice features to perform DOE’s and parametric studies. This approach brings designers and analysts with CATIA experience under one roof.
  • Next Dassault Systemes introduced SIMULIA on 3D Experience platform allowing analysts to utilize data management, process management and collaboration tools with Abaqus in the form of simulation apps and roles. The solution is now in a mature stage with incorporation of process optimization, light weight optimization, durability and advanced CFD tools. By merging SIMULIA with BIOVIA we are also talking about multi scale simulation from system to molecular level. It is further possible to perform the simulation and store the data on public or private cloud.

Siemens PLM solutions: Siemens traditional CAE tools include FEMAP user interface and NX Nastran solver. Both have been specialized tools primarily meant for analysts with little or no connectivity to CAD. More specialized and domain specific tools were added with the acquisition of LMS and Mentor Graphics.

  • In 2016 Siemens introduced its new Simulation solutions portfolio called as Simcenter that includes all Siemens simulation capabilities that can be integrated with NX environment. The popular pre-processor in Simcenter series is NX CAE that has bi-directional associativity with NX CAD. Though meant for specialists, NX CAE offers a closed loop workflow between NX CAD and NX Nastran thus making easier to evaluate re-designs and perform DOE’s.
  • Siemens also offers NX CAE add-on environments for Abaqus and Ansys thereby allowing analysis to efficiently incorporate these solvers in their NX design environment.
  • It is further possible to use Simcenter solutions with Siemens well known PLM solution Teamcenter for enterprise wide deployment of Siemens simulation tools.

This shift in approach is not limited to Dassault Systemes and Siemens. Every organization in this space be it Ansys, Autodesk or Altair are introducing such closed form solutions. One reason may be the recent acquisition of many CAE companies by bigger organizations such as Dassault, Siemens and Autodesk. Nevertheless, the change has been triggered and it will continue.

 

 

In the FEA solver world, users come across multiple numerical schemes to solve the formulated stiffness matrix of the problem. The most popular ones among all are the implicit and explicit solvers. In Abaqus terminology they are called a standard solver and explicit solver respectively. Each of these schemes has its own merits and demerits and this blog post compares these two schemes based on several parameters.

For ease of understanding, I am avoiding the use of long and complicated mathematical equations in this post. 😉

        Implicit Scheme

From an application perspective, this scheme is primarily used for static problems that do not exhibit severe discontinuities. Let’s take an example of the simplest problem: Linear static in which any physical situation can be mathematically formulated as:

[K]{x}={F}

Here K is the stiffness matrix, x is the displacement vector and F is the load vector. The size of the matrix and vectors can vary depending on the dimensionality of the problem. For example, K can be a 6×6 matrix for a 3D continuum problem or a 3×3 matrix for a 2D structural problem. The composition of K is primarily governed by material properties. F primarily includes forces and moments at each node of the mesh. Now, to solve the above equation for x, matrix K should be inverted or inversed. After inversion, we get a displacement solution used to compute other variables, such as strains, stresses, and reaction forces.

[M]d2{x}/dt2+[C]d{x}/dt+[K]{x}={F}

The Implicit scheme is applicable to dynamic problems as well. In the above equation, M is mass matrix, C is damping matrix and the rest are as usual. This equation is defined in real time. Backward Euler time integration is used to discretize this equation in which the state of a system at a given time increment depends on the state of the system at later time increment. K matrix inversion takes place in a dynamic scenario as well because the objective is still to solve for x. Abaqus standard solver uses three different approaches to solve implicit dynamic problems: quasi static, moderate dissipation or transient fidelity. Each method is recommended for specific types of non-linear dynamic behavior. For example, the quasi static method works well in problems with severe damping.

Merits of this scheme

  • For linear problems, in which K is a constant, implicit scheme provides solution in a single increment.
  • For non-linear problems, in which K is a function of x, thereby making it necessary to solve problem in multiple increments for sake of accuracy, size of each increment can be considerably large as this scheme is unconditionally stable.

Due to these reasons, implicit scheme is preferred to simulate linear/non-linear static problems that are slow or moderate in nature with respect to time.

Demerits of this scheme […]

In the simulation community, when it’s time to learn applications of software tools in real-life product development, one of the best ways to do it is to approach other users. The regional user meetings organized by SIMULIA once every year have a similar objective: bring together the user community. These meetings gather users to share their knowledge and experience in advancing methods and technology for finite element analysis, multi-physics, process automation, design optimization, and simulation management. There is also an opportunity to present a white paper, listen to keynote speakers about the value simulation brings in virtual product development, get updates on new releases from the SIMULIA product management team, and  contribute to this success by being a sponsor of the event. As SIMULIA is not so much a product but a portfolio that offers multiple products, SIMULIA regional user meetings are often a conglomeration of various product specific events: Abaqus, Tosca, ISight, Fesafe, Simpack, Simpoe, and the 3DEXPERIENCE platform.

Initially this event was called the SIMULIA community conference, with a single location at the North American Headquarters in Rhode Island. As its popularity grew, the size of the event grew as well, so there was a need to offer it in multiple locations, making it more accessible to regional users. The new series of events was named the SIMULIA regional user meetings, which are now held at different locations every year: Great Lakes, Houston, California, Toronto, Sao Paulo (Brazil).

How big this event could be in terms of public gathering! We try to never miss it because of its size and value; a snapshot below from the 2014 event held at Providence, RI speaks for itself.

The 2017 Regional User Meetings are coming up!

For all the information regarding dates, venue, agenda and registrations, please click below.

Registrations

Many of our Abaqus customers don’t know that the Computational Fluid Dynamics approach (CFD) is not the only method of modeling fluids in Abaqus. There are many other possibilities and the right approach depends on the physics of the problem. This blog post discusses the multi physics methods of modeling fluids in Abaqus.

  • CFD method: This is the well-known and traditional method for fluids modeling. It’s based on Eulerian formulation, in which material flows through the mesh and can be accessed through the Abaqus/CFD solver. Application example: Flow through exhaust systems.
  • CEL method: This is a coupled Eulerian Lagrangian method primarily used in problems involving unbounded fluids where fluids free surface visualization is required. It’s also possible to simulate interaction between multiple materials, either fluids or solids. This method is accessible through Abaqus/explicit solver. Application example: Fluid motion in washing machine.
  • SPH method: This is a smooth particle hydrodynamics approach primarily used to model unbounded fluids that undergo severe deformation or disintegrate into individual particles. This method uses a Lagrangain approach in which material moves with the nodes or particles and can be accessed through the Abaqus/explicit solver. This method can be used for fluids as well as for solids. Application example: bird strike on an aero structure.

We can compare these three methods against multiple parameters such as materials, contact, computation speed, etc. to understand their applications and limitations:

  • Material considerations:

SPH method is most versatile in terms of material support. SPH supports fluids, isotropic solids as well as anisotropic solids.

CFD is the only technique that can model fluid turbulence

CFD is the only technique to model porous media

CFD and CEL allows material flow through the mesh: Eulerian

  • Contact considerations:

[…]

Abaqus has always been first choice of analysts for modeling any form of non-linearity in the model: geometric non-linearity, material non-linearity, or boundary condition non-linearity which is large sliding contact. Within material non-linearity, the most popular model is piecewise linear plasticity used to model plastic deformations in alloys or metals beyond their yield point. This blog post primarily discusses another powerful but somewhat less known non-linear material model of Abaqus used to model elastomers or rubbers.

Before getting into Abaqus’ functionalities for rubbers, let’s see what types of rubbers primarily exist, along with their mechanical characteristics:

Solid Rubbers

They exist almost everywhere: tires, weather seals, oil seals, civil engineering equipment, etc. Their main mechanical characteristics are

  • Nearly incompressible: While it is easy to stretch these materials, it is very difficult to compress them volumetrically. It’s a common observation that a rubber band can be stretched easily but a piece of pencil eraser cannot be compressed so easily. This behavior is particularly important in elastomer modeling.
  • Progressive loading and unloading cycles show hysteresis as well as damage. As cycles continue, damage progresses.

Thermoplastics

They are a physical combination of rubber materials and thermal plastics. They can be easily molded or extruded. They are not physically as strong as solid rubbers, neither resistant to heat and chemicals. They are more prone to creep and permanent set.

Elastomeric foams

Commercially, they are referred to as porous rubbers or just foams.

  • They can undergo very large strain, as large as 500% that is still recoverable. Their counterparts, crushable foams, can exhibit inelastic strains.
  • They exhibit cellular structure that may be open or closed type. Typical examples are cushions, paddings, etc.
  • The compressive stress strain curve is as follows:

Foams exhibit a linear behavior in a compressive strain range of 0% to 5%. Subsequently, there is a plateau of severe deformation at almost constant stress. In this region, the walls and plates of cells buckle under compression thereby forming a denser structure. Post buckling, the cellular walls and plates start interacting with each other, causing a gradual increase in compressive stress.

  • Due to high porosity, foams exhibit very large axial compressive strain without any lateral strain. Due to this, the Poisson’s ratio of foams is nearly zero. This behavior is critical for material modeling of foams in Abaqus.

Material models in Abaqus for rubbers

Abaqus uses the “hyperelastic materials” terminology for its material libraries that support rubbers. This is primarily because rubbers are elastic in nature even at very high strains. The basic assumptions in modeling solid rubbers are: elastic, isotropic and nearly incompressible. Foam material libraries in Abaqus are referred as “hyperfoam” and are highly incompressible. None of the rubber material models can be represented by a single coefficient such as modulus. It rather requires a strain energy density function that can have an infinite number of terms. Therefore, in Abaqus, strain energy functions have specific forms with certain numbers of parameters to be determined. Each of these function is associated with a separate material model, as shown below. […]

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