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DEVELOP3D Reviews Abaqus 6.9

Abaqus 6.911 September 2009

Al Dean takes a look though at Abaqus 6.9, the latest

release of one of the industry’s leading analysis applications. Among the many

updates across the board there are new tools to simulate crack propagation

For years Abaqus has been one of the leading lights of the

simulation world. Its strength is in highly complex, advanced

structural simulations that feature multiple factors, including

non-linearity, large deformation and more common static

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Abaqus is the cornerstone to Dassault Systèmes’ Simulia brand

In 2005 Abaqus was acquired by Dassault Systèmes and has since become the cornerstone of the

company’s Simulia brand. The depth of the product is phenomenal, and there’s simply far too much

functionality to cover in a single review. Instead, we’re going to take a look at what’s new, what’s changed

and how some of the key parts of the system work.

Co-simulationIn Abaqus, there are two main solver codes. Abaqus/Standard is an implicit solver, most commonly used

for non-linear, static stress simulations. Abaqus/Explicit is an explicit solver, more tuned for simulations

where the product is subject to rapid events that create heavy deformations, such as impact or crash. The

big news for this release is that both of these solvers can be run in tandem. This is often referred to as co-

simulation.

While both simulation types are usually quite separate, there are many instances where it would be

advantageous for the two to interoperate and solve concurrently. For example, picture the simulation of

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The Extended Finite Element Method

(XFEM) has been implemented in Abaqus

6.9 to provide a powerful tool for

simulating crack propagation

a vehicle driving over a curb. The implicit solver would be used on the body and suspension parts, while

the explicit solver would simulate the effect of the tyres impacting the curb. The benefit of doing both

things concurrently is that the result of one event will have an impact on the other (and vice versa).

In Abaqus 6.9 the process of setting up co-simulation jobs is

relatively simple. This is done using Abaqus/CAE, the pre- and

post-processing tool in the suite. Starting with two studies, one

implicit, one explicit, the 3D data is copied into each, then

linked and the simulation inputs defined. Once the set-up has

been completed for both the implicit and explicit studies, the

co-simulation can be created. This defines how and where the

data is passed between the two analysis types in terms of

specific geometry references for load transference (typically

the point of contact). The final step is to define the time step

attributes and run the analysis.

Once the analysis is complete, the results data is fed back as

separate standard and explicit results files and an overlay

technique is used to merge the results. This allows the user to

visualise the results in a single environment.

While co-simulation is something that’s a step towards the multi-physics-based future of simulation, let’s

also step back and look at what’s changed in the usual workflow of mesh, solve, process.

Mesh preparationOne of the first things learned when training for any form of simulation (be that FEA or CFD) is that

simplification can be used to reduce the complexity of your mesh and hence reduce calculation times.

This process, commonly referred to as abstraction, is the removal of features from the underlying part

geometry that hold little influence on that part’s behaviour, but have a large affect on the mesh

complexity.

For example, fillet radii applied to edges can increase mesh element size to accommodate their form, but

for most structural simulations these are inconsequential to the results. The same can be said of small

holes that exist in the model. They increase mesh complexity but have little impact on a part’s stability.

To help with this feature removal process, Abaqus 6.9 includes new tools to help deal with more granular

geometry problems. The new Virtual Topology tool takes the source 3D geometry (from SolidWorks,

Pro/Engineer or Catia) and automatically removes all manner of small features, distorted surfaces, or

sliver surfaces. The end result is a surface that is much more suited to the process of meshing.

The interesting thing about this technology is that it retains links to the underlying source CAD

geometry. You can dive in and adapt the surface patches to merge multiple surfaces to give you the

control you need. These activities, as well as others, such as hiding or removing geometry, are stored in a

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Watching the system find the point of

failure, split the mesh and then

propagate the crack across the surface

of a part before finally failing, is quite

something

feature tree and can be edited or suppressed if needs be. This means you can have multiple states of a

single model, by simply switching features on and off (although there’s no formalised way to do this).

Abaqus also has new mesh verification checks (for distortion

of elements, aspect ratio, interior angles etc). It will give

warnings if a mesh will fail or if there are near bad elements.

The deviation of the mesh from the underlying geometry can

also be checked. This is done using a chord height value (a la

STL files) as well as minimum edge length and aspect ratio.

While for many, this won’t be critical, for those looking for the

highest quality mesh and the closest match to geometry, it’ll

prove invaluable, allowing you to tighten up your mesh to

achieve more accurate results.

Study set-upWhen preparing simulation tasks, the way contacts are defined

has been enhanced. Contacts can initially be found with a

search (within user defined tolerances). The user is then

presented with a preview list of possible areas of contact, and

can then define the type of contact and its properties – in terms of friction parameters, for example. If

the user does not want to define specific contacting pairs of surfaces, the system can also set up analyses

automatically with general contacts.

The set-up of fastener features, (used to model rivets and bolted joints) has also been made more

intelligent and patterns of fastener points can be easily created and managed. All fastener features can

now be edited or suppressed from a model.

Crack propagationWithin many industries the ability to simulate material failure is essential. This is a complex process,

particularly when using traditional techniques, but this release introduces some new tools that make it

much easier.

Abaqus 6.9 includes dedicated technology for the simulation of crack propagation. This is done using a

technique referred to as the Extended Finite Element Method (or XFEM for short).

Beginning with the 3D mesh, there are two options. The first is to predefine a crack (done easily with

geometry). The second is to allow the system to find the point of failure organically.

The next stage is to define the region in which the crack will propagate (this can be the whole model or a

selective area) followed by a few options. These include specifying whether or not to allow crack growth,

defining contact properties (including the type of contact model applied if the crack should close again),

and whether or not to allow multiple cracks. If you do allow multiple cracks the system also simulates the

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interaction between cracks when they meet.

Once this stage is complete, the simulation is run and the visualisation tools allow the user to see exactly

how and where the cracking begins and propagates. The user also has the full wealth of Abaqus’ post

processing tools to investigate and report on the findings.

Results visualisationThe big push for this release is to take advantage of the rapidly advancing world of graphics hardware

and greater caching capabilities. This means more data can be managed which makes working with very

large data-sets a reality for the majority, rather than just the ‘hardware-rich’ few. For instance, now when

you open a model database that contains a large number of parts and part instances, Abaqus/CAE loads

the parts and instances on demand, based on the parts or part instances that you display. This

enhancement provides improved performance and optimised memory usage.

For those working on 32-bit Windows systems Abaqus/CAE provides improved memory usage. When a

user-specified percentage of the total memory allocated to the kernel is reached, Abaqus/CAE optimises

the display data to enable more operations to be carried out.

By default Abaqus/CAE now uses coarse curve refinement to display the curved faces or curved edges in

a part or part instance. This modified setting provides improved performance and requires less memory,

especially when you are working with a large number of part instances or if you are working on parts with

very complex geometry. Users can override this default behaviour to use a finer curve refinement setting

if required.

There is also a new view cut (clipping plane) tool that allows instant visualisation of data. Alongside this,

users can now display the applied loads and resultant forces and moments, so there is a very graphical,

but very usable sanity check of the simulation.

ConclusionAbaqus is a huge offering that brings all manner of tools to the advanced user looking for higher-end

simulation capabilities. But let’s not misconstrue that to mean that it’s the restricted to FEA boffins and

those with PHDs. The system is easy enough to be used by designers and engineers and while there are

complexities inherent with many of the more advanced methods and techniques, it’s by no means an

exclusive product with an exclusive user community.

The thing that is essential is that the user has a solid understanding of the behaviour and the operating

conditions of the product that’s under simulation. The perfect example here is the fact that something as

deeply complex as crack propagation can be simulated with apparent ease.

Watching the system find the point of failure, split the mesh and then propagate the crack across the

surface of a part before finally failing, is quite something. It shows exactly how far simulation has moved

on in the last few years and how close we are to achieving the ability to truly simulate a product’s

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XFEM can be used with any geometry in

CAE. In this example a three-point bend

test is modelled in 2D. The XFEM crack is

defined using the create crack dialogue

Using the edit crack dialogue, an initial

mesh-independent crack location can be

specified, and the crack allowed to

grow. Contact properties for the

exposed crack faces can also be

specified

performance, with less reliance on assumption and being able to get closer to reality than ever before.

www.simulia.com

Workflow for crack propagation simulation using Extended Finite Element Method (XFEM)

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Here the XFEM-related field output

requests are made. This allows for a

visualisation of the XFEM crack following

the analysis

This and the previous image confirm that

the initial XFEM crack is independent of

the mesh

Here you can see that the XFEM crack

has clearly propagated during the

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analysis. The XFEM crack is displayed

automatically on opening the ODB

This crack is visualised using the

visualisation modules ‘View Cut’ tool. A

cut is automatically created based on

PHILSM, which is the XFEM output value

that defines the crack location

Here you can see the full (scaled)

deformation of the plate with the Von

Mises stress output plotted as the crack

propagates

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Comments on this article:

Dear sir,

I want to model debond in the interface i supposed that there is a carck at the end of bond lay er , but

abaqus asked abut the intial crack location it is ok with me but i can determine the length of it how can

determine that by guesses ????????????????????????

also when i determine the length is equal third of th elength the analy sis stop at a certain step how can

deal with this to keep the model runing

another comment if i have not toggeled on the initial crack location and leave the abaqus determine it

also there is a probelem of the runing it is stop,,,,,,, please help me

thanks

Posted by yasmeen on Tuesday 25 2010 at 09:40 AM

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