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Zoom Modelling with MSC Marc

Posted by Andy Woodward on 08-Aug-2017 09:00:00
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One of the parameters driving the age old question of “is my mesh refined enough?” is the impact it has on the model size and the knock-on effect to runtime and computer spec.  This comes into starkest relief if you have a dimensionally very large structure but where you need a fine local mesh around some detailed areas in order to refine stress intensity for e.g. fatigue life prediction. 

If you mesh the whole thing finely then you’ll need to spend ££££’s on a server and wait a long time for the results.  If you refine the mesh locally to each detail then you’ll have a challenge grading the mesh back out to the global size and still have quite a large model.  And what if you don’t know before you start where the higher stress is going to occur and where you need to refine the mesh?

This is the sort of scenario where zoom modelling, or global-local modelling as it is known in MSC Marc, is applicable.  Put simply, this is a technique where your global model can be quite coarse – refined enough to capture the overall stiffness and displacement - but not enough to capture local stress concentrations.  We convert this deformation at node information to a 3D field of displacement which is then mapped to an enforced boundary condition on a small detail mesh of one or more areas of the model.

In this example we have a box beam with a cut out.  We’ve run the load case with a relatively coarse mesh, but want to resolve the stress in the corners of the cut-out better.

Box beam with a cut out - Zoom Modelling with MSC Marc img1

We build a new model, of just one corner and map the displacement field onto its boundaries.

build new model of just 1 corner & map displacement field onto boundaries - Zoom Modelling with MSC Marc

This can be done manually with many post-processors including MSC Patran, but becomes difficult if you have a non-linear analysis or a transient event with multiple time steps.  The beauty of the method in MSC Marc is that all you need to do is define the boundary nodes in the detailed model and point the analysis at the result file from the previous global analysis and Marc does the rest.  Displacements at the new boundaries are interpolated in space and time from the previous results automatically.

Global Model       Local Model
Zoom Modelling with MSC Marc img3 Global Model   Zoom Modelling with MSC Marc img4 Local Model

You might want to perform this on multiple locations of a large structure.  I have seen this technique implemented within a scheduling environment where several local models were prepared before the global model was run and the whole package submitted to the queue together, with the local models set to run only when the global model was complete.

MSC Marc is a very capable and robust non-linear FEA solver with over four decades of development and a very experienced support team behind it.  It is available within the MSC One token licensing system which could be licensed for as little as around £5000 per year. If you want to know more, or have a broader discussion about the capabilities of MSC Marc please get in touch. 

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Topics: MSC Software, Finite Element Analysis (FEA)