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Review

A sore subject

From CAD User Mechanical Magazine Vol 22 No 2 - FEBRUARY/MARCH 2009

Dassault Systèmes’ Simulia is moving into very sensitive areas - simulating the generation of bed sores and their treatment, explains David Chadwick

I know, I know, this is not the sort of thing that you really want to think about especially as you are probably reading this during a coffee break - but how on earth do you build a digital model of bed sores? It's not as silly a question as it sounds, as unless you can build a digital materials and assemblies, such as rubber tyres comprising layers of fibre, wire and rubber, or the entire wheel assembly in impact studies. It can even be used to calculate the (surprisingly) complex forces involved in blow-moulding a plastic bottle. Multiphysics takes the process further, with A sore subject Dassault Systèmes’ Simulia is moving into very sensitive areas - simulating the generation of bed sores and their treatment, explains David Chadwick model of human tissue, even in such a sorry state, you won't be able to simulate how it will act when subject to stresses and strains.

Bed sores are a major issue for patients who spend long periods of time inactive in hospital beds. They are caused by pressure from the bones, rather than soft tissue, but can only be alleviated by supporting the outer part of the body. Any simulation of medical devices designed to provide that support, therefore, has to include not only the medical device itself, but also its effect on the underlying human skeleton and tissues. This is called, appropriately enough, bio-mechanics - the interaction of biological material with mineral.

Quite apart from the difficulty of simulating biological tissue, we are also straying into the domain of multi-body dynamics and multiphysics. We need no longer perform a single analysis on part of a structure to see how it performs under stress, but can look at all inter-related mechanical components of an assembly to see how they workm together.

Multi-body dynamics affect things like complex joints, bolted assemblies using gaskets, and welded components. Multiphysics uses combinations of FEA solutions to analyse complex materials or assemblies - such as fluid and mechanical, structural and accoustic, thermal and fluid or mechanica, piezo-electrical and mechanical.

Dealing with medical supports to alleviate bed sores is an ideal application for multi- body FEA. More common uses cover

typical applications including delivering ink in inkjet printer simulation, underwater explosions and tyre hydroplaning.

SIMULIA

Two things stand out for me - the sheer scope of manufactured products that require some form of multi-body or multiphysical simulation for, as Simulia puts it, 'Realistic Simulation', and the need to manage effectively the complex results that emerge from multi-bodt dynamics and multi-physics simulation. If successful, we enhance our ability to evaluate design alternatives, reduce physical prototypes and increase our confidence in the products we aim to build.

FEA and CFD solutions integrated into CAD programmes like SolidWorks, are essentially simple, linear solutions, focusing on single issues. They are valuable, though, in that they introduce the design engineer into the disciplines of digital simulation, enabling them to test the validity of their designs at an early stage in the design process. Where strength, cost, durability etc., are not the main factor in a design, they are perfectly adequate for a lot of products. That leaves rather a large body of simulations that need the resources of software like Simulia.

Simulia, hitherto Abaqus before it was acquired by Dassault Systèmes, provides a comprehensive solution, with scaleable FEA solutions, which can be integrated within DS' SolidWorks and other CAD solutions, or run as a separate application.

As Abaqus, Simulia was noted for its ability to handle the FEA of events such as

impact and crash scenarios, intermittent contact between parts and fractures and failures - all high speed events with large deformations. Its complex material database include thermo-plastics, human tissue and even soil composites. Simulia can also handle extremely large models, using distributed memory parallel processing, cutting simulation processes down by significant margins.

Now, as Simulia, the software comes with two important management tools - Isight and Simulation Lifecycle Management.

ISIGHT

To optimise the design of a product, many different design alternatives have to be tested, not only to find out which work best, but also to enable engineers to understand which parameters drive them towards their targets. Keeping track of all of the complex simulations is difficult, with numerous alternative scenarios under analysis.

Much of the processes can be automated, and themselves optimised, using Isight, basically an advanced job handling tool, which enables engineers to execute multiple simulation studies automatically, and evaluate multiple design options in one step. And, in the case of multi-core processors, or processor farms, it can be used to distribute different parts of a job to the different processing streams. Engineers can assign variations in simulation attributes, such as materials, loads, tolerances and operating conditions.

SIMULATION LIFECYCLE MANAGEMENT

The similarity of the name to PLM, or Product Lifecycle management, is no coincidence. SLM does for the simulation environment what PLM does for the product design and manufacturing environment. It enables all simulations, and also the people involved in them, to

manage and share the data, methods and processes, connecting them to each other, and also to the enterprise.

Previously, the anlaytical engineer, once he had his hands on the digital model requiring analysis, could, as it were, retire to his garden shed to work on his arcane and complex analyses, feeding the results back to the design engineers, and other parts of the enterprise PLM, so that they could be re-integrated into the design.

Now, however, SLM can be used to capture the data, methods and processes used for each simulation, so that they can be titled, filed and re-used for future similar simulations - leveraging the simulation intellectual property and decisions arising from them, to speed up future analyses.

An essential part of SLM, particularly as it also has to handle a number of different FEA applications in its multiphysical simulations, and their results, is its Open Scientific platform, which handles workflow

chaining and job submissions and the connector architecture for third-party applications. Part of its function is also to provide efficient web access!

SIMULATION WORKFLOWS

This is complex stuff, requiring engineers and analysts with advanced analytical skills (this is reflected in Simulia's own workforce, 66% of which have PhD's or Master's Degrees, coupled with a large R&D department). And we can see this at work in the simplest of examples - the blow- moulding of a plastic bottle.

The manufacture of the bottle involves highly non-linear, large deformation analysis with temperature dependent material properties. It is subject to conveying loads and stability tests in the manufacturing process, thermal loading for retort, filling and sealing, and package performance testing, which involves grip stiffness, pressurisation, top-load (for

stackability), drop testing and - when we finally get our grubby hands on it - opening!

Medicine is the field that seems to be benefitting the most from simulation at the moment, through bio-mechanical, or biomedical simulations. This is because we are at that happy crossroads where need and capability have coincided. Not only have we devised techniques for performing operations and building prosthetic and other mechanical devices for the human body that were inconceivable a mere decade ago, but we have developed software that can safely simulate such intrusive operations before we even approach a living body!

That is because we can now model human tissue and bone, and calculate the stresses that we are imposing on it - which brings us back to the start of this article. Perhaps talking about bed sores needn’t be such a sore point after all! www.simulia.com

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