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36 : Computational mechanics for engineers

Incontournable, la mécanique numérique s’insère aujourd’hui dans l’ensemble de la chaîne de conception rapide des produits fabriqués par l’industrie. S’appuyant sur les outils de modélisation géométrique et de visualisation, et intégrant les outils de simulation et d’optimisation, elle réduit les délais de conception, limite les erreurs et s’insère dans l’esprit du développement durable en aidant à concevoir des produits de plus en plus respectueux de l’environnement.

36 : Computational mechanics for engineers

Industrial die-stamping, between model democracy and expertise

Die-stamping of car parts at Renault factories follows suit to a series of digital design steps that run from initial design steps to final testing and certification. While some of these steps are standardized, and can be managed by less and less qualified operatives, more complex operations call for the know-how and knowledge of specialist engineers.

"Modelling, in a sense, is make-belief convincing oneself that the work at hand is "for real" and then trying to reproduce the same effects in reality", says Frédéric Mercier, research engineer with Renault, posted to the die-stamping department. This stamping operation consists of bending (deforming and shaping) a plane steel surface to obtain a 3D structure which, consequently in most cases, is no longer planar.

The geometric shaping involved is not also self-evident and, moreover, the part must comply with a series of use requirements, such as those compliant with crash-rests. "A good model must, therefore, be close to reality", explains Frédéric Mercier. The design chain for a car parts integrates the part design, the calculations needed to die-stamp them, modelling of shock characteristics. The number of full-scale tests is significantly limited, inasmuch as most of them are included in the modelling process.

Easier commonplace operations

What the engineers does is to model the car using the so-called "thin shell" finite elements method, i.e., adapted to thin structures. This kind of structure accounts for some 80% of all commonplace products, with simplified techniques and tools so that they can be implemented by operatives after a short specialist training session.

"Use of these tools has become 'democratic' and the personnel with higher level diplomas and degrees such as the PhDs and the engineers can devote more of their time to doing more complex tasks" adds Frédéric Mercier.

Optimisation is for the experts

One such complex task is optimization, which has the objective to identify the best configurations for a given list of constraints. For example, if the target is to reduce CO2 emissions, one improvement consists of decreasing the weight pf the vehicle while preserving its crash resistance and acoustic factors. Since the last mentioned criteria are antagonistic in terms of steel sheet thickness and rigidity, the optimization process consists of finding several possible solutions.

Then other arbitration criteria come into play to make the choice among these solutions, factors such as cost or ease of production of the part. The optimization calculations are often more complex to carry out when the shock resistance constraints are taken into account. It then becomes necessary to find the right models and the right tools in order to avoid excessive computer time. "It is not at all reasonable to launch a calculation process and a computer run that will last for more than 50 hours!" details Frédéric Mercier.

Adapting to special cases

Optimization does call for some precautions but this is partly due to the fact the standards applicable are multiplying, are different according to the market country, they relate mainly to safety factors, performance levels and the environment. "For shock situations, over a dozen were modelled", Frédéric Mercier, citing the pedestrian, frontal, and car side shocks ....

The needs for expertise are not just limited to optimisation programmes but relate also to processes such as the identification of digital criteria to detect possible "aspect" faults or to use of composites. Composites tend to disintegrate under shock, a transformation of the material's original features. "The software packages we use are not suitable for this kind of crash behaviour, even if it is possible to set up ways to take this into account", underlines Frédéric Mercier.

In respect to potential improvements, Frédéric Mercier also defends some personal thoughts. He expressed, for instance, the wish that software designers make more 'user-friendly', more modern interfaces. "For example, they could draw some inspiration from Apple and propose haptic tools", suggests Frédéric, who dreams of "digital model programmes for smartphones® and i-pads®".

On a more serious vein, another improvement path would be in development of models that comply the most closely possible with the laws of physics. And, even if the processing power of computer has risen tenfold and more, the issue of reducing computer times and runs is still a topical debate.