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18 August 2017

Just because you can....

Just because you can.... Just because you can....
You knew that CADCAM had taken off in the ‘80s within automotive companies because cars that had once been angular gradually gave way to a new generation of more and more curvy models. They remain so today, although the curves these days are often now refined more scientifically with the help of computational fluid dynamics (CFD).

This is at the body shell end of things, of course. Under the curves, there are still straight line struts (compression) and ties (tension) in the case of load-bearing items such as tie-rods, swinging arms, safety cages and so on.

But we have now entered the era of curvy load-bearing structures. This is driven by additive manufacturing (AM) – which allows complex structures to be easily made – and, more latterly, topology optimisation and generative design CAD software. This latter process can deliver organic structures, spitting them out in the thousands for any one set of constraining design criteria, so that we end with car space frames or motorbike frames that are organic in nature. Topology optimisation delivers less extreme organic designs, but will still produce curved load-bearing elements. Computation time for both can be lengthy.

At the recent Manufacturing and Engineering exhibition conference (www.menortheast.co.uk) in Newcastle last month, Matthew Gilbert, professor, Centre for Advanced Additive Manufacturing, Sheffield University, challenged this ‘modern’ design approach, as regards load-bearing elements. He talked about layout optimisation to achieve space-efficient lightweight AM-produced structures, with an emphasis on the use of straight line struts and ties as the optimum. Asking a curved beam to take an axial load isn’t good mechanics.

Sheffield University spin-out company LimitState’s (www.limitstate.com) Form software can quickly generate load-bearing AM structures, and Professor Gilbert, who is associated with that company, gave some examples: the much talked about GE AM engine bracket competition of 2013 (https://is.gd/fovobo), where Form can generate a solution equivalent to the winner in seconds and a better one if longer is taken; Altair’s topology-optimised AM-produced satellite frame, where Form quickly delivers an optimised, less organic, less complex and better outcome; an AM-produced aircraft rear flap pivot bracket that topology optimisation reduced in weight by 40% versus Form’s 60%, with computation taking two minutes not 24 hours; and Autodesk’s/Airbus’ ‘bionic partition’ (https://is.gd/uhabig), a generatively-designed complex AM structure where a more optimal outcome, achieved faster would again be possible by using Form, which, incidentally, draws partly on a maths paper of 1904.

Curvy organic structures look sexy and modern, sure. But for optimum performance with minimum material use and envelope in load-bearing cases, straight lines are best (where other constraints do not negate that). And the processing power required to originate the latter is a desktop PC running for a few minutes, not high power computers taking several hours, days or even weeks. Basically, just because you can doesn’t mean you should.

Andrew Allcock

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