(picture courtesy of www.digitalefolien.de)
If you study the internal structure inside the head of a human femur, cutaway above, you can see the variations of the internal structure. The structure is very dense inside the surface of the ball, where most of the stress would be from contact with socket. The fibers of the structure get long as it delivers the load to the long part of the bone. The structure is sparse in the lower left of the ball, where there would be little stress.
You could not have designed it any better. Because you don’t have the software tools.
Image from Autodesk’s Netfabb product page shows and internal structure that is of uniform size and shape in all three directions.
So as we are used to making parts solid, but then asked to make them lighter and/or use material, we should consider an internal structure. CAD programs, if they offer that option, will give you a few choices of shapes and structures. They all seem to use the 3D lattice approach, which means geometrically perfect, made of uniformly sized cells or members and extending linearly and unwaveringly in 3 dimensions.
Not at all like our femur.
Wouldn’t it be nice if the design software would have enough insight to “grow” the internal structure, varying the cell size for example, decreasing it when stresses were high, increasing it when stresses are low? And, while we are at it, can we position the structure so that it lines up along the stress path? Once it is done, it can be subjected to another load case, which adds material to compensate for a stress flow – without taking away from the first solution. Each successive load case builds up the part so, in the end, it has grown up exactly as needed. It is no more and no less of the part it needs to be.
In the world of engineering and design software, we are increasingly seeing “upfront optimization.” Programs such as Dassault System’s Tosca and Hyperworks Optistruct do exactly this – but only for solid parts. I’ve not seen anything that can optimize shells with internal structures.
Should any vendor be able to create an internal structure that can accommodate various load cases with an adaptive internal structure, it would certainly have our vote for software of the year.
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