I think I found a counterexample to the common wisdom that more walls always create a stronger part.
The pictured S shape is 1.5mm thick, so printing with 2 walls leaves no room for infill. My testing wasn’t very rigorous, but it seems that the hybrid structure of walls + rectilinear infill is 10-20% more rigid than walls alone. The infill adds strength by cris-crossing between adjacent layers.
I think it’s fine to include a concentric top/bottom layer, but multiple identical layers weaken the part. I also tried 0 walls (infill only) and that was garbage.
No, that shouldn’t be obvious. First of all, the reason a bundle of hair is stronger and stiffer than a rod of equivalent area and material, is not necessarily applicable at the scale of 3D printing layers (there should be an effect, but it gets much more prevalent in smaller scales). Then second, your example with the CFRP Sandwich Panel is not really correct. A full thickness CFRP plate is going to be stiffer and stronger than a Sandwich panel. Will it be much heavier and much more expensive? Yes! Will it be stiffer and stronger? Yes, a little bit. Only specific properties (so, per kg or per dollar) are improved (and by significant amount). Also the core has to withstand out of plane shear in addition to compression and the bending stiffness is highly dependent on this shear stiffness. My assumption for OPs effect is, that the criss-cross pattern is stiffer than the interlaminar shear stiffness of the individual 3D printed layers (but thats just a guess, could also be a quality aspect). This is not true for CFRP Sandwiches. CFRP interlaminar shear stiffness is much higher than of the core material (at least in general). Sorry, don’t want to come across as an ass :)
Everything you say is correct and, to me, obvious and was never disputed.
I think the hair analogy fits this case, obviously it is not 100 % the same.