Perhaps we should begin by considering what geometrical attributes of a knife provide:
Thinner geometries provide greater penetration and cutting capacity, and are also lighter in hand.
Blade spine-width (thickness) provides structural support against
lateral (side-to-side) forces. This is also true down the cross-section of the knife from spine to edge. A thick edge provides structural support against lateral forces, preventing the edge from bending/twisting out of alignment. However, the thick edge (and spine) require more force when pushing (cutting, then wedging) material apart.
The amount of structural support provided is a calculable value, directly proportional to measured thickness and hardness of the material (
until you reach the microscopic level of carbide-matrix structure). Bending-strength is a property of ultimate tensile and compression strengths, informed by the ductile-brittle transition. What this means is that a harder blade will resist bending better than a softer blade, and so can be made thinner. However, the thicker blade, so resistant to bending, may fracture quite suddenly before displaying much elastic deformation compared to a thinner blade. A thin blade, even a hard thin blade, will present much more elastic deformation prior to fracture. For this reason, a fillet knife can be made very hard and yet still display a high level of flexibility, but it will also fracture sooner as applied force increases. A softer blade will resist fracture longer than the hard blade, but it will take a bend (plastic deformation) sooner.
Keep in mind that this deformation is not only important along the entire width of the knife but also just at the edge, depending on the material being cut.
When considering the thickness of a blade, take into account the expected or needed lateral leverage capacity. Keep in mind that the longer the blade, the more leverage able to be applied from the handle along the blade that could bend it.
Scalpels, box-cutters, fillet-knives, and thin-machetes are not intended for high lateral-stress tasks. They can be made very thin. The first two also needn't be long, and should be harder to maximize edge retention via carbides (up to the point of brittle failure, which would be very bad in the case of surgical scalpels). Fillet knives tend to be longer, but again increased hardness reduces edge deformation until brittle failure occurs. Machetes tend to be MUCH longer and also subject to impacts forces, so they are usually MUCH softer to improve toughness. As a side-effect, the soft machete edge dulls and deforms relatively quickly, but can be restored more easily as well.
Wood-carving implements must encounter the lateral-stress applied by wandering grains of wood. A hard edge, as mentioned before, resists the forces better than a soft edge, and so can be made thinner, but as those forces increase, the hard edge can fracture if it isn't sufficiently thick, particularly on deep cuts. For this reason, wood-carving blades for deep cuts (knives & chisels) tend to have thicker edges (high edge-angle) but thin stock. This is the standard for puukkos as well. The Mora blade stock is quite thin to be light and cheap, but the scandi-grind is at a very high angle to provide more rigidity at the edge for carving wood, etc. The thicker Mora-Robust, 1/8", would benefit from a full flat grind with a secondary edge bevel rather than the narrow scandi-grind. The Mora LMF knife provides a thinner "relief" grind for improved slicing at the tip, leaving the lower portion of the blade for carving (image from rockymountainbushcraft):
Enough investigation. Personal opinion: For longer/larger knives which may double as pry-bar/shovel/hatchet, I prefer a thicker blade but <1/4" and a wide enough bevel to accommodate 4-5 dps primary-grind down to <0.030" at the should of a 1/16"-wide edge bevel, allowing ~15-dps apex. Many would call this
robust. For medium blades (~4" + 4-5" handle), I can still generate enough lateral force that ~1/8" is beneficial, but the apex angle and edge thickness should be reduced - this isn't an impact tool or shovel, etc. For small blades (<3"), I tend to keep the 1/8" spine but not for lateral support, now it is just to have a reasonably wide surface for comfortably applying force with my thumb
For this reason I prefer not to go <1/8" except on scalpels & razor-blades, instead simply modifying the grind from flat to hollow. Also, all of these knives will have a taper somewhere along the blade, these thicknesses are measured just above the guard.
There, a rather long contribution, my apologies.
