I think it would make servicing the wheel (e.g. for a flat) harder. Also, the skewer does not have as much of a cross section as many bolts.

So, first, pretty much all steels have similar stiffness, as defined by the modulus of elasticity. That is, how much does the steel deform (fractionally) per unit of stress applied.

But softness? I think that might probably relate to yield strength. That is, the amount of stress that can be applied before deformation takes place. That varies between alloys, and even between the same alloy in different states of heat treatment. Generally if you go to higher tensile strength you'll have higher yield strength to resist bending. And the stress that the bolt can take repeatedly without breaking (fatigue point) is higher too.

The right alloy, heat treated, will take more stress before deforming or breaking. SAE grade 8 bolts are alloy steel bolts heat treated to give a minimum of 150,000 psi tensile strength (proofed to 120,000). This is similar to metric grade 10.9. There's an even higher-strength standard for metric bolts, that is 12.9. That's supposed to be around 175,000psi tensile strength! A key point is that a lot of the generic fastener hardware you get at Home Depot is not made to these standards, and is made in China with leftover bean curd and rice husks. And even if the fastener has the right markings to indicate grade 8 or 10.9, it may not be.

Get SS bolts. Where did you get the idea that they were susceptible to brittle fracture?

I'm actually surprised that the hollow skewer-type axles hold up under all the weight and impacts like they do. I have had to replace a couple of bent ones, but never had one come in broken. Back in the day - when I was a kid with a paper route my rear carrier mounted to the rear axle. I would sling two heavy canvas newspaper bags over the rear carrier. No problem with the carrier mounts, but the heavy load would snap the axle inside the hub 2 or 3 times a summer. I kept a spare part on hand and got pretty good at rebuilding the coaster brake assembly.

For bending, there are engineering reports indicating the hollow tube is nearly as strong as a solid bar.

A lot more of the strength of the QR axle vs solid axle depends on the materials and heat treating.

I have broken one QR skewer. It was an older Campy NR skewer. I was towing a trailer (chainstay hitch). But, moving the trailer around a bike rack, somehow I caught the skewer clamp end, and twisted it while clamped tight, breaking the skewer.

Otherwise, so far no problems with the skewers, including some trailer towing using a skewer hitch.

I believe the strength of the hollow axles is comparable to solid axles because (a) in bending, the material near the outside diameter of the axle provides much more strength than the material closer to the inside, and (b) even if the hollow axle is slightly weaker, the skewer fills that space and adds the same amount of strength as the amount of metal missing from the axle. Unless you have exotic Ti or (*shudder*) aluminum skewers, which are both generally weaker materials than most steels for a part of the same geometry.

Maayyybe the QR axle lends a bit of support to the hollow axle if the latter bends enough, but I mostly think people overestimate the amount of strength/stiffness contributed by the "core" of a solid axle. It isn't much, otherwise we'd be riding bikes made out of solid rods rather than tubes. :D

To get a good idea of the strength of the steel cylinder absent from a hollow axle, look at a QR skewer - also made of steel, almost the exact same diameter of the hole through the axle. The steel alloys may be different, but the stiffness of all steel alloys are approximately the same, up to the point of 'plastic deformation' - weaker steels will permanently deform before stronger steels.

The strength of that part is such that a person of average strength can bend it with their bare hands. Like you said, people overestimate the strength in such a tiny rod of steel.

The skewer also loads the axle in compression, which has to be overcome to bend the axle. This compression is why bearings tighten up when the skewer is applied, the axle actually shortens under the compression, while the skewer stretches. Bolts also stretch when they are torqued; in some critical applications the bolt stretch is measured instead of implying it from the fastener torque.

And the original question was what? Smiles, MH

Back to near the OP's original question, rack mounting issues. Anytime one places a layer between surfaces that are clamped together the layer adds the chance of sloppy fit, less friction/grip between the two initial parts and any forces acting on that added layer can induce movement within the clamped assembly. So a rack mount trapped between the skewer and the drop out that has a long lever arm with a mass at it's end (the load on the rack's top) isn't my first choice for a solid and trouble free install. Andy