Question regarding flexibility

Yes it is. Knives ment to bend 90* are for the ABS test, hard edge and soft spine.

 

My understanding as a wannabe material scientist. Take with a grain of salt.

If you have a sword (say 48"), the handle can be bent 90 degrees relative to the tip and it can return to true (elastic deformation).
The stress/force is distributed over the length. As it isn't a sharp 90 degree but a radius, it can 'add up' to a 90 degree bend without too much localized stress.

With a razor (4" length), there's pretty much no way it will be able to do the same. It will encounter the yield strength and inelastic deformation will occur.

If we isolate the thickness as another variable, a piece of 4" length .0001" thickness foil can bend 180 degrees and return to true.

So length and thickness both influence what is possible, since the inside and outside of the bend are encountering opposite tensile forces of compression and tension. Thinner means less tensile force.

The hardness of the blade influences what happens when the yield strength is encountered. A largely soft or unhardened blade is likely to bend and take a set. A largely hardened blade is more likely to snap. But something needs to give to deal with that force.

 

A knife 1/8" which is fully hardened and tempered is likely not going to make it 90°. That doesn't change if it's been forged.

 

From what I have read I believe the ability to bend is more a function of geometry/cross section/taper rather than hardness. Think fillet knife @ 63 HRC.

 

if its fully hardened, i think you did well with the near 40 degrees !

 

The ABS flex/bend test is to show you have control of heat(soft spine, hard edge). You can make a knife that will pass the 90deg test and return straight but why risk it if it's not a requirement? IMO, it is not a measure of a knife's quality to flex/bend 90degs.

 

The length to thickness ratio is pretty important too. In my experience some 10 inch choppers that are 3/16 thick (fully hardened HRC 60-61) will bend 90 degrees and spring most of the way back straight but thicker or shorter knives can't quite bend that far.

 

You can see the equations for calculating surface stress for a simple rectangular cross section here: https://knifesteelnerds.com/2018/03/13/why-doesnt-heat-treating-affect-steel-flex/

If you solve for deflection instead of force you can see that the stress is divided by the length squared for a given deflection. So length is important. The stress increases linearly with thickness for a given deflection. The knife breaks when the stress has exceeded the ultimate strength of the steel.