“Flex” is one of the most commonly misunderstood topics in all of knifemaking. This is mostly because it is one of those things that your eyes are telling you one thing due to interpreting so many things at once, but when you separate out what is really happening the facts are an entirely different story. I am not saying that you may not understand the physics scherar but it is worth discussing for all those that may not know the facts.
When you pull a blade over in an arc there are several things involved- 1. how many degrees it deflects, 2. how much force it takes to deflect it that many degrees, 3. and what happens to the steel when it reaches that many degrees. You determine number 1, the thickness of the metal determines number 2, and heat treatment (hardness) can only affect number 3.
The yield point is where “flexing” ends and the metal bends. In flexing once you release the load the metal returns to true, in bending it doesn’t. It drives me crazy to see the number of makers who intentionally mislead the public by promoting their blades with outrageous “flex” tests when they are bending the darned things. The metals ultimate strength is determined by the point at which the blade breaks and this is always greater than the yield point. Think of the whole exercise of bending/flexing a blade as a line with breaking at the far end, and bending is a sliding point in the middle. All heat treatment does is slides the yield point (bend) up and down that line. The more you temper, the lower the yield point. The harder the steel, the higher the yield point as is gets closer to the ultimate strength. So a softer blade will bend at let’s say 30 degrees but not break. But a harder blade would not bend and continue to flex to 45 or 50 degrees and beyond but when it yields it will break.
In neither case can the heat treatment in any way effect the amount of force it takes to flex the blade to the yield point, that is an inherent property of the steel known as modulus of elasticity (or Young’s modulus) that is only affected by the amount or thickness of the metal. So if you are making springs and you want a weaker one, you grind it thinner, if you want a stiff spring you make it thicker, if you want a bent spring (not very useful) you make it softer. Before you start dismissing this, think about it – how do truck springs fail? Do they bend or do they break? How do you stiffen the suspension? You add more or thicker springs; you do not mess with the heat treatment at all.
Now the thickness of the metal can also affect the chances of yield or failure in a flex/bend by limiting the amount of tensile forces generated on the outside curve. Say your blade is 6” long and may fail at a 60 degree flex; this is because the thickness beyond the centerline transition from compressive to tensile action resisted enough to exceed the tensile strength of the steel for that length of blade. If you make the blade longer, or grind it thinner, you now will skew these numbers and the blade will be able to handle more degrees of flex without reaching the yield point or failing. This is why I always teach that the secret to beating the ABS performance test is as much in the grinding as it is in the heat treatment, and that somebody is going to get hurt someday with these terribly thick test blades.
Good springs don’t bend; they break, but only after extreme deformation. Good pry bars don’t do either and are thick to avoid even flexing. Good knives cut things and the physics that make the first two items work well can work against a good knife, if we don’t understand it all.