With a metal that can be work hardened, is it possible to have metal fatigue/failure without hardening happening? Is one always indicative of the other?
03-23-2017, 06:25 AM (This post was last modified: 03-23-2017, 06:32 AM by Jan.)
(03-20-2017, 02:54 PM)grepper Wrote: With a metal that can be work hardened, is it possible to have metal fatigue/failure without hardening happening? Is one always indicative of the other?
Is this a sensible question?
It is definitely a sensible question!
In my understanding the metal fatigue is conditioned by prior repeated plastic deformations of the material. If the plastic deformations were fast enough they caused that the metal was work hardened. If the plastic deformations were slow then the material was probably not hardened because the newly produced dislocations could have enough time to dissipate in the material.
The so called Deborah number tells us if the deformation is fast or slow for the material under consideration and given observation time.
I don't know how applicable this, however, when a woodworker's hand scraper is sharpened, the burr is added with a burnisher. After the burr becomes dull, the old burr is filed away, being considered as fatigued metal.
I once read or saw where someone filed away a knife edge to start again with unfatigued metal.
This is an interesting topic; thanks for starting it.
If I understand that correctly, to maintain a constant Deborah number with a piece of hot or cold steel, the rate of deformation would have to be much slower for the piece of cold steel compared to that of the the hot steel. Correct?
Is it true that even though steel is not exactly what I would consider a fluid, apparently it can actually relax enough over some period of time that fatigue could occur without hardening?
Practically, what period of deformation are we talking about with room temperature stainless steel to induce hardening? 1 per second? Five/second? 1/month?
(03-20-2017, 02:54 PM)grepper Wrote: With a metal that can be work hardened, is it possible to have metal fatigue/failure without hardening happening? Is one always indicative of the other?
Is this a sensible question?
There aren't any nonsensical questions on this site!
However, I may not always understand the question.
I understand wood scrapers plenty well, but I've never had any issues with them. I probably burnish the hook half a dozen times or more before truing the edges, and I've never seen a chip.
It's pretty rare to hear about work hardened knife steel too, and I can't say I've witnessed it. What type of application are you wondering about? Are you wondering about a specific steel or instance?
Jan, Welcome to the forum, it's great to see you jump in with your knowledge here! I've never heard of a Deborah number. From a brief search I could see that knowledge might be beyond my pay grade.
(03-23-2017, 11:24 AM)Mark Reich Wrote: What type of application are you wondering about? Are you wondering about a specific steel or instance?
While it may be more intellectually interesting than of practical consequence, or not, I don't know, but if you bend a burr back and forth during stropping to remove the burr, does that impart hardening to the edge of the blade?
Obviously the burr fatigues and fractures, but is that plastic deformation rapid enough to actually harden the edge? Hence my previous question, "Practically, what period of deformation are we talking about with room temperature stainless steel to induce hardening? 1 per second? Five/second? 1/month?"
If hardening does occur, to what depth? Is it significant enough to improve edge retention?
03-23-2017, 02:23 PM (This post was last modified: 03-23-2017, 03:00 PM by Jan.)
(03-23-2017, 09:40 AM)grepper Wrote: 1) If I understand that correctly, to maintain a constant Deborah number with a piece of hot or cold steel, the rate of deformation would have to be much slower for the piece of cold steel compared to that of the the hot steel. Correct?
2) Is it true that even though steel is not exactly what I would consider a fluid, apparently it can actually relax enough over some period of time that fatigue could occur without hardening?
3) Practically, what period of deformation are we talking about with room temperature stainless steel to induce hardening? 1 per second? Five/second? 1/month?
Ad 1) Yes, Grepper, you are correct!
Ad 3) This question is too difficult for me because I do not have the relaxation time data for stainless steels. Sorry for that!
A frequently quoted example to illustrate this behaviour is the children’s toy “Silly Putty” which is based on silicone polymers.
Pulled rapidly it fractures like a solid while pulled slowly it flows like a liquid. The relaxation time of this polymer is circa 1 s. If rapidly means during 0.1 s, then the Deborah number is De = 1 s / 0.1 s = 10, what classifies the material as solid-like. On the other hand, when slowly means during 10 s, then the Deborah number is De = 1 s / 10 s = 0.1 what classifies the material as liquid-like.
We have very striking situation when our material classification depends on the experimental time! I am attaching an after diner talk by the inventor of the Deborah number.
(03-23-2017, 11:24 AM)Mark Reich Wrote: Jan, Welcome to the forum, it's great to see you jump in with your knowledge here! I've never heard of a Deborah number. From a brief search I could see that knowledge might be beyond my pay grade.
Thanks for your welcome, Mark!
The striking, but universal, concept currently discussed here is successfully applied in Earth Sciences – e.g. seismic hazard calculations. I have only tried to apply it to steels what is not quite common. The usage of the Deborah name was inspired by the Bible verse "The mountains flowed before the Lord" (Judges 5:5). The basic idea is that everything flows, even the mountains, if you wait long enough!
I found this discussion quite interesting. I have not heard of a Deborah number and will have to research it further. Is it something that applies to geology/seismic movement? I work with several geologists and could ask them about it.
In terms of load speed, it is quite critical to how materials deform/fracture. Lay some silly putty on a clean anvil and hit it with an 8lb hammer. I've never tried it, but I have it on good authority that the results are not what would be expected, if you can still find silly putty. Our lab breaks concrete test cylinders regularly, and load rate in compression must be kept within certain limits or the results are invalid.
To the original question, Mr. Grepper, you have asked something that is not as straight forward as you might imagine. The answer is it depends on what kind of fatigue you mean; high cycle fatigue or low cycle fatigue. I would not say fatigue is always indicative of hardening, nor hardening always indicative of fatigue.
"With a metal that can be work hardened, is it possible to have metal fatigue/failure without hardening happening? Is one always indicative of the other?
Is this a sensible question?"
Yes, it is sensible. With high cycle fatigue, you have formation and propagation of cracks with no visible plastic deformation or hardening. However, at the tips of the cracks, there is some plastic deformation and therefore work hardening, though the amount is tiny, and deformation actually helps blunt the crack (increase the radius of the crack tip and lower stress) and slow it down. However, quite ductile materials fail by this mechanism virtually without warning at macroscopic stresses significantly less than the yield strength of the material. That is why it's such a big deal and prevention is key. My supposition is high cycle fatigue is rarely seen in burr removal, as it generally requires a LOT of cycles, thousands or even millions.
With low cycle fatigue, there is considerable plastic deformation with each cycle, though not always visible. Plastic deformation is required for work hardening to happen. This means the yield strength is being exceeded, in this case it's exceeded each half cycle. How much hardening happens depends on the condition of the material and how much deformation happens. Hardened steel has relatively low ductility, so I'm not sure how much deformation and work hardening can actually happen.