12-05-2017, 09:00 PM
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.
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.