Dear engineers, scientists and experts in the field of plasticity,
In the literature, the terms strain hardening and work hardening are often used under the theory of istropic hardening. Mostly these terms are used in the same context or even equated.
Is that always correct, even using Drucker-Prager flow rule?
Strain hardening and work hardening are just slightly different terms which mention the same physical phenomenon of increase of material's yield strength (and hardness) through cold plastic deformation. The term "cold" means any temperature lower than that, at which dislocations (twins, APB or other strengthening agents) disappear (annihilate).
Drucker-Prager equation is just a phenomenological description of complicated atomistic-scaled processes caused strengthening, for example, dislocation multiplication. I do not see any problem to apply this approach to strain hardening phenomena.
Thank you very much for your answer Alexander Katz-Demyanetz .
You mentioned "just slightly different terms". My question is aimed precisely at this difference. Is there one? If so, where does it appear?
As I know, the term “work hardening” or “cold work“ is an “industrial” one, namely coming to emphasize an outer work done on the workpiece to form a certain desired shape.
The term “strain hardening” emphasizes the interest to the process occurring in the sample, namely: how strains field in the crystal lattice result in dislocation generation, movement and stopping by possible atomistic mechanisms (pinning by precipitates or slowing by Cottrell atmospheres, etc.).
Despite of these etymological differences, the physical meaning of these therms is same.
Plastic deformation by mechanical working below re crystallization temperature is termed cold working (forging). This mechanical work leads to movement of dislocations and more of it leads to increase in dislocation density and these dislocations start piling at grain boundaries.This leads to increase in hardness and strength (also brittleness) as further plastic deformation require additional mechanical effort.
I guess no difference as a phenomena but the work herdening is the area under the stress-strain curve at the strain hardening status.
Thank you very much for your answer Mr. Meryl M. Hall, Jr..
Maybe that's the hint I was looking for, but I can't be sure until I've studied your comment in detail.
Strain hardening and Work Hardening mean almost the same. The difference is in the parameter used as a measure to be used in the hardening rule. In strain hardening, accumulated plastic strain is the parameter used in hardening rule, while in work hardening, it is the dissipated plastic work. As far as I remember, in an associated flow rule (plastic potential is the same as yield function), both these would give identically same results.
For me it depends on point of view. If I consider relation between stress and stress during tensile test I would use "strain hardening" term (you see that stress increases with strains usually according to Ramberg-Osgood equation) but if I consider the area under the stress-strain curve which is energy or work consumed by unit volume I rather use "work hardening" term.
Although the explanation of Mr Hall nicely illustrates some 'finesses' on terminology, I do not completely agree. What he is referring to under the term 'strain hardening' is actually based on the term 'strain hardening exponent' commonly used in material strength models, but the term strain hardening is also used in models not using the typical strain hardening exponent formulation. For me, strain hardening and work hardening mean the same thing, although the former probably refers more to the microstructural level, whereas the latter more to the macroscopic level (where 'work' is done to cause 'strain' inside the material). As far as I know, there is no formal definition of neither work nor strain hardening that would legitimate a difference (and both terms are used as synonyms in the relevant literature). However, when talking with people dealing with actual industrial metallurgical processes (e.g. rolling) one must be careful, as they sometimes tend to put all thermo-mechanical hardening under the name of work hardening (so including heating and cooling processes).
Work hardening, also known as strain hardening, is the strengthening of a metal or polymer by plastic deformation. This strengthening occurs because of dislocation movements and dislocation generation within the crystal structure of the material. Many non-brittle metals with a reasonably high melting point as well as several polymers can be strengthened in this fashion. Alloys not amenable to heat treatment, including low-carbon steel, are often work-hardened. Some materials cannot be work-hardened at low temperatures, such as indium, however others can only be strengthened via work hardening, such as pure copper and aluminum.
Work hardening may be desirable or undesirable depending on the context. An example of undesirable work hardening is during machining when early passes of a cutter inadvertently work-harden the workpiece surface, causing damage to the cutter during the later passes. Certain alloys are more prone to this than others; superalloys such as Inconel require machining strategies that take it into account. An example of desirable work hardening is that which occurs in metalworking processes that intentionally induce plastic deformation to exact a shape change. These processes are known as cold working or cold forming processes. They are characterized by shaping the workpiece at a temperature below its recrystallization temperature, usually at ambient temperature. Cold forming techniques are usually classified into four major groups: squeezing, bending, drawing, and shearing. Applications include the heading of bolts and cap screws and the finishing of cold rolled steel. In cold forming, metal is formed at high speed and high pressure using tool steel or carbide dies. The cold working of the metal increases the hardness, yield strength, and tensile strength
The attached file contains some additional thoughts on this question based on a limited search of the literature.
As mentioned above, strain hardening and work hardening means the same. The term "Work hardening" often used in small scale industries or by gold smith while, "strain hardening" widely used in the research community.
Hi.Work hardening and strain hardening are not the same in plasticity , unless Von Mises yield criterion or Prandtl-Reuss equation are used as mention completely in the fifth chapter of Akhtar S. Khan plasticity text. There are 2 ways to distinguish between these two conditions.First, from analysis the test( like tensile test and behaviour of the curves).Second, analysis yield criterion.
Hi
In my view both are same only. In strain hardening we introduce more dislocations via cold working or hot working. so i do feel both same
Hi
Work hardening is a general term that includes hardening of solid crystals,i.e., by increasing the dislocation density in the crystal structures, by plastic deformation or by heat-treatment. In contrast, strain hardening is more specifically related to hardening by heat-treatment.
In working of alloys (pressure shaping of alloys), the increase in microstructural features such as dislocation density and point defects is responsible for the work hardening observed. Strain is assessed as displacement per unit length (mm/mm) in a typical tensile test as a consequence of applied stress. We also noticed increasing strength (strain hardening) of the stressed alloy until the ultimate tensile strength is attained due to increase in dislocation density and point defects. The same scenario applies, hence there is no significant difference.
Strain hardening occurs due to repetitive bending that induces material fractures to increase whereas stress hardening is compression of material to increase surface contact between molecular or crystalline structures of the material to increase internal structural friction.
hi
Except etymological differences, the physical meaning of these therms is same.
thanks & regards,
g.sudhakar
phd(materials engineering)
hcu.
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