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Why creep is a function of Time?

by koolraj09
Tags: creep, function, time
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koolraj09
#1
Jan23-14, 08:43 AM
P: 144
Hi all,
As suggested by the title, i wanted to know as to why creep is a function of time. Since for any component the load and temperature are constant, still it creeps. What happens there at microscopic level? Why does the solid component continues to deform slowly under constant load and temperature?

Thanks
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Chestermiller
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Jan23-14, 06:32 PM
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Quote Quote by koolraj09 View Post
Hi all,
As suggested by the title, i wanted to know as to why creep is a function of time. Since for any component the load and temperature are constant, still it creeps. What happens there at microscopic level? Why does the solid component continues to deform slowly under constant load and temperature?

Thanks
The solid may not be fully crystalline, and may contain microscopic amorphous regions that are capable of deforming like fluids.
AlephZero
#3
Jan23-14, 07:26 PM
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There are several different mechanisms, depending on the material. http://en.wikipedia.org/wiki/Creep_%28deformation%29 will give you a summary, and some terms to google for more details.

koolraj09
#4
Jan24-14, 05:07 AM
P: 144
Why creep is a function of Time?

Thanks for the response.
I am not able to connect the deformation w.r.t time due to these mechanisms. Say for example, the diffusion creep. In this, the diffusion process is a function of temperature. At any particular temperature, the diffusion should occur and then stop. Why does it continue at constant temperature and thus make a material deform as a function of time? Similarly if we consider dislocation creep, we don't increase the stress, then what makes the dislocation to further move around? After we reach a particular load and temperature, these things have no reason (from my perspective, currently) to move and make the material deform continuously w.r.t time.
materials
#5
Jan24-14, 11:41 PM
P: 2
Quote Quote by koolraj09 View Post
Thanks for the response.
I am not able to connect the deformation w.r.t time due to these mechanisms. Say for example, the diffusion creep. In this, the diffusion process is a function of temperature. At any particular temperature, the diffusion should occur and then stop. Why does it continue at constant temperature and thus make a material deform as a function of time? Similarly if we consider dislocation creep, we don't increase the stress, then what makes the dislocation to further move around? After we reach a particular load and temperature, these things have no reason (from my perspective, currently) to move and make the material deform continuously w.r.t time.
Hi
Diffusion Creep involves the flow of vacancies and interstitials through a crystal under the influence of applied stress. A tensile stress increases the separation of atoms on grain boundaries that are normal to the stress axis, and the Poisson contraction decreases the separation of atoms on grain boundaries that are parallel to the stress axis. The result is a driving force for diffusional transport of atoms from grain boundaries parallel to the tensile stress to those normal to the tensile stress (there is a flow of vacancies in the opposite direction). Such diffusion produces a plastic elongation.

Diffusion not only needs temperature, but also needs time. As you may know the diffusion distance is proportional to √Dt, where D is the diffusion coefficient. In the diffusion creep, as I described above, atoms should travel the required distance to produce elongation.

Similarly, other creep mechanisms also need time. In fact, creep is an "anelastic effect".
koolraj09
#6
Jan27-14, 12:52 AM
P: 144
Hi materials,
Thanks for your response. I understood that diffusion is a function of time.
Secondly, atoms have to cross the activation energy to go to the vacancy site. But if the temperature and stress are uniform which is how the creep is defined, they why does an atom move to the vacancy?


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