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creep vs. plastic deformation

Can anyone offer a clarification of the difference between creep and plastic deformation? In various literature outside of continuum mechanics theory, the concepts of creep and plasticity seem to be used interchangeably, and thus likely erroneously.  The basic textbook definitions tell me that creep is a time-dependent, potentially irreversible deformation due to one of several microstructural mechanisms. Likewise, plasticity is a irreversible deformation above some yield stress, often atributed to fracture and slippage behaviour at the microstructural level, and time-independent. My question is two fold:

  1. Is time-dependent plastic deformation a subset of creep behaviour?  i.e. if a material is observed to creeping in a time dependent manner, but the mechanism is, for example, fiber slippage, can this be called plastic deformation?
  2. If the material is a polymer, but shows irreversible deformation above some yield stress, is this plasticity, or is this term restricted to crystalline solids?

Many thanks for the clarification or references leading to it.

 

Comments

I will put my understanding here as a reference, which may be not necessary right. The recent observation by Dr. Saif's group about 'recoverable plasticity' certainly complicates the definition of creep and plasticity.

(1) I would agree with you that time-dependent plastic deformation is one type of creep. Visco-elastic materials can also give rise to creep. Plastic deformation, however, can be either rate-dependent (viscous) or rate-independent. For the latter, you can think of plastic deformation in materials at 0K where rate-effect is completely suppressed. So there is no simple relationship between "creep" and "plastic deformation". Creep can happen at any stress (commonly seen at constant stress condition) but plasticity doesnot.

 (2) Yes. The majority of the society would call it plasticity for post-yielding strains in polymeric materials.

The term really doesnot matter as long as you know the exact physical process associated with the term.

 

Yujie Wei

 

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Thanks to both Yujie Wei and cafe4068 for your comments.  Some offline discussions with John Kolinski were also useful, and his comments are displayed here with his permission:

  • Plasticity is defined by a stress-strain relation with memory and
    irreversibility. Creep is plastic strain resulting from a constant
    load: think a beam deforming due to an attached weight over time.
  • Q: Re: creep, are the phrases "plastic deformation" and "creep" equivalent then? A: There are non-elastic plastics (e.g. plasticine, a clay-like material)
    that have essentially no elastic component to their stress-strain
    relationships.
  • Q: does plasticity also require the inclusion of a yield stress? A: Creep is encompassed by plastic
    deformation, but not visa-versa. Creep is a type of plastic deformation,
    whereby deformation occurs below the yield stress limit of the
    material. I would say creep is a narrower range of deformations
    belonging to the set of plastic deformations, which contains other
    members, e.g. super-yield stressed solids, which yield plastically
    above their yield-stress in a manner similar to, but distinct from the
    creep mechanism. Note that not all solids have a yield stress, as I say
    in the first paragraph; for these materials, creep is not defined.
  • Wikipedia's articles on Creep and Plastic deformation in solids
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