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I am going through a literature where these terms are regularly used and I am confused are they different and if yes then what does each term mean? I also want to know relation of plastic strength of a rock (say granite) with pressure. I mean which equation describes this relation?

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  • $\begingroup$ Is this geological engineering or materials engineering? You are asking specifically about a rock so I assume the former, but there is no relevant tag. $\endgroup$
    – do-the-thing-please
    Commented Oct 18, 2016 at 12:20
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    $\begingroup$ I'm voting to close this question as off-topic because it is probably better suited for Earth Science. $\endgroup$
    – Chuck
    Commented Oct 18, 2016 at 14:43
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    $\begingroup$ That said, I'd imagine that the terms are coopted from the material science meanings - plastic yielding is deformation that doesn't "recover" when the stress is removed, and I'd guess the other terms are all synonymous and refer to the point at which the stress in the material is high enough that the material begins plastic yielding. $\endgroup$
    – Chuck
    Commented Oct 18, 2016 at 14:45
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    $\begingroup$ @Chuck All these terms have their origin from engineering. $\endgroup$
    – Ather Cheema
    Commented Oct 18, 2016 at 14:47
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    $\begingroup$ This question does not belong in Earth Science. Remove the references to granite & rock & it is about material properties, which is a function of engineering. Concrete can be regarded as an artificial rock/conglomerate. Besides, geological or geotechnical engineering are fields of engineering!Hence this question is still on topic here. I'm voting to keep this question open. $\endgroup$
    – Fred
    Commented Oct 19, 2016 at 11:45

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All these terms refer to the effect of loading on the deformation of materials. Let us assume that we start with zero load and zero deformation.

Elastic deformation

If you increase the load you get an increase in deformation. During the process of elastic deformation, if you decrease the load to zero you will not have any residual non-zero deformation.

Plastic yielding

The process during which the material deforms to such an extent that when the load is removed the deformation does not return to its initial state. The material appears to flow like a fluid.

Plastic yield strength

The load beyond which elastic deformations cease to occur.

Plastic strength

This term is not specific enough and typically not used when technical accuracy is desired.

Plastic stress

This is another term that does not mean much and should be avoided. An alternative is the yield stress which means the same thing as plastic yield strength

Relation between yield stress and pressure

The term pressure can have multiple meanings depending on the context.

If you are thinking of a load applied to the surface of a rock sample, there is no straightforward way of relating the yield stress to the applied load. There are numerous models for rocks and other granular material. See https://en.wikipedia.org/wiki/Yield_surface.

On the other hand if you are thinking of the change in hydrostatic yield stress with plastic strain, crush curves are typically used to model the evolution of caps on yield surfaces. See http://www.iaea.org/inis/collection/NCLCollectionStore/_Public/15/061/15061729.pdf#page=43 for an example.

Rate-dependent plasticity If you load a rock sample in uniaxial strain compression at various speeds, the load at which it yields will appear to increase with the speed of loading. If you define the yield stress based on a low speed (quasistatic) test, the stress will appear to exceed the quasistatic yield stress.

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