# Difference between 'plastic yielding' & plastic strength?

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1. Oct 18, 2016

### Atr cheema

I am going through a literature where the terms like ( 'plastic yielding' , 'plastic yield strength', plastic strength' and 'plastic stress') are regularly used and I am confused. Are they different and if yes then how each term differs from the other? I also want to know relation of plastic strength of a rock (say granite) with pressure. I mean which equation describes this relation?

2. Oct 18, 2016

### jack action

All of these are based on stress-strain curves. The stress definition is the same as the one for pressure ($\frac{Force}{Area}$), so they have the same unit.

«Strength» is a value of stress representing a property for a given material:
• Yield strength: Stress at which a material begins to deform plastically when elongated;
• Tensile strength: Maximum stress that a material can withstand in elongation (syn.: ultimate strength);
• Compressive strength: Maximum stress that a material can withstand in compression;
• Shear yield strength: Same as yield strength, but for torsion;
• Ultimate shear strength: Same as tensile strength, but for torsion.
Saying 'yield stress' or 'yield strength' refer to the same thing, although the last one should be more appropriate.

Although I never really heard anyone use 'plastic' in engineering materials, it should refer to 'plastic deformation' which makes more sense with ductile materials. Ductile materials deform elastically at low stresses, meaning they go back to their original shape once the stress is removed. At higher stresses, they deform plastically, meaning they are permanently deformed, even when the stress is removed. The stress value that separates the two zones is the yield strength.

For brittle materials (like granite), there is no plastic deformation: It deforms elastically until it suddenly breaks.

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