Does hardness shows any relationship with % Elongations also (for a
particular composition)? I wanted to know if someone wants to plot a
curve between hardness and other meachanical properties (such as %
Elongation, YS,UTS) is it possible?
Michael Dahms wrote:
Yes, anyone can plot one property aginst another, so it is possible to
plot hardness vs "whatever you like". Many people have done this from
time to time.
An excellent example of intelligent cross plotting of data of materials
is by Prof. M. F. Ashby... "Materials Selection in Mechanical Design"
You can buy it at Amazon.com or go to a good technical library and look
at his dozens of examples of intelligent cross plotting of materials
If you are afraid of taking the time to do the work.... maybe you can
find it already done in standard reference works.
Then, who can tell what hardness mean? That is, what is the physical
meaning of hardness? Is hardness a index of materials against (plastic)
deformation? Unlike the yield strength, the elastic modulus which are
the fundmental properties of materials, hardness is only the "surface"
property of materials. The question is how to relate hardness with
those fundmental properties?
"Hardness" is the result of an experiment....
There are many ways to measure "hardness" and it has long been known
that there is not necessarily a relatinship between them.
Youngsters often demand that "Hardness" really be "SOMETHING".
Real reading in good books will often mitigate that illness.
Those who don't read will never be satisfied, confident that if they
persist, they will eventually discover the "Truth".
"Is hardness a index of materials against (plastic)
Yes, and different indentation hardness tests impose different
penetration geometries so give different results.
"Unlike the yield strength, the elastic modulus which are
the fundmental properties of materials,..... The question is how to
relate hardness with
those fundmental properties? "
There is no particular reason they should be closely related. Elastic
modulii are based on extremely small, recoverable deformations while
indentation hardnesses are based on non-recoverable macroscopic
deformations. These are completely opposite ends of a deformaton
Consider the effect of heat treatment on a carbon steel. The elastic
modulii don't change very much but hardness can be changed
Probably the most that can be said regarding the relationship between
modulus and hardness is something like a material's maximum possible
hardness is proportional to its shear modulus - unlike the minimum
hardness which is probably bounded by zero.
There is no logical relationship between a crystalline material's bulk
modulus and hardness; When a crystalline material "yields" under
hydrostatic loading it does so by changing to a different crystalline
structure - it ceases to exist.
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