The document discusses various aspects of material hardness. It describes several methods for measuring hardness, including Vickers, Knoop, Brinell, and Rockwell tests. It explains that hardness is a material's resistance to plastic deformation and penetration. The document also discusses the relationship between hardness and yield strength, as well as elasticity versus plasticity. It provides examples of using hardness models, like the Maxwell model, to understand the behavior of elastic materials under loads.

1. Dr Mumtaz ul Islam
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Resistance of a material to wear
Resistance to penetration by an hard object
Value of hardness is usually expressed in numbers
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Vickers, Knoop, Brinell and Rockwell
Shore ,Mohs’and barcol
Vickers and Knoop both involve the use of diamond
pyramid indentors
Vickers hardness, the diamond pyramid has a square
base
Knoop hardness, one axis of the diamond pyramid is
much larger than the other
Brinell hardness test involves the use of a steel ball
indentor
Rockwell direct measurement of the depth of
penetration of a conical diamond indentor
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Measurement of degree of permanent deformation
Initially stress is very high
Strain produced
Elastic limit of the materials reaches
Yield strength of the material is actually the
indicator of hardness
If any elastic deformation occurs it is of no use
Hardness is the ability to resist scratching
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The yield stress is the value of stress beyond
which the material becomes permanently
distorted
Elastic property the ability of a material to
undergo elastic recovery
When a material undergoes full elastic
recovery immediately after removal of an
applied load it is elastic
If a degree of permanent deformation remains
or recovery is slow the material is said to be
viscoelastic
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Mix model is used in dentistry
It explains well the behavior of elastic impression
materials
Such materials show an instantaneous increase in
strain due to the spring (A) followed by a gradual
increase in strain as the dashpot (B) and
spring/dashpot (C/D)system open
On removal of the load the spring (A) recovers
instantaneously followed by gradual recovery of
the spring/dashpot (C/D). Some permanent
distortion remains as a consequence of the dashpot
(B)
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Creep involves a gradual increase in strain
under the influence of a constant applied load
similar to that which takes place in the
Maxwell model
Creep implies a relatively small deformation
produced by a relatively large stress over a
long period of time
Flow implies a greater deformation produced
more rapidly with a smaller applied stress
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18.  Rheology
is the study of the flow or deformation
of materials
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