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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Rheology is the study of the flow or deformation

of materials

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