Creep of materials ~ MECHTECH GURU

Creep of materials

Creep of materials

When a member is subjected to a constant load over a long period of time it undergoes a slow permanent deformation and this is termed as “creep”. This is dependent on temperature. Usually at elevated temperatures creep is high.
x The materials have its own different melting point; each will creep when the homologous
temperature > 0.5. Homologous temp =Testing temperature/ melting temperature> 0.5

A typical creep curve shows three distinct stages with different creep rates. After an initial rapid elongation  džo,  the  creep  rate  decrease  with  time until reaching the steady state.
1)    Primary creep is a period of transient creep. The creep resistance of the material increases due to material deformation.
2)    Secondary creep provides a nearly constant creep rate. The average value of the creep rate during this period is called the minimum creep rate. A stage of balance between competing.
Strain hardening and recovery (softening) of the material.
3)    Tertiary creep shows a rapid increase in the creep rate due to effectively reduced cross- sectional area of the specimen leading to creep rupture or failure. In this stage intergranular cracking and/or formation of voids and cavities occur.
Creep rate =c1 V c2
Creep strain at any time = zero time strain intercept + creep rate ×Time
=C0 +c1 V c2 u t
Where, c1 , c2 are constants V stress
                If a load P is applied suddenly to a bar then the stress & strain induced will be double
than those obtained by an equal load applied gradually.

                Stress produced by a load P in falling from height ’h’

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