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
creep


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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