ENGINEERING AND MATERIAL SCIENCE

1. BY
Dr. K. SENTHILARASAN
ASSISTANT PROFESSOR
DEPARTMENT OF PHYSICS
E.G.S.PILLAY ARTS & SCIENCE COLLEGE
NAGAPATTINAM-611002
CREEP

2. Creep
The creep is defined as the property of a material by
virtue of which it deforms continuously under a steady load.
Creep is the slow plastic deformation of material under
the application of a constant load even for stresses below the
proportional limit. This yielding (increase of strain without
increase in load) may continue to the point of fracture.
Usually creep occurs at high temperature. In designing
of I.C. engines, jet engines, boilers and turbines creep is taken
into account as an important property.
This property is exhibited by iron, nickel, copper and their
alloys at elevated temperatures. But zinc, tin, lead and their
alloys show creep at room temperature only.

3. In metals creep is a plastic deformation caused
by slip occurring along crystallographic direction
in the individual crystals together with some
deformation of the grain boundary of the
materials.
After complete release of load, a small
fraction of this plastic deformation is recovered
with time. Thus, most of the deformation is non
recoverable.

4. Creep limit
Creep limit is defined as the maximum
static stress that will result in creep at a rate
lower than some assigned rate at a given
temperature.
Creep strength is usually defined as the
stress for 1% strain in 10,000 hours in the case of
jet turbine design while for steam turbine design,
it is the stress for 1% strain in 100000 hours.

6. Primary stage (Instantaneous elongation and transient creep) is
characterised by the relatively rapid extension but at a decreasing rate.
This primary stage is of great interest to the designer since it forms as
early part of the total extension reached in a given time and may affect
clearances provided between components of a machine.
Thermal expansion may also be included in this stage.
Secondary stage (viscous or steady creep) is characterised by the
period of extension during which the creep occurs at a more or less constant
rate having its having minimum creep rate.
The is important part of the curve which is used to estimate the
service life of the alloy.
Tertiary stage is characterised by the accelerated rate of extension
which finally lead to rupture. So the use of alloys should be avoided in this
stage.

7. Factors influencing creep resistance
1. Heat treatment: Creep resistance of steel is influenced by heat
treatment. At temperature of 300 C or higher, the maximum
creep resistance is usually produced by normalising.
2. Grain size: Creep resistance is also influenced by the grain
size of steel.
3. Strain hardening: Strain hardening of steel increase its creep
resistance.
4. Alloying additions: At temperature below the lowest
temperature of recrystallization, the creep resistance of steel
may be improved by the ferrite forming elements like nickel,
cobalt and manganese or by the carbide forming elements
like chromium, molybdenum, tungsten and vanadium.

8. Theories of creep
The creep rate for the second stage (steady state) increase with increasing stress at
a constant temperature according to the equation
𝑑ε
𝑑𝑡
= A 𝑛
Where “n” is a constant called stress exponent whose value is about 5 and A is also
a constant and epsilon and Sikma are creep strain and stress respectively.
The creep rate also increases with increasing temperature for a given stress.
𝑑
 𝑑𝑡
= B𝑒 −𝑄/𝑅𝑇
Where Q is called the activation energy for creep, R is the gas constant and T is
the temperature of the material in Kelvin.

9. One can draw a graph between the log stress and
a combined parameter of time and temperature
called Larson Miller parameter, given by P=T
(C+log t) where T is the temperature, t the
rupture time and C is a constant. We can get the
graph as a straight line. Using this graph, it is
possible to predict the repute time for lower
stresses and temperatures.