Introduction to Basic Manufacturing Processes and ... - always yours

128 Introduction to Basic Manufacturing Processes and Workshop Technology
7.3.6 Testing of Creep
Metal part when is subjected to a high
constant stress at high temperature for
a longer period of time, it will undergo
a slow and permanent deformation (in
form of a crack which may further
propagate further towards creep failure)
called creep. Creep is time dependent
phenomena of metal failure at high
constant stress and at high temperature
such subjecting of at steam turbine
blade. A schematic creep testing setup
is shown in Fig. 7.12. Test is carried
out up to the failure of the test specimen.
A creep curve for high temperature and
long time creep is shown in Fig. 7.13.
The curve shows different portions of
the primary secondary and tertiary
creep which ends at fracture in metals.
3
2
1
2
7
1. Specimen
2. Grips
3. Furnace
4. Lever
6
4
Gauge
Length
5
5. Weights
6. Thermocouple
7. Instrument for
strain measurement
Fig. 7.12 Schematic creep testing setup
Fracture
Primary
Creep
Strain
Secondary
Creep
Tertiary
Creep
Instantaneous
Elongation
Time
Fig. 7.13 Creep curve for a high temperature and long time creep test
7.4 CHOICE OF MATERIALS FOR THE ENGINEERING APPLICATONS
The choice of materials for the engineering purposes depends upon the following factors:
1 Availability of the materials,
2 Properties needed for meeting the functional requirements,
3 Suitability of the materials for the working conditions in service, and
4 The cost of the materials.