A creep test subjects a specimen to a constant load and temperature over time to measure deformation. There are three creep stages: primary creep where the rate is rapid and decreases over time, secondary creep where the rate is relatively uniform, and tertiary creep where the rate accelerates until rupture. Creep tests are used to determine design parameters like steady-state creep rate and rupture lifetime. Stress rupture tests are similar but impose higher stresses to measure time to fracture. Data from creep tests are used to extrapolate lifetimes at different conditions using methods like the Larson-Miller parameter.

1. Creep Test
A typical creep test consists of subjecting a specimen to a constant load or stress while maintaining the temperature constant ( >
o.4 Tm) ; deformation or strain is measured and plotted as a function of elapsed time as illustrated in figure 2.
For metallic materials most creep tests are conducted in uniaxial tension using a specimen having the same
geometry as for tensile tests.
Figure 1 creep test set-up

2. Fig (2) Typical creep curve of strain versus time at constant stress and constant elevated temperature

3. Creep Stages:
Following the instantaneous deformation (elastic region), there are three stages:
1-Primary Creep: starts at a rapid rate as a result of dislocations movements and slows with time
due to the work hardening (low temperature) which ties-up the dislocations at grain boundaries.
2- Secondary Creep: has a relatively uniform strain rate due to the balance between work
hardening and annealing (thermal softening). This is often the stage of creep that is of the
longest duration
3- Tertiary Creep: has an accelerated creep rate and terminates when the material breaks or
ruptures. This acceleration due to with the formation of voids and micro-cracks at grain
boundaries.

4. Figure -3- Cavity coalescence at grain boundaries
Cavitation Phenomenon:
- In tertiary region, rupture occurs due to the pore formation (cavitation) at grain
boundaries.
- Cavities form on grain boundaries that are perpendicular to the applied force
- Eliminating the grain boundaries altogether should enhance the creep-rupture life
- This was the motivation for developing single crystal alloys.

5. Creep Test. Stress Rupture Test
Measures strain versus time at constant
temperature and load or stress
Measures stress versus time to rupture at
constant temperature
Relatively low loads and creep rates. Higher loads and creep rates
Long duration, 2,000 to 10,000 hours. Not
always to fracture
.... Shorter duration, less than 1,000 hours
typically. Always to fracture
Strain measured accurately using sensitive
equipment (inductance gauges) to
determine creep rate. Strains typically less
than 0.5%.
Simpler less sensitive strain measuring
equipment (dial gauges). Time and strain to
fracture measured. Strains typically up to
50%.
Design Parameters
Most important design parameters in creep are:
•Measure Steady state creep rate
•Measure Rupture life time tr (stress rupture test). Similar to creep test except
the stress is higher
Note: both of designs strongly depend on temperature and stress applied

6. Fig (3): Effect of temperature and stress on creep behavior

7. Data extrapolation method
One of the most popularly used techniques in representing creep-rupture data is Larson-Miller Time –temperature
parameters. This parameter can be derived from the stress and temperature dependence of the creep rate or time to
rupture. The rate equation generally can be written in the form of the Arrhenius equation and expressed as
PLM= T [C + log (tr)]
where C is a constant (usually on the order of 20), for T in Kelvin and the rupture lifetime in hours, PLM is Larson-Miller
parameter.
Example: Using the Larson–Miller data for S-590 iron shown in Figure 4, predict the time to rupture for a
component that is subjected to a stress of 140 MPa at 800 C
Solution
From Figure 4 , at 140 MPa the value of the Larson–Miller
parameter is 24x103 , for T in K and in h; therefore,
24x103 = T [C + log (tr)]
24x103 = ( 800+273) [ 20+ log (tr)]
20+ log (tr) = 22.37
tr= 233 hour (9.7 days).

8. Figure -4- Logarithm stress versus the Larson–Miller parameter
for an S-590 iron.

9. Design Considerations to avoid Creep
•Reduce the effect of grain boundaries:
•Use single crystal material with large grains.
•Addition of solid solutions to eliminate vacancies.
•Employ materials of high melting temperatures.
.