"‫" الجودة والتميز‬ "‫" عراقة وجودة‬

"Quality and Excellence" "Tradition and Quality"

Al-Zaytoonah University of Jordan

Faculty of Engineering and Technology

Mechanical Engineering Department

Lab Name
Strength of materials lab

Experiment Name
creep test
Introduction :
Creep is high temperature progressive deformation at constant stress. "High temperature" is a
relative term dependent upon the materials involved. Creep rates are used in evaluating materials
for boilers, gas turbines, jet engines, ovens, or any application that involves high temperatures under
load. Understanding high temperature behavior of metals is useful in designing failure resistant
systems. A creep test involves a tensile specimen under a constant load maintained at a constant
temperature. Measurements of strain are then recorded over a period of time.
Creep occurs in three stages: Primary, or Stage I; Secondary, or Stage II: and Tertiary, or Stage III.
Stage I, or Primary creep occurs at the beginning of the tests, and creep is mostly transiently, not at
a steady rate. Resistance to creep increases until Stage II is reached. In Stage II, or Secondary
creep, The rate of creep becomes roughly steady. This stage is often referred to as steady state
creep. In Stage III, or tertiary creep, the creep rate begins to accelerate as the cross sectional area
of the specimen decreases due to necking or internal voiding decreases the effective area of the
specimen. If stage III is allowed to proceed, fracture will occur.

Objectives :
To obtain the plot of creep strain growth as a function of time

Rate of deformation is a function of:

• Applied load

• Exposure temperature

• Exposure time

• Material properties

Procedure :
The creep test is conducted using a tensile specimen to which a constant stress is applied, often by the
simple method of suspending weights from it. Surrounding the specimen is a thermostatically controlled
furnace, the temperature being controlled by a thermocouple attached to the gauge length of the
specimen, Fig.2. The extension of the specimen is measured by a very sensitive extensometer since the
actual amount of deformation before failure may be only two or three per cent. The results of the test
are then plotted on a graph of strain versus time to give a curve similar to that illustrated in the figure
next page.
Apparatus :
Creep testing machine :

Figure 1

Specimen : 25mm gauge length

Figure 2
 Theory :

Figure 3

The diagram above shows strain as a function of time. The portions are split as primary, secondary
and tertiary curve. In the beginning, the rate of strain is high and then stabilizes due to work
hardening. In the second segment, the strain is minimum and nearly constant which happens as a
result of balance between annealing and work hardening. The tertiary segment is similar to the
necking phenomenon observed when metals are tension tested in Universal testing machine.
The strain vs. time graphs are plotted with a constant load applied at a constant temperature.
Shape of the creep curve will depend on the levels of temperatures and stresses involved. If the
temperature is remained constant, the creep curves will shift upward and to the left with
increasing applied stresses. If the creep test is carried out at various temperatures but at a
constant stress level, the creep rate will increase with increasing temperatures.

The deformation process in creep which occur at elevated temperatures are due to
i. Dislocation movement known as slip
ii. Grain boundary sliding
iii. Sub-grain formation
At increasing temperature, slip systems are more available. Grain boundary sliding is a type of
shear process along the grain boundaries, providing a non-uniform amount of shear displacement.
The formation of sub grains normally in the adjacent of the grain boundaries results from lattice
distortion. This allows dislocation with opposite signs to form the sub grains

Application of creep in Engineering

While designing components, it is necessary to select the material which can withstand the
operating conditions that the components will be exposed to. Therefore, it is necessary to acquire
accurate design parameters such as creep strength from experimentation. The creep strength can
be defined as 1) the stress at a given temperature to produce a steady-state creep rate of a fixed
amount (normally at 10-11 to 10-8 s-1 or, 2) the stress to produce creep strain at 1 percent of the
total creep strain at a given test temperature (usually 1000, 10000, or 100000 hours)

Applying such a test enables the designer to calculate how the component will change in shape
during service and hence to specify its design creep life. This is of particular importance where
dimensional control is crucial, in a gas turbine for instance, but of less importance where
changes in shape do not significantly affect the operation of the component, perhaps a pressure
vessel suspended from the top and which can expand downwards without being compromised.
 Discussion

- As you can see from the plotted data (strain-time) curve , creep happens at 3 stages:
1- Primary Creep: starts at a rapid rate and slows with time.
2- Secondary Creep: has a relatively uniform rate.
3-Tertiary Creep: has an accelerated creep rate and terminates
when the material breaks or ruptures. It is associated with
both necking and formation of grain boundary voids.

- Creep rate and creep rate curve are useful in several things :
It helps predicting the life of a compenent or how long would it withstand loads applied on it
until failure occurs

Creep is sometimes used as a basis for design purposes, when the design in predicted to be
under constant stress for long periods of time , for example The creep rate of hot pressure-
loaded components in a nuclear reactor at power can be a significant design constraint, since
the creep rate is enhanced by the flux of energetic particles.

It gives you information about the material’s properties .

- Possible sources of error :

Having inaccurate readings since it was taken by us and not tabulated by a machine like the
tensile experiment .
Machine errors

We can improve the results of this test by being more careful and accurate when taking any kind
of measurements or by using advanced machines
Collected data:
Time (s) Dl (mm) strain
0 1 0.044444
10 1.02 0.045333
20 1.04 0.046222
30 1.07 0.047556
40 1.11 0.049333
50 1.14 0.050667
60 1.17 0.052
70 1.2 0.053333
80 1.23 0.054667
90 1.25 0.055556
150 1.37 0.060889
210 1.45 0.064444
270 1.55 0.068889
330 1.6 0.071111
390 1.67 0.074222
450 1.7 0.075556
510 1.77 0.078667
570 1.81 0.080444
630 1.86 0.082667

Sample of calculations :
For reading no1 :
L=22.5mm
Load(P) = 12.36 N

ɛ=deflection /length = 1 /22.5

= 0.044
 creep rate curve
0.09
0.08
0.07
0.06
Time (s)
0.05
0.04
0.03
0.02
0.01
0
0 10 20 30 40 50 60 70 80 90 150 210 270 330 390 450 510 570 630
strain

Creep rate = slope of creep rate curve at secondary stage

=
0.0555−0.0546
90−80
= 9 ∗ 10−5 𝑠−1

Conclusions :

Readings were taken every 10 seconds

The curve we got is not very accurate , as you can notice the secondary stage in not really linear
while it should be , maybe it’s because we took false readings or some other reason related to
the specimen or the machine .

We don’t know whether this rate is considered high , but the specimen didn’t take much time to
fail (almost 9 minutes) because we increased the load.

The strain rate increases rapidly due to local necking or formation of internal cavities in the
sample which results in an increase in the effective stress on the sample , eventually leading to
fracture .