Objective

The objective of this experiment is :

1. To understand the stage that is involved in the creep test.

2. To experimentally obtain the creep curve.

3. To identify the properties of material while subjected to a tensile load.

4. To measure the rate of deformation of the material to stress at a constant temperature.

Theory

Creep is defined as time-dependent plastic deformation (elongation) of the metal at a

constant tensile load. It is also defined as 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 can be studied by measuring the

permanent extension, after various time intervals, of test-pieces maintained at a constant

temperature. Measurements of strain are then recorded over a period of time as in Figure 1. After

the initial instantaneous extension, it shows that creep occurs in three stages :

Stage 1 (Primary creep) : This stage occurs at the beginning of the tests, and creep is

mostly transiently, not at a steady rate. Resistance to creep increases until stage 2 (secondary

creep) is reached.

Stage 2 (Secondary creep) : The rate of creep becomes roughly steady. This stage is often

referred to as steady state creep.

Stage 3 (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 3 is allowed to proceed, fracture will occur.

In many cases, the three parts of the curve are not clearly distinguishable. To obtain a

complete picture of the creep properties of a material, it is necessary to construct creep curves for

a range of stresses over a range of temperature. Such curves as shown in Figure 2 usually show

that, as the applied stress decreased the primary creep is also decreases, secondary creep is

prolonged, and the possible extension during tertiary creep tends to increase. Very low applied

stress may mean that tertiary creep dies not occur even after lengthy service life.

Specimen and Equipment

1. Creep measurement apparatus

2. Vernier caliper

3. Stopwatch

4. Specimens - plastic

Procedures

1. The original gauge length, thickness and width of the gauge length of the specimen was

measured and recorded in the table.

2. Then, the maximum load that can be stand by the specimen was measured .

3. The force that lower than maximum load of the specimen was used in the creep test.

4. After that, specimen was placed to the creep test machine to obtain the result

5. The force-time graph of creep test been observed on the monitor of the test’s equipment

6. Step 3 until 5 was repeated until the typical creep curve of strain versus time at constant

stress was obtained

7. When the typical creep curve of strain versus time at constant stress was obtained, the

machine was been stopped.

8. The result and graph obtained had been saved and recorded.

9. Analysis had been done.

Figure 1 : Typical creep curve of strain

versus time at constant stress and

elevated temperature

Figure 2 : Influence of stress σ and

temperature T on creep behavior

RESULTS

Specimens number Specimen 1 Load

87N

Specimen 2 Load

90 N

Specimen 3 Load

93 N

Load, m [kg] 8.869 kg 9.174 kg 9.48 kg

Specimen thickness, h

[mm] 0.17 mm 0.17 mm 0.17 mm

Specimen width of the

gauge length,

d [mm]

10.78 mm 10.78 mm 10.78 mm

Initial gauge length, l0

[mm] 10 mm 10 mm 10 mm

Final gauge length,

l [mm] 14.1 mm 14.1 mm 13.8 mm

Specimen 1 Load 87N

Specimen 1 Load 90 N

time (s) elongation (mm) strain

5 1.5 0.15

10 1.6 0.16

15 1.65 0.165

20 1.7 0.17

25 1.75 0.175

30 1.8 0.18

35 2.83 0.283

40 1.85 0.185

45 1.9 0.19

50 1.95 0.195

55 1.97 0.197

60 1.99 0.199

65 2.1 0.21

70 2.3 0.23

75 2.5 0.25

79.2 4 0.4

time (s) elongation (mm) strain

5 1.3 0.13

10 1.4 0.14

15 1.6 0.16

20 1.8 0.18

22.8 4 0.4

Specimen 1 Load 93 N

time (s) elongation (mm) strain

5 1.4 0.14

10 1.6 0.16

15 1.65 0.165

20 1.8 0.18

25 2.2 0.22

26.4 3.8 0.38

Discussion

1. From the experiment result, how is the shape of obtained creep rate curves?

Figure 1: Creep curve graph for plastic

Following the loading strain εο, the creep rate (slope of strain vs time curve), is high but decreases

as the material deforms during the primary creep stage. At sufficiently large strains, the material creeps at

a constant rate. This is called the secondary or steady-state creep stage. Ordinarily this is the most

important stage of creep since the time to failure tf is determined primarily by the secondary creep rate. In

the case of tension creep, the secondary creep stage is eventually interrupted by the onset of tertiary

creep, which is characterized by internal fracturing of the material, creep acceleration,

and finally failure.

2. How is the results compared to the theory?

A typical creep curve for a plastic specimen is shown in Figure 1. The initial part of the characteristic shows the three stages of creep. The strain has been calculated by dividing the different in length (4.1mm) over original length (10mm), which gives an elastic strain of 0.41. For the theory calculation which is 669.6E (E=Modulus of elasticity), there might be a slight different from the result obtained from the experiment. The higher the load being applied to the specimen the faster the time taken for the specimen to reach fracture period. However, this result is for a higher stress and very much shorter creep time than would normally be used for material testing

3. How do you summarize the occurred errors, and discuss the factors affected to the

experimental results?

One of the error occurred when doing the experiment is that the size measure and

the real size of the specimen is not precise, thus making the error happen. Another factor

that most likely to cause the experiment to has an error from the theory result is that the

specimen is not carefully cut from its required shape. It make the specimen have a small

unnecessary cut that make the specimen comes to fracture stage faster than usual.

Moreover, temperature of the surrounding also affects the experimental result. This is

because at high temperatures the entire creep process can occur in a matter of seconds. The last error that

might occurred is the apparatus or equipment used showed error before the experiment or the placing of

the specimen onto the universal testing machine is not correct or slightly misplace.

Conclusion

As a conclusion, we know that the creep test is conducted using a tensile specimen which

a constant stress is applied, often by the simple method of suspending weights from it.

Deformation is recorded at specified time intervals and a creep vs. time diagram is plotted. Slope

of curve at any point is creep rate. If failure occurs, it terminates test and time for rupture is

recorded. If specimen does not fracture within test period, creep recovery may be

measured. Creep is generally divided into three stages. The primary creep starts at a rapid rate

and slows with time. The secondary creep has a relatively uniform rate and the tertiary creep has

an accelerated creep rate and terminates when the material breaks or ruptures. If creep recovery

is measured, the test will determine the stress-relaxation. The rate of decrease in deformation that

takes place when the load is removed. Creep is also sometimes referred to as Stress-Relaxation

testing.

Reference

Hashemi, S. Foundations of materials science and engineering, 2006, 4th edition,

McGraw Hill, ISBN 007-125690-3

Dieter, G.E., Mechanical metallurgy, 1988, SI metric edition, McGraw-Hill, ISBN 0-07-

100406-8.

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