Lab 9 HT Creep.pdf

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ASE 324LLaboratory #9

High Temperature Creep and Creep Rupture

Purpose: Creep and stress rupture testing will be demonstrated and data will be provided for

analysis. This exercise will provide an introduction to the time and temperature dependent

deformation responses of engineering materials.

Background : So far, we have considered only the instantaneous elastic or plastic deformation

responses of solids as demonstrated in the tension tests of Labs #1 and #3. In these tension tests,

the applied displacement was steadily increased by the positive feedback control functions of the

MTS. However, many engineering applications of materials involve the application of a constant

load over a long period of time and under a variety of temperatures. In a number of cases, the

time and temperature dependent deformation responses in materials are important design

considerations and these phenomena, in part, determine the lifetime of the component of interest.

Creep: Creep is defined as the time dependent plastic deformation that occurs in a solid that is

subjected to a constant load or stress. In other words, if we load a tension specimen and hold that

load at a constant value, in time we will observe a time dependent plastic strain in addition to the

instantaneous elastic strain that occurred upon application of the load. This plastic creepdeformation increases in time and can ultimately result in failure.

A creep test per se, is simply to apply a constant load to a tension specimen at a constant

temperature and to observe the deformation in time over a sufficiently long time interval. The

stress rupture test is a useful extension of the creep test in which the specimen is allowed to creep

until it ruptures at a constant load and temperature.

Methods: We have three ATS Applied Testing Systems 2300 series 10,000 pound capacity

lever arm testers. These testers consist of a load frame, a load train, displacement and time

measuring devices and a means to maintain and control the temperature of the test specimen.

Figure 1 is a diagram of one of the testers. The specimen is loaded by applying weights to the

weight pan and the load is transmitted to the specimen and intensified by the 10:1 load ratio lever

arm load train. The specimen is enclosed in a clam shell temperature controlled furnace;

specimen displacement is measured using a mechanical extensometer and a DCDT transducer.

The mechanical extensometer preserves the specimen displacement, distance AB and carries it



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Assignment:

Read Chapter 13 in Materials Engineering, Science, Processing and Design. Do not prepare a

formal lab report this week, but answer the following questions:

1. In tension creep tests of a steel alloy, the following unit creep strains (Table 1) were

obtained.

Time Tensile Stresses in PSI

(hours) 3000 5000 8000 11000

0 0 0 0 0

50 0.020 0.045 0.099 0.135200 0.028 0.053 0.140 0.238

400 0.030 0.055 0.149 0.305

600 0.031 0.058 0.155 0.368

800 0.033 0.062 0.164 0.433

1000 0.035 0.065 0.173 0.520

Table 1

(a) Plot the creep-time curves for each stress, using the data in Table 1. UseCartesian coordinates.

(b) Determine the minimum creep rates, s using the average slope between 500 and

1000 hours.

(c) With the data obtained, determine the creep constants B and n in the equation

0/ n

s B where 0 = 1000 psi.

2. A tie bar in a chemical plant has been designed to withstand a stress of 25 MPa at 620°C.

Creep tests carried out on specimens of the same material under these conditions indicated asecondary creep rate of 3.1×10 -12 s-1. In actual service, for 30% of the time, the stress and

temperature increased to 30 MPa and 650°C. Determine the average creep rate under the

service conditions. The power law exponent of the material is 5 and the activation energy is

160 kJ mol -1.

3. A pipe with a radius of 20 mm and wall thickness of 4 mm is made of a CrMo steel carries a

hot fluid (600°C) under pressure (10MPa). The reference stress and strain rate in



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/0

0

n

Q RT s e

are 169 MPa and 3.48×10 10 , the power law exponent is 7.5 and the

activation energy is 280 kJ mol -1.

a) Determine the creep rate of the pipe wall under these conditions.

b) Determine how long it would take the pipe wall to expand by 20 m.

4. Make a Larson-Miller Plot of the data in Table 2 with C = 20.

(a) Explain how you would determine C if you had rupture data at various

temperatures and stress levels available to you.

(b) Lab tests at 840 MPa and 650°C resulted in a rupture time of 479 hrs. How long

would the material last at 840 MPa and 900°C?

Temp. Stress Rupture Time

(°C) (MPa) (hr)

650 1000 12

650 950 56

650 895 165

650 840 479

650 820 319650 790 504

705 815 23

705 695 110

705 635 254

705 590 478

760 600 28

760 515 62

760 440 291

760 415 390

815 460 15

815 395 47

815 345 91

815 260 379

815 220 773

870 260 33



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870 215 73

870 180 196

870 145 300

900 170 69

900 140 163

900 105 738

Table 2

5. A casing for a jet engine requires a series of bolts to operate at temperatures that are more

than 50% of the melting temperature of the bolt material. There is a concern that the loads in

the bolts will relax over time. The bolts will be operating in a stress/temperature regime

where dislocation creep dominates. In that case, the secondary creep rate for a fixedtemperature is n

s B , where stress is in psi and time is in hours. Determine how long it

will take the bolts to relax to 60% of the initial stress if the tensile modulus of the material is

30x10 6 psi, the initial stress is 4000 psi,

n 5 and B 10 26 .

6. Describe the various deformation

mechanisms that are shown in the

deformation mechanism map. In the creeprelated mechanisms, indicate how the

secondary creep rate depends on stress,

temperature, intrinsic lattice and

metallurgical parameters.

7. Do Problem E13.9 in the book.

8. Do Problem E 13.15 in the book

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