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Laboratory 5: Creep testing

Mechanical Metallurgy Laboratory 431303 1

T. Udomphol

L Laabboor r aat t oor r y y 55

Creep Testing

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Objectives

• Students are required to study the principal of creep testing and practice the testing

procedure.

• Students are capable of acquiring and interpreting the creep data obtained from

creep testing of lead which readily creep at room temperature.

• Students should be able to explain the causes of creep in metals, creep deformation

and be able to indicate factors influencing creep behavior in metals.

• Students can analyze the obtained creep data and use it for the selection of

appropriate engineering materials to prevent creep failures.

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1. Literature review

1.1 Creep in metals

When metals experience plastic deformation or has been mechanically deformed at room

temperature, strain hardening or work hardening takes place. We have already encountered this

phenomenon from the stress-strain curve obtained from tensile testing of materials. When the stress is

beyond the yield stress, work hardening occurs due to multiplication and movement of dislocations.

Conversely, annealing of the metals after have been plastically deformed allows annihilation of

dislocations, thus reducing the strength of the metals. However, when metals are continuously and

plastically deformed at high temperature, both work hardening and annealing take place

simultaneously, and creep in metals will result.

Creep testing aims to investigate plastic deformation of a material when subjected to a

constant load or stress at a high temperature. High temperature allows metal to deform more easily

since atoms can move more readily, hence, greater movement of dislocations or slips. New slip

systems and grain-boundary movement are also possible at higher temperatures. Therefore,

engineering alloys utilized at high temperatures is susceptible to creep as well as recrystallization and

grain coarsening. In the case of age-hardened metals, over-ageing is feasible, which results in reduced

hardness and strength due to the coarsening of the second phase precipitates. Furthermore, metals

generally oxidize at high temperatures, thus experiencing creep problems. The development of new

alloys is therefore on its way in order to combat these problems and provide materials with enhanced

mechanical properties. This is for example, the development of nickel base alloys used for aerospace

and high-performance applications.

Generally, metals creep at a temperature above approximately 0.4 T m (T

mis the absolute

temperature of the metal). Therefore, low melting point metals will creep at lower temperature in

comparison to high melting point metals. This is for example; lead having its melting point of 326oC

will creep at room temperature. Iron on the other hand having a higher melting point will creep at the

temperature of approximately 650oC.

Engineering applications such as steam engines, oil refinery and chemical industry normally

operate at temperatures around 500oC. The operating temperatures of the aeroengines, space rockets

missiles are even higher (around 1000o

C), which necessitate materials with high creep resistance.

Due to a significant degradation of mechanical properties with time, structural materials can generally

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withstand mechanical loading at high temperatures in a certain period of time (strong time dependence

of strength). Maintenance or replacing of new materials is therefore necessary for safety reasons.

Unlike creep, mechanical loading at room temperature however do not exhibit strong

dependence of hardness or strength with time. Furthermore, it should be noted that creep testing is

noticeably different from high-temperature tensile testing. For example, a high temperature tensile

test requires less time to actually finish the test, i.e., testing of materials used for missile cases. Creep

properties on the other hand are necessary for materials used for steam pipelines. The creep test in

this case might require 10,000 hrs to finish the test. Therefore, creep study is required for materials

subjected to high temperatures in extended period of time to prevent creep failure during service

operations.

Figure 1: Creep testing configuration showing specimen fitted in the testing machine coupled with a

high temperature furnace.

1.2 Creep test and creep curve

The creep test is carried out to investigate any dimensional changes of specimen with time

during high temperature test. Typically, a creep specimen is gripped at both ends (similar to that of

tensile test) encased with a furnace set at a desired test temperature as shown in figure 1 a). While a

constant load is applied, time and dimensional change are recorded and plotted to give a creep curve

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as illustrated in figure 2. In the laboratory, an SM106 Mk II Apparatus is used for creep testing as

shown in figure 1 b). The test specimens are made from lead, which readily creep at room

temperature. The specimen is attached to one end of the lever arm where the other end is hung by the

known weights. The weights provide the tensile force pulling the specimen and the force can be

calculated by taking moment about the pivot bearing as shown in equation 1.

g m F )884.2( += (1)

where F is the tensile force, Newton

m is the mass of the weight, g

g is the specific gravity = 9.8 g/m2

The dial gauge is employed to measure any dimensional change of the specimen during the

test. However, the position of the dial gauge from the specimen is twice the distance of the specimen

to the pivot. Therefore, the actual dimensional change of the specimen is approximately haft of the

extension indicating from the dial gauge. Moreover, if the test is intended for higher or lower

temperature conditions, the hot pack and the cold pack are used respectively. The hot pack is placed

in the hot water for 15-20 minutes before fitting in the perspex encapsulation attached with a

thermometer for temperature control. The temperature should not exceed 70oC to avoid hot pack

damage. Similarly, the cold pack is freezed before putting in the compartment before testing is

carried out. The temperature should be stabilized for at least 10-15 minutes prior to testing.

Nevertheless, each metal creeps at different rate and thus require different time to finish the

test, ranging from minutes, hours, days, weeks or months. According to the typical creep curve in

figure 1, it should be noticed that the creep curve can be divided into three main stages; primary,

secondary and tertiary creeps. Each stage of creep behavior is influenced from both work hardening

and annealing mechanisms occurring at the same time. However, work hardening and annealing will

take place at different rates depending on response of metals to applied tensile force with time. The

creep rate therefore changes accordingly. This should now be mentioned in details as follows.

The primary creep or transient creep exhibits a decreasing creep rate with time as shown in

figure 1. A very sharp increase in the initial stage is observed with the original strain, ε o, taking place

before the creep rate starts to decrease. The creep rate then diminishes until reaching the secondary

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creep region as detailed in figure 1. This diminished creep rate in the primary creep region accounts

from work hardening mechanism of the metal. Multiplication and interaction of dislocations rule out

the annealing effect at this stage, resulting in increasing the creep resistance of the metal. Metals,

such as lead that can creep readily at room temperature, exhibit an obvious primary creep.

Beyond the primary stage, the creep rate is reaching a steady state where the creep rate is said

to be relatively constant with time and gives the minimum creep rate of all the three regions. This

minimum creep rate is used to represent the creep rate of the metal being tested at particular test

temperature and load. The constant creep rate is due to balancing of strain hardening and annealing

(recovery) processes according to the applied stress and temperature. The amount of dislocations

being generated by work hardening is equal to the number of dislocations being annealed out. The

secondary creep exhibits a relatively linear relationship between strain and time, and the slope

obtained represents the creep rate,•

ε , of the metal, as expressed in equation 2.

dt

d ε ε =

•

(2)

The secondary creep rate depends strongly on stress and temperature, which can be expressed

in the equation of the type

RT E ne A −

•

= σ ε (3)

where A and n are constants

E is the activation energy for creep in the metal

R is the universal gas constant (8.31 J/mol K)

According to equation 3, it can be seen that raising either stress or temperature results in

increasing creep rates. The stress exponent n can be determined by taking the natural logarithms of

this equation as shown in equation 4 and then plotting ln•

ε and ln σ . The relationship between ln•

ε

and ln σ is essentially linear and its slope represents the stress exponent n value.

RT

E n A −+=

•

σ ε lnlnln (4)

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For engineering design point of view, materials subjected to creep in operation should limit

its use in the secondary creep stage and should never enter the tertiary creep stage. Since the

secondary creep gives the longest operating time without creep failure, the materials whose secondary

creep rate is as small as possible are good candidates for engineering applications subjected to creep

failure.

The tertiary creep region gives a rapid creep rate approaching failure. This is due to the

formation of necking. Load bearing capability decreases due to the simultaneous reduction in the

cross-sectional area of the specimen, which is related to local stress acting on this area. Furthermore,

tertiary creep is associated with microstructural alterations due to increasing temperature such as

coarsening of precipitate phases, recrystallization and diffusion of phases. These mechanisms

effectively increase the tertiary creep rate, and eventually leads to fracture under creep.

Figure 2: Creep curve.

However, factors influencing the shape of the creep curve depend on the levels of the stress

and temperatures involved. If the temperature is remained constant, the creep curves will shift

upward and to the left with increasing applied stresses as shown in figure 3. Similarly, if the creep

test is carried out at various temperatures but at a constant stress level, the creep rate will increase

with increasing temperatures.

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Figure 3: Effects of stress levels on the shape of creep curves at a constant temperature.

A plot of creep rate and strain in a log-log scale as illustrated in figure 4 shows a curve which

can be again divided into three stages. It can be noticed that the second stage exhibits a linear

relationship of creep rate and strain while the first and third stages shows decreasing and increasing

creep rates with strain respectively. Since the stress and temperature are kept constant, the variation

of the creep rate is therefore owing to microstructural alteration due to strain and increasing time.

This type of curve is normally applied for engineering design.

Figure 4: Relationship between creep rate and total strain in a log-log scale.

1.2 Deformation processes at elevated temperature

Microstructural changes of metals due to plastic deformation at high temperature results

primarily from 1) dislocation movement (slip), 2) subgrain formation and 3) grain boundary sliding.

High temperature deformation normally produces coarser and wider-spaced slip bands than those

achieved at room temperature. Local strains occur for example at grain boundaries, and cannot be

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observed from the creep curve. At increasing temperatures, slip systems are more available. For

aluminium alloys, slip planes {111}, {100} and {211} are operative at temperatures above 260oC

whereas, zinc and magnesium have non-basal plane slips which are working at high temperature.

The formation of subgrains normally in the adjacent of the grain boundaries results from

lattice distortion. This allows dislocations with opposite signs form the subgrains. The formation of

the subgrains is usually observed in materials with high stacking fault energy. The size of the

subgrains depends on the level of stress and temperature applied. Grain boundary sliding involves a

shear process along the grain boundaries, providing a non-uniform amount of shear displacement. It

was found that the increasing strain due to grain boundary sliding varies directly to the total strain

observed in the creep test. Apart from the processes mentioned previously, plastic deformation of

metals at high temperatures also results from multiple slip, course slip bands, kink bands, fold

formation at grain boundary and grain boundary migration.

Figure 5: Fracture mechanism map for nickel [1].

1.3 Fracture at elevated temperature

High temperature deformation of metals can initiate a crack, which leads to final failure. Slip

planes operating at ambient temperature require relatively lower strength within the grain than those at

the grain boundaries. Cracks are then initiated at inclusions or defects, which act stress concentration

and eventually cause transgranular brittle fracture. At higher temperatures, grain-boundary movement

is prevalent, giving the strength of the grain boundaries to be lower than that of grain interiors. This

in turn results in grain boundary fracture or intergranular fracture at high temperatures. The fracture

energy is lower in this case. Figure 5 shows fracture mechanism map of nickel at various levels of

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stresses and temperatures where the homologous temperature is the test temperature divided by the

melting point of the metal. The fracture mechanism map is useful for the prediction of fracture

mechanisms in engineering materials at particular stress and temperature ranges.

1.4 Interpretation of engineering creep data

In order to select proper materials for high temperature applications, it is necessary to acquire

accurate design parameters such as creep strength from experimental. 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).

The influence of stress levels on minimum creep rate is shown in figure 6. The minimum

creep rate is found to increase with increasing stress levels, giving a linear relationship. The test

temperature is also shown to affect the creep strength of 316 stainless steel observed. As the

temperature increases, the stress level that gives the minimum creep rate reduces accordingly.

Figure 6: Stress vs minimum creep rate of a 316 stainless steel.

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2. Materials and equipment

2.1 Creep specimens made from lead

2.2 Micrometer or vernia caliper

2.3 Permanent pen

2.4 Creep testing machine

2.5 Hot and cold bags

2.6 Thermometer

3. Experimental procedure

3.1 Measure and record lead specimen dimensions for the calculation of stress and strain from the

creep test.

3.2 Fit a lead specimen on a creep test machine as shown in figure 7 with a dial gauge positioned

in a mid range of the specimen gauge length for the calculation of specimen extension (creep

strain).

3.3 Hung the weights of known values at the end of the sample to determine the applied stress.

Specimen extension will be read on the dial gauge and time is recorded using a stopped

watch.

3.4 When the weight is left hanging, the specimen will be immediately strained. The time will be

recorded at every specimen extension of 0.5 mm. Repeat the test using a new specimen

having a similar test condition to give an average value of the creep rate obtained.

3.5 Repeat the creep tests but now using different weights provided (to give different stress

levels) to investigate the effect of stress levels on the creep rate of the specimen.

3.6 Construct the creep curves from the obtained experimental data similar to that shown in

figure 2.

3.7 Analyze and discuss the creep rate of each experimental set as well as fracture surfaces. Give

conclusions.

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4. Results

SpecimenDetails

1 2 3 4

Specimen length (mm)

Specimen width (mm)

Cross-sectional area (mm2)

Weight (g)

Stress (MPa)

Creep rate (sec-1)

Fracture details

Table 1: Creep test data of lead specimens.

Figure 7: Creep curves of lead specimens at different stress levels used in the test conditions.

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6. Conclusions

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7. Questions

7.1 Why do you think that lead creeps at room temperature? Explain.

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7.2 What would have happened if this experiment was set at temperatures below subzero and

water boiling point?

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7.3 Explain the differences between creep testing and stress rupture testing.

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8. References

8.1 SM106 Creep machine, TQ education and training ltd product division.

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

100406-8.

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

edition, McGraw-

Hill, ISBN 007-125690-3

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