Lesson 62 and 63 rupture test

Lesson-62 and 63Rupture Test

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ObjectivesObjectives

On completion of this session, you would be able to know:• Stress – rupture test.• Structural changes during creep.• Creep test Vs stress – rupture test.

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RecapRecap

In the previous session, we have learnt about.• Mechanisms of Creep.– Dislocation glide.– Dislocation creep.– Diffusion creep.• Nabarro-Herring Creep.• Coble Creep.

– Grain-boundary sliding.

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Stress – Rupture Test

• The rupture test in carried out in a similar manner to the creep test, but at a higher stress level until the specimen fails and the time at failure is measured.

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Creep test Stress rupture test

Stress-rupture data is plotted as Stress Vs Rupture time on a log-log scale

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Fig.1 Method of plotting stress-rupture data Fig.1 Method of plotting stress-rupture data

• Rupture strength and failure time are plotted, normally showing a straight line.

• Changing of the slope indicates structural changes in the material,– i.e., Transgranular intergranular fracture, oxidation,

recrystallisation, grain growth, spheroidization, precipitation.

• Direct application in design.• The basic information obtained from the stress-rupture test is

the time to cause failure at a given nominal stress for a constant temperature.

• The elongation and reduction of area at fracture.

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Structural changes during creepStructural changes during creep

• Different creep rates result from changes in internal structure of the materials with creep rate and time.

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Fig.2 Creep rate and total strain relationshipFig.2 Creep rate and total strain relationship

• There are three principal deformation processes at elevated temperature.– Deformation by slip– Sub-grain formation– Grain boundary sliding

Deformation by slip• More slip systems operate at high temperature.• Slip bands are coarser and widely spaced.

Sub-grain formation• Creep deformation produces imhomoginiety especially around

grain boundaries, allowing dislocations to arrange themselves into a low-angle grain boundary.

• Easy for metals with high stacking false energy.8

Grain boundary sliding• Produced by shear process and promoted by increasing

temperature or decreasing strain rate.• Results in grain boundary folding or grain boundary migration.

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Difference between Creep test Vs Stress – Rupture Test

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S.No CREEP TEST STRESS RUPTURE TEST

1 Test may not be carried out to

the failure of the material.

Test is always carried out

to the failure of the

material.

2 Loads used are less Higher loads are used

3 Creep rates are low Creep rates are higher

4 Creep tests are frequently

conducted for periods of

2,000h and often to 10,000h.

Creep tests are terminated

in 1000 h.

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5 Important design parameter

derived is minimum creep rate

at a given stress and

temperature.

Important design

parameter derived is

rupture time at a given

stress and temperature.

6 The total strain is often less

than 0.5 percent

The total strain may be

around 50 percent

7 Simpler strain-measuring

devices cannot be used.

Simpler strain-measuring

devices, such as dial gages,

can be used.

SummarySummary

In this session, we learnt about:• Stress – rupture test.• Structural changes during creep.• Creep test Vs stress – rupture test.

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Multiple Choice Question(MCQ)Multiple Choice Question(MCQ)

1. which of the following is correct regarding Stress – Rupture test.

a. Test carried to the failure,

b. Creep rates are low,

c. Loads are less.

2. After prestressing process is completed, a loss of stress is due to ______________ .

a. Elastic shortening of concrete,

b. Creep of steel,

c. All the above.

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Multiple Choice Question(MCQ)Multiple Choice Question(MCQ)

3. The steel generally used in R.C.C. work, is _________ .

a. Stainless,b. Mild steel,c. High carbon steel.

4. Plastics as a material of construction suffer from the drawback of low.

a. Machinability,b. Density,c. Strength.

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Frequently Asked Question(FAQ)Frequently Asked Question(FAQ)

1. Collect the Stress rupture data of at least five alloys which are important in high temperature applications.

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