Creep & Stress Rupture

8/4/2019 Creep & Stress Rupture

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Creep is defined as thetime-dependent strain that occurs

under load at elevatedtemperature and operates in mostapplications of heat-resistant high-alloy castings at normal service

temperatures.The stress that produces a

specified minimum creep rate ofan alloy or a specified amount of

creep deformation in a given timeis referred to as the limiting creepstrength, or limiting stress.



Since materials have itsown different melting point,

each will creep when the

homologous temperature > 0.5





1. Deformation by slip

- More slip systems operate at high

temperature

2. Subgrain formation

- Creep deformation produces

inhomogeneity especially around grain

boundaries, allowing dislocations toarrange themselves into a low-angle grain

boundary.

3. Grain boundary sliding- Produced by shear process and promoted

by increasing temperature/or decreasing

strain rate.





Q = the activated energy for therate-controlling process

A = the material structuralconstant

T = the absolute temperature

R = the universal gas constant



• Superplasticity is the ability to

withstand very large

deformation in tension withoutnecking.

• Give elongation > 1000%.



For grain boundary diffusion

For lattice self-diffusion

L(bar) is the mean linear intercept measure

of grain size. In this case n = 2, m =

0.5



Creep strength is defined as

the stress at a given temperature,which produces a steady-statecreep rate (10-11 to 10-8 s-1.)



Creep data can also be

presented as a plot of stress andtime to produce different amountsof total strain.



Creep occurs under load at hightemperature. Boilers, gas turbine engines, and

ovens are some of the systems that havecomponents that experience creep. Anunderstanding of high temperature materialsbehavior is beneficial in evaluating failures inthese types of systems.

Failures involving creep are usuallyeasy to identify due to the deformation thatoccurs. Failures may appear ductile or brittle.Cracking may be either transgranular orintergranular. While creep testing is done atconstant temperature and constant load actual

components may experience damage atvarious temperatures and loading conditions.



High temperature progressivedeformation of a material at constantstress is called creep. High temperature

is a relative term that is dependent on thematerials being evaluated. A typicalcreep curve is shown below:



Stress rupture testing is similar to creeptesting except that the stresses used are higherthan in a creep test.

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



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Creep & Stress Rupture