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Creep failure and related question

by student_
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student_
#1
Aug4-09, 12:53 PM
P: 9
I don't know if the answers by me are correct or not to these questions. Can someone please check them and give me some suggesstions. thanks

QUESTION 1

Explain the distinctive stages of a creep failure. (5 marks)

ANSWER

Creep is the slow deformation of a material when it is under the influence of stresses. It comes about as a result of long term exposure to levels of stress that are below the yield strength of the material.

There are three stages of creep:

1. Primary where plastic deformation occurs at decreasing strain rate. In this stage the strain rate is relatively high, however it slows down with increasing strain. This is due to work hardening.
2. Secondary where the strain rate eventually reaches a minimum and becomes near constant. This is because of the equilibrium between work hardening and recovery.
3. Tertiary where the strain rate exponentially increases with strain eventually leading to fracture.

QUESTION 2

Outline the ways that creep may lead to an engineering component ceasing to perform its designed function. (3 marks)

ANSWER

Creeping can slowly deform the engineering component which reduces the components efficiency. As time goes on the component become less and less efficient. This will also mean that the number of functions that the component can perform reduces and/or cannot perform them properly. Eventually this leads to a fracture which totally disables the component from doing the job it was designed for.

The rate of this deformation is a function of the material properties, exposure time, exposure temperature and the applied structural load. Depending on the magnitude of the applied stress and its duration, the deformation may become so large that a component can no longer perform its function for example creep of a turbine blade will cause the blade to contact the casing, resulting in the failure of the blade.

QUESTION

Explain, in the context of a practical example, the reasons why rigorous control of operating temperatures and regular NDT are necessary to prevent service creep failures. Suggest an appropriate steel specification for good creep resistance. (12 marks)

ANSWER

Creep is more severe in materials that are subjected to heat for long periods, and near the melting point. Creep always increases with temperature.
Unlike brittle fracture, creep deformation does not occur suddenly upon the application of stress. Instead, strain accumulates as a result of long-term stress. Creep deformation is "time-dependent" deformation.
The temperature range in which creep deformation may occur differs in various materials. For example, Tungsten requires a temperature in the thousands of degrees before creep deformation can occur while ice formations will creep in freezing temperature.
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student_
#2
Aug5-09, 02:03 PM
P: 9
Anyone??
FredGarvin
#3
Aug5-09, 03:19 PM
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Pretty good except answer #2. The efficiency of a component may be influenced but that is because the object is undergoing a displacement. You should just concentrate on that and say something to the affect that a part can fail under stresses less than yield strength and can change dimensions to a point that it may exceed its given geometric envelope.

Mapes
#4
Aug6-09, 03:03 AM
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Creep failure and related question

Seconding Fred's comments. "Efficiency" is vague here; the more fundamental problem is that the material is a different shape than it was fabricated to be (and that the shape is continuing to change). The most obvious problem is that a moving part could contact other parts.

By careful about talking about creep beginning at a certain temperature. Creep always occurs; it's better to talk about a given rate of creep, or measurable, noticeable, or observable creep.
student_
#5
Aug6-09, 08:52 PM
P: 9
I will recheck my answers...

Thanks guys


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