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If an element has undergone failure below its yield point on the application of constant load at constant elevated temperature, then which of the following has it undergone?

If an element has undergone failure below its yield point on the application of constant load at constant elevated temperature, then which of the following has it undergone? Correct Answer Creep

Explanation:

Creep:

  • Materials subjected to constant load at an elevated temperature (> 0.4 - 0.5 times of Melting Temperature) will Creep i.e exhibit time dependant deformation.
  • It occurs in three stages.

Stages of Creep:

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Primary Creep

Secondary Creep

Tertiary Creep

Creep rate decreases with time (decreasing slope) due to the strain hardening process resulting from deformation.

Creep rate becomes linear (Constant slope)

There is a balance between strain hardening 

and recovery (softening) of the material because of recrystallization.

Creep rate increases with time (Increasing slope) leading to necking and finally fracture because of the structural changes occurring in the material.

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Factor affecting Creep:

  • Material Properties (Melting point, Young’s Modulus, Grain size)
  • Exposure time
  • Exposure temperature
  • Structural load

Fatigue strength: 

It is the highest stress that a material can withstand for a given number of cycles without breaking.

Practically, levels of stress are not held constant as in S – N tests, but can vary below or above the designed stress level.

Overstressing: The initial applied stress level is higher than the fatigue limit for a short period of time beyond failure, then cyclic stressing below the fatigue limit. This overstressing reduces the fatigue limit.

Understressing: The initial applied stress level is lower than the fatigue limit for a period of time, then cyclic stressing above the fatigue limit. This under-stressing increases the fatigue limit (might be due to strain hardening on the surface).

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