Q. Explain various stages of creep?
Ans. CREEP: Creep can be defined as the process by which plastic flow occurs when a constant stress or load is applied to a material for a prolonged time. Turbine rotor in jet engines and steam generators that experience centrifugal stress and high pressure steam line for a long time then deformation under these circumstances is termed as creep. Creep is time dependent strain occurring under stress. Some material like iron, nickel, copper, and their alloy exhibit creep at higher temperature whereas Zinc, Tin, Lead and their alloy exhibit creep at normal temperature.
Creep Curve: Creep curve is plotted after creep test, in this test the specimen is heated in an electric heater at definite temperature and constant load is applied on the specimen.
The strain produce in the specimen is measured by strain gauge and a curve is plotted between time and strain i.e. known as creep curve.
The creep curve is plotted as the strain or elongation of the test material under strain of constant loading the curve can be divided into three stages.
(1) Primary Stage: In this stage very low increase in creep rate due to its elastic after effect and the starting of deformation process producing work hardening.
(2) Secondary Stage: In the second stage creep rate is continuously increase in a steady rate during this process there will be a balance between the rate of work hardening and rate of softening because of recovery or Recrystallization.
(3) Third Stage: In this stage stress rate is sharply increases due to internal crack formation.
Factors Effecting Creep:
Following factors affect the creep:
(1) Load: Creep strain is directly proportional to the applied load.
(2) Grain Size: Fine grains have higher creep resistance at low Recrystallization temperature whereas coarse grains have creep resistance at high crystallization temperature.
(3) Strain Hardening: Creep produced by applied load depends on yielding of material and strain hardening yielding rate increase is due to strain hardening.
(4) Heat Treatment: The maximum creep resistance can be obtained in normalising process and minimum creep resistance by quenching and drawing.
(5) Effect of Alloying Element: Creep resistance can be improved by adding Nickel, Chromium, and Manganese etc.
Mechanism of Creep: Basically four mechanisms are used to describe the creep these are:
(1) Sliding of Grain Boundaries: In the polycrystalline material the grain boundaries slide past each other creeping vacancies. After sliding a small distance at the boundary interface, the movements can be arrested by some other dislocations. This process weakens the crystals structure and producing a creep.
(2) Dislocation Climb: At higher temperature atomic movement case dislocations to climb to another plane otherwise known as jog thus are relieving the piped up dislocation. The diffusion rate of vacancies may produce a motion in response to the applied stress. The climb dislocation gives way for all slip dislocation to enhance this deformation by slip continues as creep.
(3) Vacancy Diffusion: The migration of vacancies from one side of a grain to another side, vacancy move in response to the applied stress in the directions. The diffusion of vacancy controls the creep rate but does not involve the climb of edge dislocation.
(4) Slip dislocation: When stresses are applied continuously at constant temperature dislocation goes on increasing and thermal energy activate slip resulting in creep.