Mumbai University > Mechanical Engineering > Sem 4 > Material Technology

Marks: 5M

Year: May 2015

Under many service conditions, materials are required to sustain steady loads for long periods of time and different temperature conditions (e.g.) furnace parts, bladed of a turbine rotor, timber beams in the roof of a building, filament in vacuum tube. In these cases time-dependent deformations can grow large and result in fracture of the member without any increase in load.

Creep can be defined in the following ways:

• Time dependent strain occurring under stress.
• Process of which plastic flow occurs when a constant stress is applied to a metal for a prolonged period of time.
• Viscous flow in metals involving applied stress, time, and temperature.
• It takes place and lead to fracture at static stresses much smaller than those which will break the specimen when loaded quickly.

Creep Curve:

• It is plotted between the % of elongation/ strain and the time for the entire duration of test.

1. Primary creep/ Transient creep:

• It is a decreasing creep rate because of the work hardening process resulting from deformation. It is similar in its mechanism to delayed elasticity. It is recoverable one.

2. Secondary creep/ steady state creep:

• The deformation continuous at an approximately constant rate. It may be essentially viscous (or) plastic in character, depends upon stress level & temperature. In this stage, a balance exists between rate of work hardening and rate of softening because of recovery (or) recrystallisation.

3. Tertiary Creep:

• If the stress is sufficiently high & temperature also high there is a tertiary stage in which the creep rate accrderates until fracture occurs. It is because of structural changes occurring in the metal.