Time Dependent Stresses: Creep

         

        If the stress in a material exceeds the yield point, the strain caused in the material by the application of load does not disappear totally on the removal of the load. The plastic deformation caused to the material is known as Creep. At high temperature, the strain due to creep is quite appreciable.

         Creep is the property by which, at elevated temperatures, metals continue to elongate continuously under a constant stress. The stress may be much less than the ultimate tensile stress. Creep is measured by the rate of strain per hour at a certain temperature under a given stress. When a tensile test is carried out in a shorter period of time on a meter specimen at some specific temperature, a definite value of ultimate tensile stress can be obtained. However, under creep a material can also fail even at lower stress values but at higher temperatures if enough time is allowed to pass. It is also observed that there is limiting stress value below which creep does not take place at a certain temperature and the material does not fracture even if the stress is applied for indefinite time. Thus, it becomes necessary that the materials which are exposed to high temperatures for longer periods at certain stress values, the design must be based on limiting creep stress.

        To determine the limiting creep stress requires very long periods which may be months or even years for investigations. It is also observed that the rate of creep diminishes with time. Thus, to have a fair value of limiting creep stress at a certain temperature, effort is made to find the stress at which, after a shorter period of time, a definite very small rate of creep takes place. Usually, a typical stress value is obtained which causes a creep of 1 millionth per hour after 40 days.         

       Some creep resistant alloy steels have been developed by adding small percentages of molybdenum, cobalt, vanadium and tungsten for high-temperature applications such as gas turbines and high-pressure steam fitting.

Classical Creep Curve:

The Classical creep curve is shown in following diagram.

The rate of deformation is called the creep rate. It is the slope of the line in a Creep Strain vs. Time curve.

As per the diagram, Creep has following three stages;

Primary Creep: starts at a rapid rate and slows with time.

Secondary Creep: has a relatively uniform rate.

Tertiary Creep: has an accelerated creep rate and terminates when the material breaks or ruptures. It is associated with both necking and formation of grain boundary voids.