Creep deformation

Creep is the tendency of materials to deform permanently.  Creep can occur at ambient temperature or at temperatures below the melting point.  The deformation can happen when the material is not loaded.  For instance, a pipe left lying can deform even without any load. 

Creep increases as the temperature increases.  Hence, the study of creep is important for components which will operate at high temperatures.

Soft metals such as lead, aluminium or solder can creep even at room temperatures.  Material such as tungsten can resist creep deformation even at high temperatures.  As a rule of thumb, creep can occur at around 35 percent of the melting point for metals. 

Creep deformation can be classified into three stages

Stage 1
This is the primary stage.  At this stage, the strain rate or the deformation rate is very high.  The strain rate slows gradually and stabilizes.

Stage 2
At this stage, the strain rate stabilizes and is almost constant. 

Stage 3
In this stage, the strain rate again increases.  This is due to failure processes such as necking or the formation of cracks.  The material is permanently deformed at this stage. 

Creep can be prevented by choosing materials which have higher melting point.  Special alloys which resist creep can be chosen for critical applications.  Materials with bigger grain size will also have lesser creep.