Creep in materials is a slow continuos process where the material slowly deforms over time.
To creep the material has to be under load - temperature only accelerates the process. Creep will occur under any load - the stress does not have to be over the yield strength of the material.
A common example of creep is light bulbs - when lit, the tungsten filament heats up to around 2000 degrees C. While this is way below the melting point of tungsten (above 3000 deg C), it is hot enough to initiate creeping in the material. At this temperature it will slowly bend downwards under the force of gravity, and permanently elongate. When it reaches it's maximum elongation, it breaks off - and the light goes out.
The critical temperature when creep starts is typically 30-40% of the melting temperature.
Therefore, ice creeps rapidly even at very cold temperatures (-30 C) this is because -30 C is really 88% of the melting temperature of ice. At room temperature we also see that lead is at 50% of melting temperature, which explains why lead pipes deform with age.
There are two main consequences of creep:
- At constant load, the material deforms over time. This is typical with glaciers, turbines, and structures. The material is subject to the same load (most often plain old gravity), and therefore will bend permanentely - given enough time.
- At constant displacement, the load will decrease. Bolts and other pre-tensioned fasteners are often affected by this. The bolts doesn't go anywhere, but they get longer - and therefore relaxes. Bolts on engine blocks needs tightening from time to time. A funny thing here is that the more you tighten the bolt, the quicker it will creep, and the faster it will need re-tightening....
Final fracture occurs when the creep has elongated the defects inside the material so much that the rest material cannot handle the load. This is why most turbine blades are made out of a single grain. A single grain structure minimizes the defects inside the material, and it becomes much less suceptible to creep fracture.
To avoid creep you need to select materials with a high melting point. As stated, the working temperature needs to be under 30% of the melting temperature. If this cannot be avoided, one should alloy the material to make it less creepy (the technical term is to maximise obstructions to dislocation motion...).
As a quick glance in the theory behind creep, here is the formula for steady state creep (power-law creep):
εss = B · σ n
ε = Strain (Elongation)
ss = steady state
B = a material constant
σ = Stress in material
n = A material constant - usually between 3 and 8.
A temperature dependent equation for creep can be found in Arrhenius's Law.