Creep strength (also known as creep limit) is a measurement of the resistance to creep that a material exhibits.
Before examining creep strength in detail, it’s important to understand exactly what creep is, how it occurs and the problems that creep deformation may cause.
What is creep?
Creep is the natural tendency of a material to gradually move or permanently deform as a result of mechanical stress.
When materials are subject to increasingly high stress levels over a long period, but not exceeding the yield strength, creep can become severe. This especially applies to materials that are frequently exposed to high heat and can even permanently deform materials as temperatures reach the melting point.
Types of creep deformation
The type of deformation depends on the material and structure. On a railway track for instance, continuously welded rail heated in direct sunshine can buckle. This is caused by increasing stress in the steel and the resultant creep. Concrete may crack under moderate levels of creep, but sometimes this is desirable as it can reduce tensile stresses in the structure.
Creep strength in alloys
Alloys, particularly aluminium alloys, are prone to creep-fatigue and fracture, due to their low melting point. However, there are other factors that affect the creep strength of alloys, as follows:
- Substructure composition – a stable substructure that has uniform dislocations provides resistance to creep-fatigue. However, a substructure that coarsens under strain will result in reduced creep strength. Nickel-base alloys are a good example of this phenomenon. A fine grain size within the structure can also increase creep strength.
- Heat treatment – heat treatment leads to the creation of austenites. Some of these, such as bainite, help to increase creep strength.
- Solid solution strengthening – lessens movement and deformation through the lattice structure. E.g. molybdenum increases creep strength significantly.
- Hard precipitate quantity and dispersion – carbides and intermetallics such as VC (Vanadium Carbide), when dispersed finely and evenly in an alloy reduce movement and creep.
Therefore, the creep strength inherent in an alloy differs considerably depending on the chemical composition, the spacing of particles in the material, and the treatment or strengthening of the metal.
Measurement of creep strength
Creep strength is measured using a creep-testing machine; a device that measures the distortion of a material at various stress levels. It can be used to plot how much stress and strain a material can take, with temperature or loading as variables.
The resulting graph will show three distinct stages of creep:
- Primary creep – creep which rises rapidly at first over a short space of time, but then slowly decreases.
- Steady-state creep – a constant rise in creep, at a less rapid pace than the primary stage, shown as a straight line on the graph.
- Tertiary creep – the final stage of creep, as the material reaches its fracture point. This is normally a very steep upward curve on the graph as the stress and creep escalate rapidly towards breaking point.
The graph can be assessed to pinpoint the temperature and time interval for the various stages of creep. The creep strength or creep limit can thus be ascertained from the tertiary creep stage of the graph.