Learning topic
Creep of Materials
Creep of materials: the three stages of the creep curve, creep rate, effects of temperature and stress, long-term strength, and stress relaxation.
Creep is the time-dependent deformation of a material under a sustained load. Unlike instantaneous elastic deformation, creep strain develops gradually even when the load remains constant.
When creep becomes important
The intensity of creep depends on the material, temperature, stress level, and duration of loading. For metals, creep becomes particularly significant at elevated temperatures. For polymers and some other materials, time-dependent deformation may also be noticeable at ordinary temperatures.
Creep testing
In a typical creep test, a specimen is held for a long time under specified load and temperature, while strain $\varepsilon$ is recorded as a function of time $t$. The resulting relationship is called the creep curve.
Creep Curve: Strain vs Time
Show strain as a function of time and how the strain rate changes through the three stages of creep.
Stages of creep
- Primary creep. The strain rate decreases with time.
- Secondary, or steady-state, creep. The strain rate is approximately constant.
- Tertiary creep. The strain rate increases due to accumulated damage; this stage ends in rupture.
The creep rate is characterized by the derivative $\dot{\varepsilon}=d\varepsilon/dt$.
Effect of temperature and stress
As temperature and stress level increase, creep generally accelerates and the time to rupture decreases.
Long-term strength
Long-term strength characterizes a material’s ability to resist rupture for a specified time at a given temperature. Long-term design must also limit accumulated deformation and prevent failure during the required service life.
Stress relaxation
Creep is related to stress relaxation. If the total deformation of a component is kept constant, the stress in the material may decrease with time.
Engineering example
Superheated-steam pipelines at thermal power plants operate for years under the combined action of high temperature and internal pressure. The pressure produces mechanical stresses in the pipe wall, while the high temperature makes creep of the metal significant.
Even if the pressure and temperature remain nearly constant during a given operating regime, deformation of the metal continues to accumulate gradually. At an early stage, the creep rate decreases; then, during a substantial part of the service life, it remains relatively constant. Over time, damage such as microvoids and microcracks may accumulate in the material. Tertiary creep then begins: deformation accelerates and can eventually lead to pipe rupture.
For such equipment, checking only the instantaneous strength condition $\sigma < [\sigma]$ is therefore insufficient. The engineer must account for temperature, stress, and the duration of their action when assessing long-term strength and the remaining service life of the material. The practical conclusion is that a component may safely carry a given load today, but this does not mean it can safely carry the same load for tens of thousands of hours at high temperature.
What you should be able to do
After studying this topic, a student should be able to explain the physical meaning of creep, distinguish the three stages of the creep curve, determine creep rate, explain the effects of temperature and stress, distinguish creep from stress relaxation, and understand the need for long-term design calculations.
About this topic
Creep of materials is time-dependent deformation under sustained loading. This topic covers the creep curve and its three stages, creep rate, the effects of temperature and stress, long-term strength, and stress relaxation.