What is Creep?
In materials science, creep (sometimes called cold flow) is the tendency of a solid material to move slowly or deform permanently under the influence of persistent mechanical stresses.
It can occur as a result of long-term exposure to high levels of stress that are still below the yield strength of the material. Creep is more severe in materials that are subjected to heat for long periods and generally increases as they near their melting point.
The rate of deformation is a function of the material’s properties, exposure time, exposure temperature, and the applied structural load.
Depending on the magnitude of the applied stress and its duration, the deformation may become so large that a component can no longer perform its function, for example, creep of a turbine blade could cause the blade to contact the casing, resulting in the failure of the blade.
Creep is usually of concern to engineers and metallurgists when evaluating components that operate under high stresses or high temperatures. Creep is a deformation mechanism that may or may not constitute a failure mode.
For example, moderate creep in concrete is sometimes welcomed because it relieves tensile stresses that might otherwise lead to cracking.
Unlike a brittle fracture, creep deformation does not occur suddenly upon the application of stress. Instead, strain accumulates as a result of long-term stress. Therefore, creep is a “time-dependent” deformation.
Stages of Creep
Creep is a type of metal deformation that occurs at stresses below the yield strength of a metal, generally at elevated temperatures. Creep occurs in three stages:
- Primary or Stage I
- Secondary or Stage II
- Tertiary or Stage III
Stage 1: Primary Creep
Primary creep occurs first during the deformation process. At this stage, elastic deformation is initialized. Elastic deformation occurs from atomic bond stretching and is not permanent. Following the elastic deformation, permanent plastic deformation starts to take place.
During the primary creep stage, this deformation occurs more rapidly at first and then slows with time. The reduction in the creep rate that occurs near the end of the primary creep stage is due to work hardening.
Stage 2: Secondary Creep
Secondary creep begins once the strain rate begins to stabilize and becomes constant. The strain during secondary creep occurs relatively slowly when compared to the first stage and the third stage of creep. The creep rate remains constant and relatively slow because no microstructural damage has taken place yet.
Stage 3: Tertiary Creep
Tertiary creep is the final phase of the creep deformation process. This stage of the creep process begins once damage to the microstructure of the metal takes place. The strain rate accelerates as more and more deterioration of the microstructure continues to happen. After enough microstructural voids have been created, the metal eventually fractures and fails completely.
What is creep strength?
When exposed to high instantaneous stress or constant stress for a certain period, the material behaves differently. It appears to move slowly or to deform permanently when the material is under continuous mechanical strain.
This inherent propensity is known as the Crawl. The introduction and development of Creep in a material involve various variables, including temperature, time, stress, and composition of alloys. The slipping percentage is called the Creep deformation rate.
Creep must study various engineering applications, especially high-temperature and stress-related applications. Disk & blade are just a few examples in turbine, spacecraft, and steam lines of creeping impact.
The Creep’s strength, also called the creep limit, tests the material’s resistance to the Creep. The environmental conditions that result in a constant creep rate are known as stress in particular. It means that crack resistance is the most incredible stress the material has experienced without significant deformation for a specific time.
Types of creep deformation
There are various types of creep deformation, including dislocation creep, diffusion creep (bulk diffusion or grain boundary diffusion), dislocation climb-glide creep, and thermally activated glide creep.
All these different creep mechanisms depend on the temperature at which deformation is happening, the stress level experienced by the material, and the material’s microstructure and composition.
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.
Polymers, when exposed to constant stress, undergo a time-dependent strain growth, commonly known as viscoelastic creep.
Common Instances of Creep
Creep is commonly found in some applications more than others. For instance, automobile frames are designed more with impact strength in mind since their static loads are small and normal operating temperatures are low. On the other hand, certain automobile engine components subjected to high loads and temperatures from engine combustion may experience creep if the right material is not selected.
Typically, applications that have high heat and high stress can be susceptible to creep. Examples include nuclear power generation, industrial engine components, heated metal filaments, jet engine components, and pressurized high-temperature piping.
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 the 3 distinct stages of creep, primary creep, steady-state creep, and tertiary creep.
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.
It is crucial to control the temperature of the chamber within which the creep test is performed in order to keep the effects of thermal expansion minimal.
How to minimize or avoid creep deformation?
It is clear now that creep deformation is generally an undesired phenomenon. In order to reduce its effect or prevent it from happening, certain design considerations can be taken into consideration, some of which are:
- Minimize grain boundary sliding by using single-crystal materials with large grains and remove microstructural vacancies by adding solid solutions
- Use materials with high melting points
- Minimize diffusivity by using face-center cubic (FCC) metals rather than body-center cubic (BCC) metals due to their lower diffusion coefficients
- Use materials with high shear modulus or make use of convenient alloying
- Reduce the working temperature at which the material is utilized (application-specific)