Creep, sometimes known as cold flow, is the tendency of a solid material to move slowly or undergo irreversible deformation when subjected to ongoing mechanical loads. It can happen as a result of prolonged exposure to high-stress levels that are still below the material’s yield strength. Long-term heat exposure makes creep more severe, and it often gets worse as materials get closer to their melting point.
When examining components that operate under high loads or high temperatures, engineers and metallurgists typically have concerns about creep. A deformation mechanism called creep may or may not be a failure mode. In this article, the answers to the following questions will be answered:
- What is creep in a material?
- What are the stages of material creep?
- What is creep strength?
- What are the types of creep deformation?
- What are the instances of creep?
- How to measure a creep strength?
- How to minimize or avoid creep deformation?
Now let’s dive in!
Contents
What is creep in a material?
Just as earlier mentioned, creep, which is sometimes known as cold flow, is the tendency of a solid material to move slowly or undergo irreversible deformation when subjected to ongoing mechanical loads.
It can happen as a result of prolonged exposure to high-stress levels that are still below the material’s yield strength. Long-term heat exposure makes creep more severe, and it often gets worse as materials get closer to the melting point. The qualities of the material, exposure period, exposure temperature, and the applied structural load all affect how quickly a material deforms.
Based on the intensity of the applied stress and how long it lasts, the deformation may increase to the point where a component can no longer serve its purpose. For instance, the creep of a turbine blade may cause the blade to come into contact with the casing and fail. When examining components that operate under high loads or high temperatures, engineers and metallurgists typically have concerns about creep. A deformation mechanism called creep may or may not be a failure mode.
For instance, some concrete engineers like modest creep because it reduces tensile strains that could otherwise induce cracking. Creep deformation doesn’t happen instantly when stress is applied, unlike a brittle fracture. Instead, persistent stress leads to strain accumulation. Creep is a “time-dependent” deformation as a result.
What are the stages of material creep?
Creep is a kind of metal deformation that takes place at loads below a metal’s yield strength, usually at high temperatures. There are three phases to creep:
Primary Creep, also known as Stage I or Initial Creep, starts the tests and is typically transient rather than steady. Up until Stage II, there was resistance to creep. The level of creep is essentially consistent during Stage II, often known as Secondary Creep. A steady-state creep is a name given to this phase. Stage III, also known as Tertiary Creep, is characterized by an acceleration in the creeping rate when the specimen’s cross-section area reduces as a result of nesting or internal nesting and its useful size shrinks. If stage III is allowed to advance, the fracture will take place. The creep test is typically useful to establish the stage II minimum creep rate. When developing systems, engineers must take this anticipated distortion into consideration.
What is creep strength?
The material responds differently when subjected to high immediate stress or sustained stress over an extended period. When a material is continuously mechanically strained, it appears to move slowly or permanently deform. In addition, creep is a time-dependent deformation under a certain applied load. Generally occurs at high temperatures (thermal creep), but can also happen at room temperature in certain materials (e.g. lead or glass), albeit much slower. As a result, the material undergoes a time-dependent increase in length, which could be dangerous while in service.
The Crawl is the name for this innate tendency. Temperature, time, stress, and alloy composition are some of the factors that affect the beginning and development of creep in a material. The creep deformation rate is the name given to the slipping percentage. Creep needs to learn about many engineering applications, particularly those that deal with high temperatures and stresses. A few instances of creeping impact in steam lines, spaceships, and turbines are disk and blade.
The creep limit, also known as the creep strength, measures how well a material can withstand creep. Stress, in particular, refers to the external factors that cause a steady creep rate. It implies that the highest stress the material has undergone without significantly deforming for a given period is what causes crack resistance.
What are the types of creep deformation?
Dislocation creeps, diffusion creep (bulk diffusion or grain boundary diffusion), dislocation climb-glide creep, and thermally triggered glide creep are a few examples of creep deformation. These many creep mechanisms are all dependent on the temperature at which the material is deforming, the amount of stress the material is under, and the microstructure and composition of the material.
Continuously welded rail heated by direct sunlight, for example, can buckle on a railroad track. This is brought on by the steel’s growing tension and the ensuing creep. Under moderate creep, concrete may fracture, but this is occasionally advantageous since it helps lower tensile strains in the structure. Constant stress on polymers results in a time-dependent strain increase process called viscoelastic creep.
What are the instances of creep?
More often than not, creep can be noticed in some applications. Because of their low static loads and low operating temperatures, automotive frames, for example, are more focused on impact strength. On the other side, if the wrong material is chosen, certain car engine components exposed to high loads and temperatures from engine combustion may undergo creep.
Applications with high heat and extreme stress are frequently prone to creep. Examples include the production of nuclear energy, the parts of industrial engines, heated metal filaments, the parts of jet engines, and pressurized high-temperature pipes.
How to measure a creep strength?
A creep-testing machine, a tool that gauges a material’s distortion under various stresses, is used to assess creep strength. With temperature or loading as the variables, it can be used to plot how much stress and strain a material can withstand. The 3 unique stages of creep—primary creep, steady-state creep, and tertiary creep—are displayed in the following graph.
The temperature and time interval for each step of creep can be determined from the graph. Thus, the graph’s tertiary creep stage can be used to determine the creep strength or creep limit. To minimize the effects of thermal expansion, it is essential to regulate the temperature of the chamber where the creep test is conducted.
How to reduce or avoid creep deformation?
Creep’s effects can be avoided or lessened via a variety of techniques. Lowering the operating temperature of the metal or material being used is one technique to reduce creep, though this is not always possible. The constant load that the metal must withstand can be reduced as an alternative, but again, this may not be possible depending on the application. Because there is less grain boundary sliding when metal with large grains is used, creep can be reduced. By removing microstructural voids, certain metals with appropriate alloying element additions can prevent creep.
It is now obvious that creep deformation is typically a bad thing. Certain design considerations can be made to lessen its impact or prevent it from occurring, some of which include:
- Utilize single-crystal materials with big grains to reduce grain boundary sliding and add solid solutions to eliminate microstructural voids.
- Utilize substances with a high melting point.
- Use face-center cubic (FCC) metals rather than body-center cubic (BCC) metals to reduce diffusivity because of their lower diffusion coefficients.
- Utilize materials with a high shear modulus or practical alloying.
- Lower the material’s operating temperature while using it (application-specific).
Watch the video below to learn more about creep in metals:
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Conclusion
It is clear now that creep deformation is generally an undesired phenomenon. To reduce its effect or prevent it from happening, the above step should be followed. In materials science, creep is the tendency of a solid material to move slowly or deform permanently under the influence of persistent mechanical stresses.
That is all for this article, where the following questions are being answered:
- What is creep in a material?
- What are the stages of material creep?
- What is creep strength?
- What are the types of creep deformation?
- What are the instances of creep?
- How to measure a creep strength?
- How to minimize or avoid creep deformation?
I hope you learn a lot from the reading, if so, kindly share with others thanks for reading, and see you around!
