Industrial Engineering

Understanding creep in a material

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. 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 common instances of creep?
  • How to measure a creep strength?
  • How to minimize or avoid creep deformation?

Now let’s dive in!

creep material

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What is creep in a material?

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 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.

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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 or Stage I
  • Secondary or Stage II
  • Tertiary or Stage III

 

Primary or Stage I

During the deformation process, primary creep first occurs. Elastic deformation has just started at this point. Atomic bond stretching results in temporary elastic deformation. Permanent plastic deformation begins to occur after the elastic deformation. This distortion starts more quickly during the primary creep stage and then slows down over time. Work hardening is the cause of the decrease in creep rate that happens near the end of the primary creep stage.

Secondary or Stage II

Once the strain rate starts to stabilize and become constant, secondary creep starts. When compared to the first stage and the third stage of creep, the strain during secondary creep happens very slowly. Since there hasn’t been any microstructural damage, the creep rate is steady and quite slow.

Tertiary or Stage III

The process of creep deformation ends with tertiary creep. Once the metal’s microstructure is damaged, this step of the creep process starts. As the microstructure continues to deteriorate more and more, the strain rate increases. The metal eventually fractures and fails once enough microstructural voids have been produced.

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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.

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.

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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.

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What are the common 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.

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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 on 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.

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How to minimize or avoid creep deformation?

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).

 

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In summary

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 common 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!