Creep Deformation

What is Creep Deformation

Creep deformation is a type of plastic deformation that occurs when materials are subjected to continual stress over a long period of time. This phenomenon can happen if high loads are applied for extended periods that are still below the yield point of the material. Creep can be observed in many materials, including metals, plastics, and rocks.

Creep deformation occurs due to a combination of several mechanisms, including dislocation motion, grain boundary sliding, and diffusion. The rate and extent of creep deformation can be influenced by a number of factors, including temperature, stress level, time, and material microstructure.

One example of creep deformation can be seen in metals such as steel when they are used in applications where they are constantly exposed to excessive heat or pressure for long periods of time. For instance, turbine blades used in jet engines experience temperatures up to 1600°C, which can cause them to gradually deform over time until they break down completely.

Another example is plastic pipes used for plumbing; even though these pipes may not be under constant stress like turbine blades, their molecular structure will still slowly shift over time due to thermal cycling from hot water running through them repeatedly.

Stages of Creep Deformation

Creep deformation typically progresses through three stages known as primary, secondary, and tertiary creep. The behavior of materials during each stage can be described as follows:

Primary Creep: During primary creep, the deformation rate is relatively high and decreases over time. This is because the material is undergoing internal changes to adjust to the applied stress, such as the movement of dislocations or the rearrangement of grain boundaries. Primary creep is often referred to as the “transient” stage of creep deformation, as it is characterized by significant changes in deformation behavior.
Secondary Creep: In the secondary creep stage, the deformation rate becomes relatively constant and continues at a steady rate over time. This stage can last for an extended period and is often referred to as the “steady-state” stage of creep deformation. During secondary creep, the material undergoes a balance of deformation and recovery processes, which can include dislocation motion, grain boundary sliding, and diffusion.
Tertiary Creep: In the tertiary creep stage, the deformation rate begins to increase rapidly until the material ultimately fails. Tertiary creep is often associated with the development of microstructural defects or damage within the material, such as voids, cracks, or cavities. These defects can lead to a significant decrease in the material’s strength and contribute to the accelerated deformation seen during tertiary creep.

The progression through these stages can vary depending on a variety of factors, including the material’s composition, temperature, stress level, and time under load. Understanding the behavior of materials during creep deformation is important for predicting their long-term performance and ensuring the safety and reliability of engineering systems that operate under high temperatures or stresses.

Creep Deformation Mechanisms

There are several mechanisms by which creep deformation can occur in materials. These mechanisms are related to the movement of atoms, dislocations, and other defects within the material. The main creep mechanisms are:

Dislocation creep is one of the most common mechanisms of creep deformation in metals and alloys. In this mechanism, the movement of dislocations through the crystal lattice causes the material to deform over time.
Diffusional creep is a mechanism that occurs in materials where the movement of atoms through the crystal lattice is the primary source of deformation. In this mechanism, atoms move from areas of high stress to areas of low stress, causing the material to deform over time.
Coble creep is a type of diffusional creep that occurs in materials where the grain size is small. In this mechanism, atoms move through the grain boundaries, causing the material to deform over time.
Nabarro-Herring creep is a type of diffusional creep similar to Coble creep, except that the movement of atoms occurs within the interior of the grains rather than in the grain boundaries.

Effect of Temperature and Stress on Creep Deformation

Diagram showing the creep strain vs time curve for a typical material at low vs medium vs high temperature and stress values

An important aspect to consider when analyzing creep is the surrounding environment’s conditions and their effect on creep rate. Two of the most important factors that affect creep deformation in materials are Temperature and stress.

Temperature: Increasing temperature has a significant effect on the rate and extent of creep deformation in materials. This is because higher temperatures increase the mobility of atoms and dislocations within the material, making it easier for deformation mechanisms such as dislocation motion, grain boundary sliding, and diffusion to occur. As a result, materials exhibit a greater tendency to undergo creep deformation at higher temperatures.
Different materials have different temperature ranges where creep deformation becomes significant, and the temperature at which creep deformation becomes important is often characterized by a parameter called the creep activation energy. This parameter quantifies the energy required for the material to undergo creep deformation, and can be used to predict the behavior of the material under different temperature and stress conditions.
Stress: The stress applied to a material during creep deformation also plays a significant role in determining the rate and extent of deformation. As stress levels increase, the rate of deformation also increases, with higher stresses leading to more rapid deformation.
However, there is a limit to the amount of stress that a material can sustain before failure occurs. This limit is known as the creep rupture strength and is often characterized by a parameter called the creep rupture life. This parameter quantifies the time that the material can sustain a given stress level before failure occurs, and is an important consideration in the design of engineering components that are subject to high-temperature and high-stress conditions.

Understanding the effects of temperature and stress on creep deformation is essential for predicting the long-term performance of materials in high-temperature and high-stress environments and for developing new materials with improved creep resistance.

Design Considerations to Avoid Creep

Creep is a significant factor to consider during the design phase of products and structural components, as it can lead to costly mistakes. Accurately predicting and accounting for creep in the design process entails analyzing the environmental and stress conditions that may influence creep, evaluating the quality of the raw materials utilized, and assessing any manufacturing-related factors that could affect creep behavior.

In general, there are three methods of preventing creep in materials:

Reduce the effect of grain boundaries by: a) Use single crystal material with large grains. b) Addition of solid solutions to eliminate vacancies.
Use materials with high melting temperatures.
Consult Creep Test Data during materials selection by checking the type of service application and setting adequate inspection intervals according to life expectancy.

What causes creep?: Creep deformation is caused by the movement of dislocations and the diffusion of atoms through the crystal lattice of a material. The rate of dislocation movement and atomic diffusion increases with stress and temperature, leading to accelerated creep deformation.

What is creep rate?: Creep rate is the rate of creep or slow-gradual displacement caused by a constant applied force over time. It can be found by taking the slope (strain / time) of the creep strain versus time curve.

What factors affect creep rate?: The rate of creep deformation is affected by several factors, including stress, temperature, and material properties such as grain size and dislocation density.

What are the stages of creep deformation?: The stages of creep deformation are primary, secondary, and tertiary. During the primary stage, the strain rate is high and decreases over time. In the secondary stage, the strain rate becomes more constant and continues at a steady rate over time. The tertiary stage is characterized by a rapid acceleration of creep deformation leading to failure, often due to necking, void formation, or material instability.