Creep in materials
Creep is a phenomenon in materials science and engineering, where materials deform gradually over time when subjected to sustained mechanical stress. Researchers and engineers are intrigued by this subtle yet impactful process, as it can compromise the structural integrity and long-term performance of materials.
What is creep in materials ?
Creep leads to permanent deformation or even failure if not properly considered in the design and use of materials.
Creep becomes more pronounced as the temperature of a material increases, typically above 40% of the material's melting temperature in degrees Kelvin or when temperature > 0.3 - 0.4 Tm.
Creep occurs under a constant load or stress, which distinguishes it from other forms of deformation, such as elastic or plastic deformation, which occur under changing loads or stresses.
Creep deformation is time-dependent, meaning it occurs gradually over an extended period. It doesn't happen instantly when a load is applied but evolves over time.
Note that :
- Temperature is a factor that affect on the creep in materials.
- Higher temperatures accelerate the creep process.
- Creep is permanent deformation (plastic deformation).
- The stress at which the creep occurs is below the yield strength of the material.
- Creep deformation does not occur suddenly.
Not all materials exhibit the same degree of creep. Materials like metals, ceramics, and some polymers are often chosen for their ability to resist creep in specific applications. The creep resistance of a material depends on its composition, microstructure, and thermal properties.
Engineers use various testing methods to assess and characterize a material's creep behavior. These tests involve subjecting a specimen to a constant load or stress at a specific temperature and monitoring its deformation over time. Creep tests provide data on the material's creep rate, which is the rate of deformation with respect to time.
This phenomenon is particularly important in engineering and materials science, especially in applications where materials are exposed to high temperatures for extended periods.
Creep behavior is often represented by creep curve, which show the relationship between strain (deformation) and time under constant load and temperature conditions. Creep curves typically exhibit three stages: primary creep, secondary creep, and tertiary creep, each with distinct deformation characteristics.
- Primary Creep (Transient stage): is the stage of what is called unstable. In this initial stage, the material experiences rapid deformation, but the creep rate or deformation rate decreases over time.
- Secondary Creep (Steady stage): This stage is characterized by a relatively constant creep rate, often described by a linear or power-law relationship between strain and time.
- Tertiary Creep (Fracture stage): Eventually, the creep rate accelerates, leading to material failure or rupture. This third stage is, as a rule, short and should be avoided, since quick failure.
Creep deformation can occur through various mechanisms, depending on the material's composition and structure. Common mechanisms include dislocation motion, grain boundary sliding, and diffusion-controlled processes.
Primary Creep
This is the initial stage which also called transient creep. It is unstable stage where the creep rate decreases with time and the effect of work hardening is more than that of the recovery process. It typically involves the accommodation of stress through the rearrangement of dislocations and other mechanisms. This stage is mainly due to dislocation movement.
Secondary Creep
Secondary creep, also called steady stage is characterized by a relatively constant creep rate. During this stage, dislocation glide, grain boundary sliding, or other mechanisms continue at a steady rate, resulting in a linear or power-law creep behavior.
The rates of work hardening and recovery during this stage are equal, so the material creeps at a steady rate. Depending upon the state level and temperature, steady state creep may be essentially viscous or plastic in character. Structural observations reveals that polygonization is an important recovery process during secondary creep.
Note that:
- The minimum creep rate is in the steady stage.
Tertiary Creep
The specific type of creep that predominates in a material depends on factors such as temperature, stress level, microstructure, and the nature of the material itself. Engineers and materials scientists study these different types of creep to better understand the behavior of materials under various conditions, which is crucial for designing reliable components and structures for high-temperature and long-duration applications.
Effect of the stress and temperature on the creep curve
The development of each stage of creep depends on the temperature and stress. For the same stress, an increase of temperature shortens the time of the second stage and accelerates failure. An increase of stress at the constant temperature has a similar effect.
Types of creep in materials
Creep in materials can be categorized into several types based on the nature of the deformation and the underlying mechanisms. The primary types of creep in materials are logarithmic creep, recovery creep, dislocation creep, diffusion creep, coble creep, and power law creep.
At low temperature the creep rate usually decreases with time and one obtains a logarithmic creep curve.
At higher temperatures in the range of 0.5 to 0.7 of TmK (melting point temperature on the absolute scale), the influence of work hardening is weakened and there is a possibility of mechanical recovery.
Dislocation Creep:
Dislocation creep is one of the most common types of creep in crystalline materials, particularly metals and alloys. It occurs when dislocations, which are defects in the crystal lattice structure, move through the material in response to an applied stress at elevated temperatures. The motion of dislocations allows the material to slowly deform over time.
Diffusion Creep:
Diffusion creep, also known as Nabarro-Herring creep, involves the movement of atoms or molecules within the material. At elevated temperatures, beyond 0.7 TmK these atoms or molecules diffuse through the crystal lattice, leading to a gradual change in the material's shape. Diffusion creep is more pronounced at higher temperatures.
Coble Creep:
Coble creep is a type of grain-boundary diffusion creep that occurs in polycrystalline materials. It involves the sliding or shearing of grain boundaries, which are interfaces between individual crystal grains. Coble creep can significantly affect the overall deformation behavior of materials, especially ceramics.
Power-Law Creep:
Power-law creep, also known as steady-state creep, is characterized by a relatively constant creep rate over time. This type of creep is described by a power-law relationship between strain rate and stress, often expressed as ε ∝ σ^n, where ε is the strain rate, σ is the applied stress, and n is a constant exponent.
Factors affecting creep
The most important factors affect the creep in materials are grain size, thermal stability of the microstructure of alloys, and the purity of metals.
The major factor in creep is grain size. Usually, coarse-grained materials exhibit better creep resistance than fine-grained ones, since the latter have a great amount of grain boundary materials and grain boundaries behave as a quasi-viscous material with a high tendency to flow at elevated temperatures.
The thermal stability of the microstructure of alloys and its resistance to oxidation at high temperatures is another important factor. An annealed specimen for having greater thermal stabilities is far superior in its creep resistance to a quenched steel for its poor thermal stability.
Pure metals with high melting points and a compact atomic structure generally exhibit good creep resistance at high temperatures. By alloying the pure metals with suitable elements, the creep resistance can be increased considerably.
Creep in materials examples
High-Temperature Alloys in Jet Engines:
Jet engines operate at extremely high temperatures. The materials used in their construction, such as nickel-based superalloys, are specifically designed to withstand these conditions. Over time, even these high-performance materials can experience creep deformation, which must be carefully considered in engine design.
Boiler Tubes in Power Plants:
Power plants generate steam at high temperatures and pressures to produce electricity. Boiler tubes, which are often made of materials like stainless steel, must resist creep to maintain structural integrity over the long term.
Welded Joints in Pressure Vessels:
Pressure vessels used in the chemical, petrochemical, and nuclear industries are exposed to elevated temperatures and pressures. Creep in welded joints can lead to deformation or cracking, making creep-resistant materials and proper welding techniques critical.
Aerospace Components:
Aircraft and spacecraft components, such as turbine blades, rocket nozzles, and heat shields, are exposed to extreme temperatures during flight. Creep-resistant materials are essential to ensure the reliability and safety of these components.
Ceramic Materials in Furnace Linings:
Ceramics are often used as refractory materials in high-temperature industrial furnaces. Creep resistance is crucial to maintaining the structural integrity of furnace linings over time.
Steam Turbine Blades:
In power generation plants, steam turbine blades are subjected to high temperatures and mechanical stresses. Creep-resistant materials are used to prolong the service life of these critical components.
Creep in Glass:
Even materials like glass can exhibit creep behavior when subjected to elevated temperatures over extended periods. Stained glass windows in historic buildings, for example, may gradually deform over centuries due to the influence of gravity.
Creep in Plastics:
Certain plastics and polymers can exhibit creep when exposed to sustained loads at elevated temperatures. This property is important to consider in applications like automotive components or plastic piping systems used for hot fluids.
Geological Creep:
In geological contexts, the slow flow of rock or soil over time due to sustained stress or pressure is referred to as geological creep. This can lead to phenomena like landslides or the gradual deformation of rock layers in the Earth's crust.
