Creep (Deformation): Definition, How It Works, Importance, and Graph
Learn more about this mechanical deformation and how it can be prevented.
Creep deformation is time and temperature dependent and can occur in metals and polymers. In general, materials will permanently deform well below their yield point when exposed to long-term stress and ill-suited temperature. Creep deformation is a slow process and can be prevented by selecting materials with inherent creep resistance and high melting points.
This article will describe what deformation creep is, how to read a creep graph, what the form of the general creep equation is, as well as the various creep mechanisms. It will focus primarily on the creep mechanisms observed in metals with brief explanations of creep in nonmetals.
Creep deformation is a form of slow mechanical deformation that occurs when a material is exposed to high-stress levels for a long period of time. Creep is time-dependent. The deformation occurs at a relatively slow rate that can cause a material to fail below its yield point. The creep rate can be affected by material properties, stress levels, and temperatures. Materials like steel will only experience significant creep levels at temperatures that approach their melting points. Creep rupture is the term used when a material has failed due to creep load.
From a materials-science perspective, creep is a complex failure mode that describes how materials deform on an atomic scale when exposed to constant stress and/or elevated temperatures. These deformations are typically in the form of dislocations where voids are formed either on the grain boundaries or within the crystal structure of the grain due to the applied stress. These voids or dislocations then travel throughout the material over the lifetime of the part, resulting in permanent deformation known as creep. These deformations can occur well below the yield point of the material. Creep typically presents itself in three stages, namely, primary, secondary, and tertiary creep.
Creep deformation in concrete can cause structures to permanently deform over time. The creep mechanism in concrete is very different from that present in metals and polymers. One important difference is that creep in concrete can occur at any stress level. The level of aggregate in the mix can help reduce the creep rate.
Creep deformation in steel is only considered a problem when its operating temperature reaches 40% of its melting temperature for long periods of time. Various creep mechanisms can present themselves depending on the load conditions and the type of material.
Creep deformation works by localized dislocations forming either within the grain structure of a metal or on the grain boundaries. For polymers, creep works by molecular chains sliding past each other. Creep deformation is highly dependent on applied stress and operating temperatures.
Creep deformation in 3D printing depends on many factors, such as the technology used to print the part, the material used, and what post-processing techniques were followed. When 3D printing in plastic using FFF (Fused Filament Fabrication), the normal viscoelastic behavior of polymers applies. This means that if the part is exposed to constant stress, the molecular chains within the material will slip past each other resulting in creep. This is especially a problem as 3D printing plastics generally have lower melting temperatures and are therefore more readily affected by environmental temperatures which can accelerate creep.
A creep test is important because it allows engineers to design parts while understanding the relationship between stress, temperatures, and creep rate to ensure that a part will not fail at loads below its yield strength at elevated temperatures. For metals, a creep deformation test is performed by subjecting a sample to a constant tensile load and temperature in order to plot the strain developed as a function of time.
For brittle materials, compressive creep tests are used to develop the behavior of the material under prolonged loads and increased temperatures. Creep tests provide insight by defining the secondary creep rate which is used to design components for multi-decade service life as well as the time to rupture which is used to design relatively short-term components like turbine blades.
A creep deformation curve can be broken down into three sections: primary, secondary, and tertiary creep. For metals, these refer to three distinct behaviors on the curve as indicated in Figure 1 below:
Strain vs. time.
Image Credit: http://www.totalmateria.com/
The first part of the curve refers to an elastic region that is developed when a material is first exposed to the load and begins to strain and harden over time. The second linear area refers to a steady state deformation during which the creep rate is constant. The final section refers to the failure region in which the material's creep rate rapidly increases until it reaches the rupture point.
A more common way of indicating the creep behavior of a metal is to plot the creep strain rate of a material at different temperatures, as per Figure 2 below:
Strain rate at different temperatures.
Image Credit: https://practicalmaintenance.net/
This gives an extensive overview of a material’s behavior in relation to a range of different temperatures. It can be seen that increased temperatures result in higher strain rates and reduced time to rupture.
Creep can be represented as the change in strain over time as seen in the equation below:
- C - Constant that changes depending on the creep mechanism and the material
- σ - Stress applied to the material
- m, b - Exponents that depend on the specific creep mechanism
- d - The materials’ average grain size
- Q - Activation energy of deformation
- k - Boltzmann’s constant
- T - Absolute temperature
Creep can be categorized into three stages as listed below:
Also called transient creep, this is the first stage of creep and occurs on the instantaneous application of load. This is an elastic region in which the creep rate will slowly reduce as a function of time due to strain hardening in the material. This slowdown in creep rate can be observed by the decreasing gradient of the curve in Figure 3 below:
Strain vs. time.
Image Credit: http://www.totalmateria.com/
This implies the material is experiencing increased creep resistance in this region.
Also called steady-state creep, this stage is characterized by a constant creep rate defined by a linear curve, (Figure 3 above), and has the longest duration during creep deformation. While there is still a level of strain hardening, this is balanced by the material undergoing a recovery stage that softens the material and enables deformation. This secondary creep rate is what is used by engineers as a parameter to inform their designs.
This is the final stage of creep deformation and culminates in the eventual rupture of the material. The mechanism of failure usually manifests as the development of micro cracks, internal voids, and grain boundary separation. These failures ultimately result in a decrease in an effective cross-sectional area which then results in increased stress which accelerates the failure rate. The tertiary stage of creep results in an accelerated creep rate as observed by the increase in the gradient of the curve in Figure 3 above.
The various mechanisms by which deformation occurs on an atomic level are described in more detail below:
Nabarro-Herring creep can be generally categorized as diffusion creep. This creep mechanism occurs primarily at low stress and high temperatures. This type of creep results when atoms diffuse within the crystal lattice of grain due to vacancies that form as the temperature increases. These vacancies are formed where the grain boundary is in tension, i.e., in line with the applied stress. The grain boundaries perpendicular to the applied stress will be in compression. The larger the grain size the slower the creep rate.
Polymers can also experience creep when exposed to constant stress and elevated temperatures; however, in some cases, creep can occur at room temperature. The primary mechanism for creep in polymers is the sliding of individual chains in relation to each other. Creep in polymers is more likely with amorphous polymers instead of crystalline ones as amorphous polymers’ molecular chains can more easily slip in relation to each other.
Also called power law creep, dislocation creep is a mechanism whereby creep occurs due to atomic dislocations. The strain rate is determined by the motion of vacancies which can present as either glide or climb dislocations. Glide is when dislocations move parallel to their glide plane and climb is when dislocations move perpendicularly to their glide plane.
Coble creep is a type of diffusion creep. This creep mechanism forms primarily at lower temperatures as it is easier for vacancies to occur on the grain boundary than in the grain. Unlike nabarro-herring creep, coble creep occurs where the vacancies move from the grain boundary perpendicular to the applied stress to the grain boundaries parallel to the applied stress instead of through the grain itself.
This creep mechanism is commonly observed in alloyed elements whereby the solutes in the alloy impede the formation of dislocations in the crystal lattice of the material. This ultimately increases the creep resistance of the material at high temperatures. These alloys are often used in aerospace applications, one example of the typical use cases of Inconel®.
Harper-Dorn creep is a type of dislocation creep. For Harper-Dorn creep, grain size has no effect on the strain rate. However, in order for this form of creep to present itself in a material, the grain size must be large (for example 0.5 to 3.3 mm), the material must have a high elemental purity (99.95%), and there must be a low initial dislocation density. Harper-Dorn creep typically occurs from 0.35 to 0.6 times the material's melting point with relatively low stresses.
During sintering, metal particles are heated to a high temperature. The voids present between these particles will begin to shrink. However, at a certain stress level, this void shrinkage can stop. This stress is called the sintering limit stress. During sintering, the density of the material will increase over time—which essentially is a form of creep. This process is governed by temperature, strain rate, and density.
The most common materials that experience creep are metals and polymers. However, creep is highly dependent on applied stress and operating temperatures. As such, some metals may never creep in most usual situations. For example, structural steels will only creep at temperatures far higher than normal operating conditions. When materials are required to withstand long-term stress at high temperatures, then creep-resistant super alloys are preferred as they are highly creep resistant.
The temperature at which creep becomes important depends entirely on the material. For example, some polymers can experience creep at room temperature, whereas metals generally only experience creep from about 40% of their melting temperature.
Creep failure is a time-dependent plastic deformation of a material that has been exposed to constant stress, with higher temperatures increasing the likelihood of creep failure. Creep failure occurs at the tertiary-creep stage. It normally follows an extended stage of steady-state creep. The failure occurs relatively quickly when compared to the steady state phase and occurs with the formation of internal voids, grain boundary separation, and micro cracks.
In general, creep deformation can be prevented by selecting materials with high melting points and high creep resistance. A highly creep-resistant material like Inconel® is common in high-performance applications. The exact mechanisms for creep prevention are described in more detail below.
Creep deformation can be easily eliminated by following the three suggested methods below:
Creep occurs in three stages: primary, secondary, and tertiary. In most cases, the secondary stage of creep is what is used to determine if a material is compatible with a specific stress and temperature combination. This secondary stage takes the longest time and is defined by having a constant stress rate. The material must remain in this second phase during normal operating conditions to prevent creep.
Creep deformation can be reduced or eliminated by selecting the correct material for the application. Materials with large grains are more resistant to certain types of creep, specifically diffusion creep. Materials without any grains can be highly creep-resistant. A metal without grains can be produced by directionally casting a part to ensure it is made up of a single homogenous crystal. Some iron alloys can be made to be creep-resistant with specific precipitate. Carbide, for example, tends to collect at the grain boundaries to stabilize them, thereby preventing dislocation from occurring at these points. Selecting materials that have undergone dispersion strengthening—where alloying elements have been added to create a second phase within the material—helps prevent dislocations from forming.
Creep requires time and temperature. The easiest way to prevent creep deformation is to ensure that the operating temperature is as low as possible. If this is not possible, design the part with a lower service life to ensure it can be replaced while creep deformation levels are still low. Alternatively, materials with higher melting points can be selected.
Creep deformation is permanent and cannot be reversed as the material would have deformed plastically. The only way to fix creep deformation is to change the part or use a material that will not creep under normal operating conditions.
Creep is a relatively slow form of failure that is dependent on prolonged stress at elevated temperatures. Creep can occur well below the yield point of a material. Brittle failure occurs rapidly and is primarily caused by high stress and can be accelerated by defects such as cracks and inclusions. Brittle failure occurs at the ultimate tensile strength of a material.
This article presented creep (deformation), explained what it is, and discussed the various examples of it. To learn more about creep (deformation), contact a Xometry representative.
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