Intro to Steady-state creep rate

Last Updated on May 17, 2022

The steady-state creep, also known as the “secondary creep,” takes place when the strain rate settles into a constant state. Therefore, the strain is relatively low because the microstructural damage has not been achieved yet.

The steady-state creep is the second of the three consecutive states of creep. Although the mechanics of the “creep” can be deemed as complex, these three stages simplify the whole process.

It is also important to explain the two broad mechanisms by which steady-state creep rate appears, the diffusion creep and the dislocation creep.

Diffusion creeps: This phenomenon appears by the motion of material via the diffusion of atoms within a grain. A gradient of free energy drives diffusion creep, as all diffusional processes. Diffusion creep is divided into two different processes based on the diffusion paths. The “Coble creep” is favored at lower temperatures, and the diffusion paths are mainly through the grain boundaries. The other type would be “Nabarro-Herring creep,” which is favored at higher temperatures and the diffusion paths are through the paths themselves.

Dislocation creeps: As their name suggests, dislocation creep is a mechanism that involves the motion of dislocations. It mainly dominates at lower temperatures and high stresses most of the time.

Steady-state creep rate calculation

Before the explanation of steady-state creep rate calculation, it is important to define what “creep” is.

“Creep” refers to a time-dependent material that deforms under constant stress underneath the material’s yield strength. This term is also referred to as material creep or cold flow. It’s important to note that several factors play a key role in the commencement and continuity of creep in a determined material. Some of these aspects can be:

  • Temperature
  • Time
  • Stress
  • Alloy composition

Now, the “creep rate” is defined as the “rate of deformation” as a result of creep.

As for the steady-state creep rate calculation, it is necessary to follow the following equation:

In this equation, we can identify the following elements:

Q = Activation energy                                n = Stress exponent                         A = Constant

Additionally, the same formula can be arranged in the following manner:

As an extra note, the activation energy (Q) can be defined via experimentation through the plotting of the natural record of creep rate in contrast to the reciprocal of temperature.

What is the minimum creep rate?

The minimum creep rate is determined thanks to the slope of the portion of the creep against the timing diagram that corresponds to secondary creep.

The minimum creep rate is often treated as the most essential parameter that describes creep behavior. However, minimum creep rates are often examined, taking a small portion of the creep curve into account. Therefore, the minimum creep rates can be observed from different creep curves, even if their shapes are different from each other.

It is possible to obtain an accurate determination and description of creep behavior, taking into account the minimum creep rates and the shape of the creep curves.

What is creep constant?

“Creep” or “deformation,” sometimes known as “cold flow,” is defined as the propensity of solid materials to move slowly or perhaps “deform” permanently while under the influence of continuous mechanical stress.

The general equation of creep is the following:

ε = Creep strain

C = Constant dependent on the material and the specific creep mechanism

Q = Activation energy

σ = Applied stress

d = Grain size of the material

k = Boltzmann’s constant

T = Absolute temperature

Although the “creep” phenomena are quite common in our daily lives, the term itself is not popular among the general population. It is especially usual due to the presence of plastic materials.

Why does creep rate increase with temperature?

Creep temperature refers to the time-dependent deformation at high temperatures and constant stress. As the temperature increases, the stress-strain curve starts shifting upward. This way, the axial pressure increases, and so does the creep strain deformation. Hence, we can say that a larger axial pressure translates into a larger axial pressure.

Contrary to this, the decrease rate decreases at a low temperature because the activation energy becomes unavailable.

What is the difference between a stress rupture test and a creep rupture test?

Stress rupture and creep rupture are synonyms. Hence, both terms refer to the same processes and are essential to understanding and preventing potential product failures. These tests can be performed in metals, composites, and even plastics so you can determine the long-term stress on materials.

For starters, creep testing is completed via the utilization of a tensile specimen along with an extensometer specifically designed for this purpose, as well as applied constant stress and temperature. This test is often completed in furnaces or perhaps in environmental chambers. It is typically recorded on a graph of strain against time to determine the creep rate.

Stress rupture is all about the abrupt and total failure of a particular substance or material under stress. The test consists in holding the sample at a determined load level and temperature for a pre-established period. These tests may imply the application of loads via tensile bending or flexural, biaxial, and hydrostatic procedures.

It’s possible to find many companies specializing in stress rupture and creep test services in the modern-day.

The “creep” phenomenon is present in our daily lives. However, this term is still unknown to the common population. Understanding how these processes work is quite important if you want to prevent potential accidents within a metallic structure or a certain item, for instance.