What Is Creep In Materials? – And How Does It Work?

When material is subjected to constant load over a prolonged period of time, its deformation gradually progresses leading to failure of the material. This phenomenon of material failure is called creep. The creep deformation will be permanent even if the stresses are below yielding point of the material. The effect of creep rapidly increases at elevated temperatures due to an increase in diffusion rate. Soft materials can creep at room temperatures while denser materials creep at elevated temperatures. Some of the examples of soft materials that undergo creep are lead, tin, zinc, aluminum, polymers (polypropylene, polystyrene, polyethylene) and soft alloys (bronze and brass). Examples of denser materials for creep are iron, copper, nickel, titanium, alloy steels, cobalt and tungsten.

It is very important to account for creep failure during design phase to ensure reliability and safety of the engineering materials and components. In this article let’s discuss physical aspects of creep like mechanism and stages of failure. In the article, we will focus on numerical aspects of creep like different mathematical models to predict transition and failure in Abaqus.

Mechanisms Of Creep

Creep is caused by various complex internal mechanisms occurring within the material’s microstructure. The exact mechanism depends on the type of material, microstructure, stress level and temperature. Even though the mechanisms are complex and different, the common physics behind all of them is diffusion of some sort. The time dependency of creep evolves from the fact that diffusion processes gradually progress over time.

Diffusion Creep

This mechanism occurs when the material is subjected to low stress and high temperatures. As the temperature increases, the energized atoms within the crystal lattice diffuse creating vacancies. These vacancies are created in areas where the grain boundary is under tension.

Polymer Creep

In polymers, creep typically occurs due to sliding of individual polymer chains relative to one another. As the molecular links in amorphous polymers can slip more easily with respect to each other, this phenomenon is more common in amorphous polymers compared to crystalline polymers.

Creep Failure in Materials

Grain boundary sliding

This typically occurs when the fine-grained material is subjected to high temperatures and moderate stresses. In this mechanism, the grains slide past one another along their boundaries. Finer grains can more easily slide and adjust under applied stresses compared to larger grains, as they experience lower friction.

Creep Failure in Materials

Dislocation Creep

In this mechanism, the creep occurs due to the movement of atomic dislocations. The strain rate in this mechanism is controlled by the motion of vacancies, which facilitate dislocation movement under the action of applied loads. The dislocations can either move along a slip plane or out of slip plane in this mechanism. This typically occurs when  the material is subjected to high temperatures or stresses.

Stages Of Creep Failure

During creep mechanisms, the rate of deformation is not constant throughout. Based on how rate of deformation progresses, the total span of creep failure can be divided into 3 stages as described below.

Creep Failure in Materials

Primary Creep

During this first stage, the rate of deformation is high. As the material is deforming under the applied load initially at high rate, initial alignment of molecular chains (in polymers) or strain hardening occurs due to increase in the dislocation density (in metals). This results in an increase in resistance to further deformation because of which the rate of deformation slows down and the material stabilizes under the applied load.

Steady-State Creep

As the material deformation is stabilized at the end of primary stage, the rate of deformation reached a steady state. There is still microscopic diffusion or slip occurring in the material, but the overall macroscopic behavior can be assumed to change linearly over the time. There is a balance between strain hardening and recovery during this stage. It is the longest phase in terms of time and is used to determine whether the material is appropriate for the specific stress and temperature loading conditions. The steady-state creep strain rate can be calculated using the Norton’s power law given below.

Where, K is a material constant, sigma is stress applied, n is exponent of stress, T is temperature, R is universal gas constant and Q is activation energy which is material dependent.

In the above equation, K, n and Q are constants based on material and R is universal constant. The only variables for a material are stress sigma and temperature T. Hence, the steady state strain rate is constant for a given (sigma, T ).

Creep Failure in Materials

Tertiary Creep

This is the final stage of creep failure with shortest duration, in which micro defects like voids and cracks form leading to localized weakening and necking of the material. The strain rate increases rapidly leading to catastrophic failure of the material.

Final Thoughts

Hopefully this article has provided insights into different physical aspects of creep failure. In the next article we will discuss how to capture the creep failure in Abaqus using time power law by defining the required material constants.

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