How do you design and conduct a creep test to obtain reliable and accurate data?

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Creep deformation and rupture are common failure modes of materials subjected to high temperatures and stresses over long periods of time. In this article, you will learn about the mechanisms and characteristics of creep, and how to design and conduct a creep test to obtain reliable and accurate data for failure analysis.

本文章的要点总结

Choose your load condition:

Deciding between a constant load or a constant deformation condition tailors the creep test to reflect real-world usage, ensuring the data is more applicable and reliable.
Apply the Omega method:

This technique evaluates creep strain rate against allowable rates, providing a clear assessment of component life expectancy and aiding in preemptive maintenance strategies.

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1 What is creep?

Creep is the gradual and permanent deformation of a material under a constant or varying load at high temperatures. Creep occurs when the material is exposed to stresses that are lower than its yield strength, but high enough to cause microstructural changes and damage over time. Creep can affect the mechanical properties, dimensions, and shape of the material, leading to reduced performance, cracks, and ultimately failure.

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2 Why is creep important?

Creep is important for failure analysis because it can determine the lifetime and reliability of components and structures that operate at elevated temperatures, such as turbines, boilers, engines, pipes, and reactors. Creep can also influence the design and selection of materials for high-temperature applications, as different materials have different creep resistance and behavior. Understanding the factors that affect creep, such as temperature, stress, time, and environment, can help engineers and analysts predict and prevent creep failures.

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Ryan Erickson

"Fixer" and Technical Sales at Polysource
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Creep is critical for viscoelastic materials (such as plastics) since the material's structure and thereby properties will slowly shift over time. Polymer chains slowly disentangle to reduce stress and that can significantly decrease the mechanical strength over time. Plastic failure WILL occur eventually in every scenario undergoing creep - the question is just if it is in 5 minutes, 5 years, or 5 millennia if the loading is humorously low compared to the strength.

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3 How does creep occur?

Creep occurs through three main mechanisms: diffusion, dislocation, and grain boundary sliding. Diffusion is the movement of atoms from regions of high concentration to regions of low concentration, driven by thermal energy. Diffusion can cause creep by creating vacancies, voids, and precipitates in the material, which reduce its strength and ductility. Dislocation is the movement of defects or irregularities in the crystal structure of the material, driven by applied stress. Dislocation can cause creep by allowing the material to deform plastically without breaking the atomic bonds. Grain boundary sliding is the relative movement of adjacent grains in a polycrystalline material, driven by shear stress. Grain boundary sliding can cause creep by changing the orientation and shape of the grains, which affects their ability to resist deformation.

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4 How do you measure creep?

Creep can be measured by conducting a creep test, which involves applying a constant or varying load to a specimen of the material at a constant or varying temperature, and monitoring the deformation and strain over time. A creep test can provide useful information about the creep behavior and properties of the material, such as the creep rate, the creep strain, the creep modulus, the creep life, and the creep rupture strength. A creep test can also reveal the creep mechanisms and damage modes of the material, such as cavitation, cracking, necking, and fracture.

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5 How do you design a creep test?

Designing a creep test requires careful consideration of several factors, such as the objective, the material, the specimen, the loading, the temperature, the environment, and the equipment. The objective of the creep test determines the type and duration of the test, as well as the data and analysis required. The material of the specimen affects the creep behavior and properties, as well as the selection of the loading and temperature conditions. The specimen should have a uniform cross-section and geometry, and be free of defects and residual stresses. The loading should be within the creep range of the material, and be applied uniformly and accurately. The temperature should be representative of the service conditions of the material, and be controlled and measured precisely. The environment should be similar to the actual or simulated operating conditions of the material, and be monitored and regulated carefully. The equipment should be capable of applying and maintaining the desired loading, temperature, and environment conditions, and recording the deformation and strain data.

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Maxime Belanger

Sr Principal R&D engineer at Medtronic
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One important factor in designing a creep test is to decide if you are interested in a constant load use condition (the load remains constant throughout the test, regardless of the deformation) or a constant deformation use condition (the displacement remains constant throughout the test, but the stress in the sample will decrease over time.

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6 How do you analyze a creep test data?

Analyzing a creep test data involves plotting the deformation or strain versus time or logarithm of time, and identifying the different stages of creep. Creep typically consists of three stages: primary, secondary, and tertiary. Primary creep is the initial stage of creep, where the deformation or strain increases rapidly at first, then slows down as the material adjusts to the applied stress. Secondary creep is the steady-state stage of creep, where the deformation or strain increases at a constant rate, which is called the minimum or steady-state creep rate. Tertiary creep is the final stage of creep, where the deformation or strain accelerates rapidly due to damage accumulation and microstructural instability, leading to rupture or failure. Analyzing a creep test data can also involve calculating or estimating the creep properties and parameters of the material, such as the activation energy, the stress exponent, the strain exponent, and the rupture time.

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Shahab Zangeneh, PhD

Fitness for Service (FFS) assessment
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The Omega method in API579 for creep damage assessment is a best way to creep life assessment . This method involves evaluating the creep strain rate and comparing it to the material's allowable creep strain rate to determine the remaining life of the component.

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How do you design and conduct a creep test to obtain reliable and accurate data?

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1 What is creep?

2 Why is creep important?

3 How does creep occur?

4 How do you measure creep?

5 How do you design a creep test?

6 How do you analyze a creep test data?

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