Inspectioneering Journal

FFS Forum: Creep in Heater Tubes and Other Components – Part I: Understanding Creep

By Greg Garic, P.E., Senior Staff Consultant at Stress Engineering Services, Inc. This article appears in the May/June 2019 issue of Inspectioneering Journal.
This article is part 1 of a 2-part series on Creep.
Part 1 | Part 2

Editor’s Note: This is the first of a two-part column on “Creep”. It presents an engineering overview of the phenomena, answers the question: “What is Creep?”, and provides readers with foundational knowledge for evaluating and managing remaining equipment life. Part two of this series, to be published in the July/August issue of Inspectioneering Journal, will discuss how to assess creep damage in plant equipment.

What is “Creep”?

It’s probably good to start with a definition.

Creep is the time dependent deformation of material under stress.

Creep is not yielding; creep can occur at stresses well below yield. The onset of creep can occur over a wide range of temperatures for different materials. For example:

  • Lead can creep near room temperature
  • Ice can creep below 30°F
  • Steel can begin to creep around 650°F, or up to 1200°F for some high temperature alloys

Generally, for metals, the effects of creep begin to become noticeable at around 35% of the absolute melting temperature of the metal. For ceramics, creep begins around 45% of the melting temperature.

To get a good mental picture of creep in action, consider a carbon steel rod with a weight hanging on the bottom, inducing a stress of 5 ksi (Figure 1a).

Weight on a rod
Figure 1: Weight on a rod

In Figure 1(b), where load is applied at low temperature (i.e., below the creep range), there is no time-dependent aspect to the deformation – it’s all elastic. It can sit for 50 years, but when you remove the load, the bar will elastically return to its original length. Contrast that with Figure 1(c), where the same load is applied at a high temperature (i.e., above the creep range, but still below yield at temperature). Over time, the bar continues to stretch. Given enough time and/or stress, the bar will continue to stretch until it breaks. For this particular material, temperature, and stress, the creep life is predicted to be a bit over 100 years.

For a given material, increasing the stress and temperature results in increasing creep strain over time. A typical creep-time relationship is shown in Figure 2.

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Comments and Discussion

Posted by Hervandil Santanna on July 3, 2019
Another excellent topic! And you have answered a... Log in or register to read the rest of this comment.

Posted by Josh Yoakam on July 24, 2019
Greg, this is yet another good article which can... Log in or register to read the rest of this comment.

Posted by Greg Garic on July 24, 2019
Thanks so much Josh. I appreciate the feedback.... Log in or register to read the rest of this comment.

Posted by Mohamerd Shaaban on January 14, 2020
It is excellent data and best practice, I think... Log in or register to read the rest of this comment.

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