Inspectioneering
Inspectioneering Journal

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

By Greg Garic, Managing Principal at Stress Engineering Services, Inc.. This article appears in the May/June 2019 issue of Inspectioneering Journal
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.

This content is free for registered users

Register today to read this article for free.

Create your free account and you'll also get access to:

  • Unlock one premium article of your choosing per month
  • Exclusive online content, videos, and downloads
  • Insightful and actionable webinars
GET STARTED
Interested in unlimited access? VIEW OUR SUBSCRIPTION OPTIONS

Current subscribers and registered users can log in now.


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.

Add a Comment

Please log in or register to participate in comments and discussions.


Inspectioneering Journal

Explore over 20 years of articles written by our team of subject matter experts.

Company Directory

Find relevant products, services, and technologies.

Job Postings

Discover job opportunities that match your skillset.

Case Studies

Learn from the experience of others in the industry.

Event Calendar

Find upcoming conferences, training sessions, online events, and more.

Industry News

Stay up-to-date with the latest inspection and asset integrity management news.

Blog

Read short articles and insights authored by industry experts.

Acronyms

Commonly used asset integrity management and inspection acronyms.

Asset Intelligence Reports

Download brief primers on various asset integrity management topics.

Videos

Watch educational and informative videos directly related to your profession.

Expert Interviews

Inspectioneering's archive of interviews with industry subject matter experts.