Inspectioneering Journal

FFS Forum: Primary & Secondary Stresses – What’s the Difference & Why Do We Care?

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


The concepts of “Primary” and “Secondary” stresses are frequently encountered in pressure system codes. Yet there seems to be a good bit of confusion about what these terms actually mean. In this issue’s column, we’ll try to shed some light on this important engineering concept.

A definition is probably a good place to start. The following basic definitions are from Part 1 of API 579-1, Fitness-for-Service:

Primary Stress: “A [stress] developed by the imposed loading that is necessary to satisfy the laws of equilibrium of external and internal forces and moments.” [API 579-1 (2016), para. 1A.79]

Secondary Stress: “A [stress] developed by the constraint of adjacent parts or by self-constraint of a structure.” [API 579-1 (2016), para. 1A.90]

Primary Stresses

Since a primary stress must satisfy equilibrium, the structure will fail – or at least, experience significant distortion – if the primary stress gets too high. This stress is NOT self-limiting and failure can occur on a single application of load.

The simplest example of primary stress is a weight on a rod. I illustrate this example in Figure 1.

Perhaps the most important thing to realize is that a structure can fail from a single application of a primary load. Secondary loads are more of a problem under repeated cyclic loading, i.e., fatigue.

In the figure, the stress throughout the cross-section is well represented by the nominal stress. When the external load, P, increases to the point that the material can no longer react that load by the nominal stress, snom, then equilibrium can no longer be maintained and the rod will fail.

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
Interested in unlimited access? VIEW OUR SUBSCRIPTION OPTIONS

Current subscribers and registered users can log in now.

Comments and Discussion

Posted by Khalid Ahmed Khan on April 9, 2019
Nice and simple definition of stress.... Log in or register to read the rest of this comment.

Posted by Hervandil Santanna on July 3, 2019
Hello, Very nice article. Congratulations. I... Log in or register to read the rest of this comment.

Posted by Greg Garic on July 3, 2019
Hi Hervandil...The bolt load has both primary and... Log in or register to read the rest of this comment.

Posted by Hervandil Santanna on July 3, 2019
Thanks for your replay, mr. Greg! As another... 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.

Talent Solutions

Discover job opportunities that match your skillset.

Case Studies

Learn from the experience of others in the industry.


Inspectioneering's index of mechanical integrity topics – built by you.

Industry News

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


Read short articles and insights authored by industry experts.

Expert Interviews

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

Event Calendar

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


Downloadable eBooks, Asset Intelligence Reports, checklists, white papers, and more.


Watch educational and informative videos directly related to your profession.


Commonly used asset integrity management and inspection acronyms.