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Fitness-for-Service Forum: Primary & Secondary Stresses – What’s the Difference & Why Do We Care?

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

Introduction

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 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.

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