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FFS Forum: Using Fracture Mechanics to Lower Risk and Improve Plant Reliability - Part 2, A Closer Look at the FAD

By Greg Garic, Managing Principal at Stress Engineering Services, Inc. This article appears in the November/December 2020 issue of Inspectioneering Journal.
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This article is part 2 of a 3-part series on Fracture Mechanics.
Part 1 Part 2 Part 3

Editor's Note: This article is the second in a 3-part series on the application of fracture mechanics in a plant environment. Part 1 provided an introduction to fracture mechanics and its basic principles. This article will take a closer look at the failure assessment diagram or FAD.

Introduction

Every fitness-for-service (FFS) assessment must have a well-defined acceptance criterion. For many damage mechanisms, it’s the “remaining strength factor” (RSF). But for crack-like flaw assessment, it’s the Failure Assessment Diagram, or FAD.

A Quick Review

In the last Fitness-for-Service Forum, published in the September/October 2020 issue of Inspectioneering Journal, I discussed the basics of the FAD and included the diagram reproduced in Figure 1.

Figure 1. Failure Assessment Diagram (FAD)
Figure 1. Failure Assessment Diagram (FAD)

In Figure 1 you can see an “Operating Point” plotted in red. Because the point falls below the “Failure Locus” (the solid blue line), it passes the assessment. You can also note the different “cut-offs” for various materials (vertical dotted lines at the right end of the plot).

The operating point is plotted in terms of two ratios: Lr and Kr, representing a load ratio and a fracture ratio, respectively, and are defined as:

         Eqn. (1)

           Eqn. (2)

Where:

KI = Calculated stress intensity in the material, from both primary and secondary stresses.
KIc = Critical stress intensity (i.e. fracture toughness), a material property.
σP = Calculated reference stress in the material, due to primary stresses.
σys = Yield stress

But, What Does it Mean?

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Comments and Discussion

Posted by Mohamerd Shaaban on February 9, 2021
Thank you Mr.. Garic for this very valuable... Log in or register to read the rest of this comment.

Posted by Greg Garic on February 9, 2021
Mohamerd...The next issue will include the 3rd... Log in or register to read the rest of this comment.

Posted by Greg Garic on February 9, 2021
Mohamerd...The next issue will include the 3rd... Log in or register to read the rest of this comment.

Posted by ARNAB GUPTA on February 10, 2021
Mr. Greg.. thanks for the nice article.. and... Log in or register to read the rest of this comment.

Posted by Greg Garic on February 10, 2021
Arnab - The sharp drop in the FAD curve that... Log in or register to read the rest of this comment.

Posted by Benjamin Mina on February 15, 2021
Thanks a lot Mr. Greg Garic for this valuable... Log in or register to read the rest of this comment.

Posted by Greg Garic on February 15, 2021
Part 3 in this series, coming out around March... Log in or register to read the rest of this comment.

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