Introduction

The Material and Construction (M&C) Defects chapter provides an overview concerning defect response to increasing pressure, which is largely focused on ductile response. But as the failure response is only fully ductile when the loading occurs well above the ductile to brittle transition temperature (DBTT), it is instructive to also consider cases where brittle fracture dominates due to rate effects acting in combination with lower temperatures.

In such cases where rate effects acting in combination with lower temperatures lead to brittle response the resistance to crack advance is low, such that little to no blunting develops, and under plane strain conditions crack initiation is immediately followed by propagation. This is much different than the circumstances addressed in the M&C Chapter. More critically, such scenarios open to the possibility of running brittle fracture – wherein cracking propagates at high speeds down the pipeline. Such fracture-controlled failures typically occur at much lower pressures as compared to ductile failures because the toughness that acts to resist that propagation is much reduced in relative terms. For further details view the video below:

video: Failure Analysis – Defect Response to Pressure

Further detailed discussion of fracture initiation and propagation can be found in References [1] and [2].

Click here for more on Failure Mechanisms & Pipeline Properties.

Click here for details concerning Collapse and Fracture Controlled Failure.

The Segment titled Collapse and Fracture Controlled Failure expands the scope of this discussion specific to the ductile and brittle modes of failure.

The Segment titled Failure Mechanisms and Pipeline Properties provides background and details concerning those modes of cracking, and the role of the mechanical and fracture properties in regard to collapse-controlled and fracture-controlled failure.

Summary

Failure localized at defects can occur either by plastic collapse or fracture, with the properties, particularly the toughness, and the pipe’s geometry determining which of these controls the failure process and the sizes of the defects that will fail at a given pressure. Tougher steels will blunt initially sharp defects, leading to collapse-controlled failure which occurs at a higher pressure as compared to fracture-controlled failure – all else being equal.

References

  1. B. N. Leis, “Arresting Propagating Shear in Pipelines: Part I and Part II,” Steel in Translation, Volume 45, Issue 1 (Part I), January 2015, pp, 5-20, and Issue 3 (Part II), March 2015, pp 161-174.
  2. Leis, B. N. and Eiber, R. J., “Fracture Control Technology for Transmission Pipelines”, PR-003-084506, December, 2014: PRCI Member Access only