Definition(s)
Critical Thickness
The critical thickness for API couplings is defined as the thickness at the root of the thread at the middle of the coupling, based on the specified coupling diameter and the specified thread dimensions.
Source: API TR 5C3, Technical Report on Equations and Calculations for Casing, Tubing, and Line Pipe Used as Casing or Tubing; and Performance Properties Tables for Casing and Tubing, Upstream Segment, First Edition, December 2008. Global Standards