In order to achieve the large pipeline strains required for strain-based design, it is essential that the material be on the upper shelf of the toughness transition curve at the lowest anticipated service temperature and the pertinent strain rate. Full-scale testing is normally quasi-static and at ambient temperature, so upper-shelf behavior is not necessarily demonstrated by a successful test. The approach under development is to use Charpy V-notch testing to ensure upper-shelf fracture toughness is maintained when the project-specific conditions exist. Through use of empirical relationships, the CVN test temperature is adjusted to reflect the pertinent strain rate. This adjustment is based on crack-tip opening displacement test results at both quasi-static and intermediate strain rates.
Strain-based design (SBD) is often required to make pipelines economical in arctic climates and/or earthquake/landslide zones. In areas with freeze-thaw ground heave the strain rate is quasi-static (usually taken to be a nominal strain rate of about 10-4 s-1), whereas in earthquake/landslide zones the strain rate is intermediate (usually taken to be a nominal strain rate of about 10-2 s-1). In order to achieve large strains, it is essential that the fracture toughness be on the upper-shelf of the toughness transition curve at the lowest anticipated service temperature (LAST) and the pertinent strain rate. The usual approach used to confirm strain capacity is through wide plate or full-scale tests (Fairchild et al., 2008, Gioielli et al., 2007). However, such tests are normally conducted at ambient temperature (at least 20°C) and with quasi-static strain rate. Hence, they do not by themselves guarantee that upper-shelf behavior is maintained at LAST with the strain rate of interest. Lower temperature and/or a higher strain rate could cause the toughness to be in the transition region, potentially well below the upper-shelf, as depicted in Fig. 1.