The crack growth resistance of 20 mm thick (complying with American Petroleum Institute 5LX65 specifications pipeline steel) in the longitudinal orientation has been measured employing the yielding mechanics parameters J and crack opening displacements (COD) on the three-point bend and center-cracked tension specimen configurations.

The values of these parameters at the initiation of cracking for the two geometries were essentially identical. The resistance curve derived for the tension geometry, however, was appreciably steeper than its bend counterpart.

A comparison of J estimation methods is presented and a simple relationships suggested for the determination of COD resistance for the center-cracked tension geometry.

Energy balance considerations are extended to describe instability in the elastic-plastic regime and J driving force curves estimated for the three-point bend and center-cracked tension geometries.

In addition, the analysis was employed to derive the Paris instability parameter T and the results compared with the existing formulations.