The initial ring stiffness or ring stiffness to ISO 9969 or ASTM D2412 characterizes the resistance of a pipe section to radial compressive forces. The measurement occurs at low deformations of 3%.

The ring flexibility to ISO 13968 describes the ability of a pipe to withstand deformation without undergoing structural damage. The test arrangement is similar to the test arrangement for measuring ring stiffness, however deformations of 30% are applied.