Ring stiffness, ring flexibility and initial specific ring stiffness

The ring stiffness of a pipe describes its force-deformation behavior under a radially acting external mechanical load. Ring stiffness corresponds to a gradient in the force-deformation diagram. In the case of thermoplastic pipes this property is usually measured in accordance with ISO 9969 or ASTM D2412. Glassfiber-reinforced pipes are measured as per EN 1228. In this test sections of pipe of a defined length are deformed between two flat platens. The length of the section of pipe is based on the nominal diameter. For pipes with a diameter of up to 1500 mm, sections 300 mm in length are tested. In the case of special section pipes and corrugated pipes, deformation is measured on the inside diameter of the pipe because of the risk of flattening the ridges. Special extensometers are available for this; they are either permanently installed on the machine or can simply be placed in the pipe.

Measurement is performed at a deformation of 3% of the initial diameter. The result of the measurement is the initial ring stiffness, which is expressed as force per unit of radial deformation and per unit of pipe length. Ring stiffness is usually stated in kN/m² in the SI system; lbf/in² are the normal units in the American inch-pound system. Ring stiffness is used to assign pipes to SN classes. Thus an SN 8 pipe has a minimum ring stiffness of 8 kN/m² in the nominal diameter range from DN100 to DN800.

Ring flexibility describes the ability of a pipe to withstand deformation without incurring structural damage. Measurement employs the same test arrangement as for ring stiffness. The test to ISO 13968 is continued up to failure of the pipe or until deformation of 30% of the pipe outside diameter has occurred. The result of the test is maximum force and deformation values, or force and deformation values at which a damage criterion defined in the standard first appeared.