The ring stiffness of a pipe describes the force-deformation ratio under a radially acting external mechanical load. Ring stiffness corresponds to an upward slope in the force-deformation diagram. This characteristic is typically measured to ISO 9969 or ASTM D2412 for thermoplastic pipes, and to EN 1228 for glass fiber-reinforced pipes.
This test compresses pipe sections of a defined length between two flat platens. The length of the section depends on the nominal diameter of the pipe. In the diameter range up to 1500 mm, sections with a length of 300 mm are tested.
In the case of profile pipes and corrugated pipes, deformation is measured by analyzing the inner diameter of the pipe to prevent flattening of the ridges. For this case, we offer special extensometers that are either mounted onto the testing machine, or simply positioned inside the pipe.
The measurement is performed under a deformation of 3% of the initial diameter. The result of the measurement is the initial ring stiffness, which is the force per unit of radial deformation and per unit of length of the pipe. In the SI system, the ring stiffness is usually indicated in kN/m²; in the American inch/pound system it is typically indicated as lbf/in². Ring stiffness is used to classify pipes in the SN classes. 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 structural damage. The measurement is performed in the same test arrangement as for ring stiffness. According to ISO 13968 the test is run until the pipe fails or until deformation of 30% of the outer diameter of the pipe is reached. The test results of the test are maximum values of force and deformation, or force and deformation values, in which a damage criterion defined in the standard, occurs for the first time.
The test frame can be equipped with two test areas. For example, the upper test area accommodates tensile tests on dumbbell specimens and the lower test area accommodates compression tests to determine ring stiffness or ring flexibility.A load frame with two test areas is user-friendly because there is no need to change test fixtures.
The testXpert III Standard Test Programs control the test sequence making testing is easy and reliable. All important test results are already preprogrammed and automatically calculated.
Many types of pipes require measurement of deformation for the inner diameter. ZwickRoell offers a special extensometer for this, as well as a manual displacement transducer developed specifically by ZwickRoell for pipe testing.