John M. Kurdziel & Bill Vanhoose
Papers # 2012 Barcelona
Major standards organizations, such as ASTM International, ISO (International Standards Organization) and AASHTO (American Association of State Highway and Transportation Officials), have a test method for evaluating a plastic pipe’s wall stability and load capacity. The name of the performance test may be called buckling, ring flexibility, failure point or ultimate load. The specific value for the design limit may vary, but in all cases, the test apparatus used is a parallel plate test. This test method, however, is inherently flawed for determining the actual wall stability or profile capacity. The parallel plate provides an accurate assessment of pipe stiffness at 5 percent deflection, but the test’s applicability drops precipitously as the deflection value exceeds 10 percent. Depending on the pipe’s profile geometry, stiffness and material composition, the load application dramatically changes negating the validity the test. A pipe with a higher stiffness, profile height, and modulus will provide greater resistance to bending resulting in severe reverse curvature at a lower deflection than a solid wall pipe with a low stiffness and modulus. Structurally, the higher stiffness pipe will likely have significantly better in-field performance. The parallel plate test, however, would show otherwise as the load points move from a two point load configuration to four points as reverse curvature occurs at the crown and invert changing both the failure mode and increasing the overall stress on the now shorter vertical arch section. The new test protocol presented in this paper eliminates the four point loading condition and maintains a uniform two point load, which is also more indicative of field conditions. The maximum strain obtained at the crown, invert and springline can be directly correlated to the structural design evaluations required for buckling and maximum strain evaluations. While a standard parallel plate machine may be used with an adaptor plate, a revised test apparatus may also be used which is also portable and much less expensive than the current parallel plate machine.
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