Plastic Pipes Conference Association # 2023 Lake Buena Vista
Kevin Copher, Patrick Vibien
High Density Polyethylene (HDPE) conduit is a durable product designed to protect cables for multiple generations buried underground or encased in concrete. Conduit is subject to coiling stresses, installation stresses and in-service stresses, such as those generated from installed curvature, soil loading, or rock impingement. These stresses are typically low enough that they do not generate ductile failure; however, these stresses can produce brittle failures if the material is not sufficiently resistant to this failure mode. The North American HDPE conduit industry has adopted a material performance requirement based on the ASTM D1693 Standard Test Method for Environmental Stress-Cracking of Ethylene Plastics Environmental Stress Crack Resistance (ESCR). This requirement has served the industry well with few, if any, brittle failures in service. The current stipulated requirement is a failure time (F10) greater than 96 h. The ESCR test has been anecdotally reported to have significant interlaboratory variation. This variability is supported by the ASTM D1693 test method stated precision and bias. As this test is frequently used as a QC test for outgoing and incoming materials, the minimum test time of 4 days can result in significant delays in material shipping by the HDPE material producer or in accepting and transferring materials into inventory for the conduit manufacturer. The ASTM F2136 Standard Test Method for Notched, Constant Ligament-Stress (NCLS) Test to Determine Slow-CrackGrowth Resistance of HDPE Resins or HDPE Corrugated Pipe has been adopted with success by the North American HDPE corrugated pipe industry as a replacement for the ESCR test. The NCLS test is reported to provide improved reproducibility and acceleration over the ESCR test, both desirable properties for the HDPE conduit industry. A round-robin test program was conducted based on 12 materials, with a range of expected ESCR performance, with multiple labs completing both the ESCR and NCLS tests on samples generated from these materials. This paper provides the analysis and results of the correlation study between both methods. Significant variability within the ESCR test results was confirmed. Based on the correlation, the HDPE conduit industry may adopt an NCLS based requirement shown to be equivalent to the existing ESCR requirement. Additionally, the findings may be used to support the introduction of recycled HDPE in conduit applications.
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