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STRESS CRACKING BEHAVIOR OF HDPE GEOMEMBRANES AND ITS PREVENTION
Citation:
Koerner, R. M., Y. Hsuan, AND A. R. Lord. STRESS CRACKING BEHAVIOR OF HDPE GEOMEMBRANES AND ITS PREVENTION. U.S. Environmental Protection Agency, Washington, D.C., EPA/600/R-93/092 (NTIS 93-196616), 1993.
Impact/Purpose:
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Description:
Geomembranes made from high density polyethylene (HOPE) have a high percent crystallinity and are therefore of concern with regard to stress cracking. A review of the literature plus our field exhuming of various sites-of-opportunity gave rise to twenty-five (25) situations where stress cracking of HDPE geomembranes has occurred since the introduction of this liner material in 1980. The stress cracks varied from very small lengths of a few centimeters to large shattering types of failure. All failures have occurred on exposed geomembranes where ultraviolet light exposure, high temperature oxidation and low temperature contraction are continuously ongoing processes. The currentiy used ASTM D1693 bent strip test method for quantifying HDPE geomembranes contains some generally acknowledged limitations. Therefore an alternative (and more challenging) test method was investigated. The result of this search was the development of a notched constant tensile load (NCTL) test. In this test dumbbell shaped test specimens are centrally notched and stressed to a prescribed percentage of their yield stress. They are then incubated in a wetting agent at 50°C constant temperature and their failure times recorded. Evaluation of a series of test specimens at different applied stresses produces a ductile-to-brittle response curve where the onset of brittle behavior is termed the transition point. The test method and procedure was also verified for reproducibility and was evaluated in a series of interlaboratory tests with satisfactory results. Utilization of the above described NCTL test on 18 commercially available sheet materials and 7 field exhumed geomembranes led to the recommendation that for an acceptable stress crack resistant HDPE geomembrane the test results should indicate a transition time of greater than 100 hours. The above recommendations have been shown to be clearly more challenging than those resulting from the bent strip test. It is recommended to discontinue sole reliance on the bent strip test for qualification of HDPE geomembranes in favor of the proposed NCTL test. In consideration of the length of testing time required to completely develop the full NCTL test response, a single point (SP) test has also been developed. In the SP-NCTL test, five notched test specimens are stressed at 30% of then- yield stress and evaluated in a similar procedure as the NCTL test The recommendation is that none of the test specimens should fail within 200 hours. Still further, a seam constant tensile load (SCTL) test has been developed whereby five notched seam specimens are stressed at 30% of their yield stress and, again, are evaluated in a similar procedure as the NCTL test The recommendation is that none of these test specimens should fail within 200 hours. This report was submitted in fulfillment of Cooperative Agreement CR 815692 by the Geosynthetic Research Institute of Drexel University under the partial sponsorship of the US Environmental Protection Agency. This report covers a period from May 11,1989 to May 10,1992 and was completed as of July 31,1992.