Strain Measurement in HDPE Geomembrane Tensile Tests
According to ASTM D 638, the tensile strain in an HDPE geomembrane dumbbell specimen subjected to a tensile test should be evaluated using an extensometer that records the elongation of a defined length of the central portion of the specimen. However, the current practice in the United States for HDPE geomembrane quality control and conformance testing consists of following the 1991 NSF standard that recommends to conventionally evaluate the tensile strain by dividing the change in grip separation by an arbitrary gauge length. A theoretical analysis and test results presented in this paper show that the strain derived from grip separation using the gauge length specified by the 1991 NSF standard is significantly different from the actual strain, which is obtained using an extensometer as recommended by ASTM D 638. The discrepancy between the conventionally calculated strain and the actual strain may lead to unconservative design of HDPE geomembrane applications. A modification to the NSF standard is proposed to obtain a strain derived from grip separation that is close to the actual strain.