A comparison of the response of geosynthetics in the multi-axial and uniaxial test devices

The wide strip tension test imposes boundary conditions that vary from a un iaxial stress state near the middle of the specimen to a plane-strain, biax ial stress state at the clamps. The multi-axial tension test imposes bounda ry conditions that vary from a plane-strain, biaxial stress state at the re straining ring to a nearly isotropic, biaxial stress state at the center. T o evaluate the influence of the stress state induced during testing on the stress-strain response of geomembranes, strain-controlled multi-axial and w ide strip tests were performed on specimens of elastic latex, polyvinyl chl oride (PVC), and high density polyethylene (HDPE). The ratio of the secant Young's modulus in the multi-axial test to that in the wide strip uniaxial test was approximately 1.2 for the nearly linear, elastic latex membrane, w hich is substantially less than the theoretically derived value of 2.0. Thi s ratio was approximately 1.4 and 1.9 for PVC and HDPE geomembranes at 1% s train, respectively, indicating greater differences between measured multi- axial and uniaxial responses with materials exhibiting more nonlinearity. S trength values measured in the tests were similar, but the uniaxial test ov erestimated the ductility (i.e. failure strain) of the HDPE geomembrane. Wi der use of the multi-axial test device is recommended for cases where the g eomembrane deforms in a biaxial stress state, and material ductility and st iffness are important.