A Digital Image Analysis Technique for Improved Strain Measurement in Geosynthetic Tensile Testing

Tensile testing of geosynthetics for measurement of strength and stiffness plays a critical role in the selection and specification of these materials for civil engineering applications. Measurement of the tensile properties of geotextiles is commonly performed via the widewidth strip method, where wide-width specimens are gradually strained to failure while being held in tight-gripping jaws or wrapped roller grips. During testing, material strains are inferred from the relative displacement of the grips. This approach does not properly account for slippage of the material in the gripping jaws or around the roller grip windings. Moreover, it inherently assumes a uniform distribution of strain across the tested specimen, which is often not the case at failure, as can readily be observed from visual examination of tested specimens. To address these issues, this paper utilizes a simple digital image analysis technique, implemented using open source software tools, to improve strain measurements in geosynthetic tensile testing. The presented technique is simple to use, low-cost, and can be deployed easily using readily available technology, making this technique useful for geotechnical engineering practitioners and laboratory testing managers in a production environment. Results from wide-width geotextile tensile tests on a polypropylene woven fabric are presented and analyzed using this technique; tests conducted using both tight-gripping jaws and wrapped roller grips show significant slippage, stress concentrations, and nonuniform strains. The technique that was utilized allowed for improved strain characterization across the entirety of a given tested specimen, which in turn yielded enhanced interpretation of geotextile test results.