Measuring strain in geosynthetics

Q: Currently, what is the best-known method to measure strains in geosynthetics? We are having arguments between results generated from crosshead movement and extensometers.

A: This question arises often when comparing ISO 527/ASTM D638 and ASTM D6693 results. The former requires an extensometer, where the latter utilizes crosshead deflection and assumes gauge lengths. This discussion first needs to start with preload on the specimen and the assurance that there is no slack in the specimen at the start of the test. The next point is that the extensometers are generally set up with a gauge length of 1 inch (25 mm), where ASTM D6693 assigns a gauge length for yield at 1.3 inches (33 mm). Therefore, the elongation (strain) at yield is rarely going to be comparable. In short, the strain for high-density polyethylene (HDPE) geomembranes will be in the low teens for ISO 527/ASTM D638 and in the high teens for ASTM D6693. This infuriates people who oversee specification review but is an artifact of the testing procedure.