Geomembrane Residual Stress

Q: We are getting a lot of uncertainty with our ASTM D5397 single-point notched constant tension load (SP-NCTL) stress crack testing. Can you shed some light on the situation? This uncertainty is frustrating our field personnel and holding up a rather large HDPE geomembrane liner installation for a massive landfill liner.

A: You may be experiencing residual stress in the geomembrane from fabrication. This often occurs on thin geomembranes 0.75 mm (29.5 mil) thick or less that are run fast in production and where stretching may occur during the cooling and take-up operations. Residual stresses are those stresses that remain in a product even in the absence of external loading. In extreme cases, residual stresses result in plastic deformation, leading to distortion of the geomembrane. One can get rid of residual stress in geomembranes by annealing the material to relieve or redistribute the residual stresses. This is generally done to a SP-NCTL sample prior to testing in the conditioning phase of the method at 212 degrees Fahrenheit (100 degrees Celsius) for one hour in a forced air oven.