Stress Cracking Morphology of Polyethylene (PE) Geomembrane Seams

The high crystallinity of polyethylene (PE) geomembranes offers an excellent chemical resistance to harsh chemical leachates, but can be problematic with regard to stress cracking. Particularly vulnerable are field seams due to their overlapping geometry, necessity for surface grinding for removal of oxide layers, and possibility of poor quality. Using laboratory test specimens under constant load (ASTM D 2552 [modified]) it was shown that field seams can be subjected to such stress cracking.

This paper presents a fracture surface morphological study of the cracked specimens from those particular tests. Five different seam types were evaluated, compared, and contrasted under different applied stress levels. Qualitatively, the morphology patterns were grouped into five categories: long fibrous, short fibrous, flake, hackle, and lamellar. These morphologies appear to be related to the magnitude of the applied global stress. The short fibers occurred at low stress, the long fibers at moderate stress, and the flake at high stress. Hackle often appears before the plastic failure as the cross-sectional area of the specimen decreases. Lamellar, the least common structure, may be caused by a combination of a low stress cracking with local plastic failure.

What remains now is to see if this tentative classification applies to stress-cracked specimens from large laboratory tests and for field situations. If the nature of the field failure can be ascertained by comparison with the laboratory-generated categories (small and large sizes), implications to optimal seam types and/or polymer compound type may become apparent.