Deformation Instabilities and Cavitation in Plastic Deformation of Semicrystalline Polyethylene

Abstract

Tensile deformation behavior of semicrystalline highdensity polyethylene (HDPE) was investigated and compared with compression to identify the main mechanisms and instabilities that are important for the process. Deformation behavior and the associated structure evolution were investigated using SAXS, SEM, and DSC. Samples of HDPE, neat and modified with a low-molecular liquid, were used. Unlike neat HDPE, the modified materials demonstrated strongly suppressed cavitation. It was found that, irrespective of cavitation, the tensile deformation of all samples was governed by crystallographic mechanisms in crystalline lamellae, supported by shear deformation in the amorphous layers, the same as those operating also in other deformation modes. In addition, two important deformation instabilities were always observed in well-defined strain ranges: (1) microbuckling followed by formation of lamellar kinks, beginning at a true strain of 0.3−0.4, and (2) slip instability leading to extensive lamellar fragmentation at true strains above 0.6. These instabilities were found to be common and very important steps in the deformation sequence, occurring in both compressive and tensile deformation modes, in similar strain ranges, regardless of cavitation, which appeared to be a tension-specific phenomenon that does not actually influence the deformation sequence very much. Cavitation is not able to replace or notably modify the main deformation mechanisms and moreover does not compete with main instabilities associated with crystalline lamellae, microbuckling and slip instability, and thus can be considered rather as a tensile-specific side effect that does not seem essential from the point of view of plastic deformation behavior, although still significantly affecting the final properties and appearance of the drawn material, as it introduces a substantial number voids into the structure.