Characterization of the spherulitic deformation in equatorial region and cavitation in HDPE materials submitted to mixed-mode oligo-cyclic tensile loading

• Published in: Polymer testing 2021-07, Vol.99, p.107208, Article 107208
• Main Authors: Guo, Hang, Rinaldi, Renaud G., Tayakout, Sourour, Broudin, Morgane, Lame, Olivier
• Format: Article
• Language: English
• Subjects: Cavitation; In-situ SAXS measurements; Oligo-cyclic tensile loading condition; Semi-crystalline polymers; Spherulite
• ISSN: 0142-9418; 1873-2348
• DOI: 10.1016/j.polymertesting.2021.107208

The characterization of the mechanical response of semi-crystalline high-density polyethylene (HDPE) materials submitted to mixed mode oligo-cyclic tensile loading condition is attempted. More precisely, HDPE materials of varying microstructures triggered via thermal treatments, are cycled to a prescribed strain in the vicinity yet beyond the elastic limit and then retracted to zero stress. Macroscopically, a Mullins-like effect can be observed. At the meso- and micro-scales, in-situ SAXS measurements have been carried out so that the local deformation of the equatorial region of the spherulites can be estimated along with the broad evolutions of the crystalline spherulitic structures and the cavitation. Upon loading, the local strain is found proportional to the macroscopic one in the equatorial region of the spherulites. The shearing of the crystallites initiates during the first cycle and slightly accumulates with increasing cycles. Moreover, nano-sized cavities are only observed in the isothermal samples exhibiting higher crystallinity and a lower density of stress transmitters. These micro-voids are nucleated during the first loading step. Assuming that the cavities do not merge, it is found that the accumulation-saturation of the cavity volume fraction is essentially resulting from their dimension increase, especially along the direction perpendicular to the macroscopic elongation. •The deformations of nano-sized lamellar stacks located at the equatorial region of micron-sized spherulites remain proportional to the macroscopic ones.•At the equatorial region, the first cycle induces the most important shearing/fragmentation of the crystallites and the remaining cycles cause the accumulating evolution and stabilization of the crystalline structure.•The micro-voids were nucleated during the first loading step, and partially closed upon the first unloading step. With consecutive cycles, only the dimensions of the cavities evolve whereas no additional nucleation appear in the observation window.•It is very interesting to noticed that all the microstructural modifications seems to saturate very quickly in few cycles under mixed mode oligo-cyclic loading conditions.•The tendency for the PE to “cavitate” was highly dependent on the initial microstructure.