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

Abstract

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.

Keywords

• Semi-crystalline polymers
• Oligo-cyclic tensile loading condition
• In-situ SAXS measurements
• Cavitation
• Spherulite