Non-linear viscoelastic properties under cyclic fatigue for oriented high-density polyethylenes (HDPEs) with different molecular aggregation states were discussed in terms of the non-linear viscoelastic parameter (NVP). Non-linear viscoelasticity of the oriented HDPEs became dominant with an increase of imposed strain amplitude, and fatigue strength decreased with an increase in the magnitude of NVP. Also, in the case of the same magnitude of NVP, the fatigue strength of the oriented HDPE drawn at the crystalline relaxation temperature, Tαc (drawing temperature, Td = 353 K) was greater than that of the oriented HDPEs drawn at the other temperatures, because PE drawn at Tαc had the most stable aggregation structure. Higher-order structural change during the fatigue process by cyclic straining for the oriented HDPE at Td = 353 K was not so apparent compared with that for the oriented HDPEs prepared at the other temperatures. In the case of the oriented HDPE at Td = 300 K, composed of crystallites with small dimensions, the larger the magnitude of imposed strain amplitude, the greater the increase in crystallite size and/or the orientation of molecular chains to the direction of cyclic deformation occurring with cyclic straining. Also, that became more dominant with an increase in the magnitude of imposed strain amplitude. In the case of the HDPE drawn at a higher temperature than Tαc (Td = 383 K), the HDPE was composed of crystallites with large dimensions, and the crystalline disordering accompanying the decomposition of lamellar crystals into the small fragments occurred at the initial stage of cyclic fatigue. The magnitude of NVP for the oriented HDPEs increased with an increase in the degree of strain concentration in the amorphous and/or crystallite boundary regions in the case of fatigue experiments at 300 K. Thus, it is reasonable to conclude that the non-linear viscoelasticity of the oriented HDPEs under cyclic fatigue at 300 K mainly originated from the deformation of the amorphous and/or crystallite boundary regions. Also, it was clarified that the appearance of non-linear viscoelasticity remarkably reduced the fatigue strength.
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