Fiber-grade high-density polyethylene (HDPE) was melt spun into a mult
ifilament yarn that, after quenching in air, was wound at speeds in th
e range 450 m/min to 1000 m/min. Tensile tests on these anisotropic ya
rns showed two yield points of varying intensity, similar to those rep
orted in the literature for compression molded, isotropic HDPE sheets
and films. However, unlike the sheets, the second yield in the case of
fibers always occurred at a stress higher than that for the first yie
ld. More detailed studies on the yarn spun at 450 mi min showed that t
hese characteristics persisted at different strain rates and test temp
eratures and were also present in the heat-set as-spun samples. The ap
plicability of the existing models to explain the double-yield phenome
non has been critically examined in the light of the results obtained.
For this, the necessary structural and morphological characterization
of the samples was also done.