| Abstract: | The phenomenon of stepwise creep has been discovered and studied earlier for unoriented polymers by a new laser interferometer method. This same technique has been used for studying long-term menhanical properties of UHMWPE gel-spun fibers of various draw ratios. A variability of creep rate has been observed. The creep rate variability has been characterized by two parameters: the typical period of deformation increment, with enhanced deformation rate (step), and the variability (inhomogeneity) of the deformation rate over a deformation interval. Despite a large scatter of data, at least three levels of deformation steps could be derived: (i) microns, (ii) tens of microns, (iii) hundred and more microns. The larger levels of deformation steps decrease with increasing draw ratio. Their dependence on strain is not observed. The creep rate variability significantly depends on the creep strain. First, it increases with strain and then decreases just prior to rupture. The effect of draw ratio on creep rate variability is not significant. It is assumed that the creep rate variability is a cooperative deformation. It is suggested that interfibrillar slip and slip between fibrillar layers occurs via a slip-stick motion making a significant contribution to total deformation. The mechanism does not change with draw ratio. |
