In 1926 Ostwald found for structural viscous fluids' a critical flow rate at the transition from the laminar to a new type of irregular flow, far below the rate for Reynold's established eddy turbulence. Tordella localized his LDPE melt flow instabilities in 1956, observing the partition of soft markers at the constrict of a glass pipe and tracing it to a crucial value of the shear rate in a corresponding capillary die. He also introduced the label 'melt fracture' after hearing a related crackling noise. Jet the expression 'flow instability' is to be recommended. In 1960 Reiner predicted, that the polymer fluid must loose structural coherence after the strain rate becomes larger than the rate of relaxation. In 1972 Hurlimann found, that a LDPE melt strand at the inlet of a die is stretched at a nearly constant elongational force, and that the flow transition occurs at a critical value of elongational stress. This dynamic property is much less dependent on temperature and other physical parameters than formerly used kinematic criteria. In extensive investigations, Cogswell also drew attention to the correlation of die inlet pressure loss, elongational stress and strain rate at the transition to flow instability. For now sixty years many more researchers ventured to find the key to these phenomena, but even an extensive compilation of 2011 spotted no conclusive interpretation. Let us return to the basic physics.
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