Cold-drawing rate dependence

Fig. L(a) Three common working definitions of the yield point for metals. (1) Load maximum (2) tangent method, (3) firoofistress" or "strain-offset method. (The proof-strain is commonly taken to be 0 /%, but is quite arbitrary.) (b) Load elongation curves for polymers. (I) Brittle, (2) strain softening, (3) cold-drawing, (4) strain-hardening, (5) rubbery. Typical definitions of the yield point are marked by arrows on curves (2), (3) and (4). Any one polymer can show behaviour raiding from (1) to (5) depending on test conditions, e.g. temperature, strain-rate, tension...

It is important to note that this peculiar time-dependence of the deformation rate is associated with a simple law for the evolution of stress. Since the external force during cold-drawing is constant, the tensile stress acting on the volume element follows from... 

The growth rate dependence of the work suspended in plastic flows is conceivable if we recall, for example, the strain rate dependence of the force applied for the shear yielding during cold-drawing. Experimentally one finds a power law for the relation between Ki and c in the range of subcritical growth... 

The PEE was prepared on a semicommercial scale [29] from PBT and PEG of molecular weight 1000 in a weight ratio of 1 1. Neat PBT and PEE were cooled in liquid nitrogen, then finely ground and blended (1 1 by wt). Films of the blend and of the respective neat components were prepared by means of a capillary rheometer equipped with metal rolls for immediate pressing the extruded bristle. The films obtained were 3-4 mm wide and 100-150 pm thick depending on the extrusion rate and the force applied to the rolls after cold drawing (A = 4-8) they were annealed with fixed ends for 6 h at 170°C (neat PBT and PEE) or at 170, 180, 190, and 200°C (PBT/PEE blend) in a vacuum oven [28]. 

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