Geometric strain hardening

Comparison of GUR 1050 in three conditions as described by the respective load-displacement curves. Longevity and DURASUL are commercially available formulations of highly crosslinked UHMWPE, which exhibit the characteristic geometric strain hardening in the drawing phase of the small punch test. Free radical quenching by remelting UHMWPE above its f>eak melt transition typically reduces the observed peak load as exhibited here (Edidin and Kurtz 2001). 

From the microscopic picture for the craze growth it seems clear that one important microscopic variable must be the mean number n of entangled strands within each fibril which survive the geometrically necessary strand loss associated with the interface formation. If the number of such strands is zero, the fibril will fail, since the polymer fluid which flows from the active zone into the fibril has no strain hardening capability and will not be able to support the relatively high tensile stresses necessary to propagate the interface. To obtain n one first estimates n, the total number of strands in the undeformed phantom fibril from which a craze fibril is drawn and which is given by ... 

A ductile material can be stretched uniformly only when stable flow occurs. The stable flow of materials has been investigated by Hart who described the transition from the stable to unstable flow. The beginning of geometrical instability and localisation of strain is the limit of the stable flow. At temperatures above 0.5 T (at equilibrium between recovery and hardening) the strain rate sensitivity parameter "m" may be derived from the expression ... 

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