Anisotropic Creep in Semi-crystalline Polymers

The first detailed study on anisotropy of stiffness behaviour in LDPE was reported by Raumann and Saunders for material drawn at room temperature to a range of draw ratios from 10 (undrawn, isotropic) to 4-6 (very highly drawn). These initial measurements, reproduced in Fig. 7, were carried out in cyclic repeated loading and highlighted at least two 

Mention has been made above of the possible importance of an easy shear mechanism, associated with a high value of S44, and corresponding to laminar shear parallel or perpendicular to the fibre axis. The relevance of this to the data presented merits discussion. 

Wide angle X-ray diffraction studies on the highly drawn sheets show that the crystallographic c-axes are highly aligned parallel to the fibre axis and the easy shear therefore occurs parallel or perpendicular to the c-axes, although its exact nature is not known from these studies. 

In tensile loading of a specimen, the maximum shear stresses occur on planes with their normals at 45° to the tensile stress direction. In tensile loading of a 45° specimen we are interested therefore in laminar shear on planes with their normals parallel to the sheet surface which either contain or are perpendicular to the fibre axis i.e. are at 45° to the tensile stress direction). Since easy shear on either set of planes is consistent with the observed high value of S44, additional measurements (such as X-ray diffraction) must be made during deformation in order to determine the relative importance of the possible molecular deformation modes. Such measurements were not attempted in the above study. 

The quantitative significance of the easy shear mechanism in the highly 

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