Single crystal plasticity approach

It is not yet possible to establish a clear link between the approach of Ward and co-workers to the problem of molecular reorientation, which is discussed above at length, with many theories in the literature which take as their starting point the crystalline nature of the material. Some of these ideas have already been discussed in relation to the yield criterion for anisotropic polymers, but it is worth looking at them again to assess how they relate to the structure of the material which has been deformed in the 

Specific crystallographic deformation mechanisms have been proposed by many other workers, e.g. Refs. 29, 38, 42, 43, 55, but the distinction between elastic and permanent deformation has not always been made. Indeed the question of whether this is in general possible, and whether the yield behaviour is related in any way to the elastic behaviour is still to be settled. 

Another possible link between the two schools may be established by observations of the X-ray photographs published by several workers (Refs. 22, 34, 37, 48, 59-62) which show indications, at least, of a loss of crystallinity with plastic strain. Jajima and Seto measured the change in lamellar orientation and c-axis orientation on redrawing oriented HDPE. They commented that the WAXS scattering from specimens experiencing a further tensile strain in the draw direction became too difluse to measure 

TAe o/ increasing axial compressive strain on the IVAXS photographs from oriented polypropylene. Increasing disorientation and distortion of the Bragg reflection is apparent at strains greater than 10% (after Zihlif ). 

The break-up of crystallites and the reformation of the lamellar fragments into microfibrUs is the basis of a theory for the cold-drawing of isotropic semi-crystalline polymers due to Peterlin. (See also Hosemann et and Robertson .) Both Peterlin and Hosemann assert that the main mechanism is the break-up of each crystallite into approximately twenty smaller units which lie like pearls on a string with their chain axes parallel to the IDD. Many aspects of these theories would seem to be relevant to the deformation of oriented polymers of modest draw ratios. 

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A further study used this resonance to ascertain the anisotropic components of the hyperfine coupling in the radicals formed by irradiating bicyclo[2.2.1]hept-2-ene (norbornene) in its plastic phase. The tensor had already been derived from a single-crystal FT-,aSR investigation but the ALC experiment was able to determine the (axial) anisotropy in a powder sample from the asymmetry of the shape of the resonance. The results agreed well with the FT-,aSR data and confirmed effective partial averaging above (129 K) and a rapid approach to complete isotropy at (320 K). 

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