Plastic deformation of polyethylene crystals by dislocation motion

Solvent-free polyethylene single crystals were plastically deformed by depositing the crystals on carbon replicas obtained from the surfaces of deformed copper single crystals. The observations by transmission electron microscopy suggest that plastic shear parallel to the molecular chains of the polyethylene crystals ([001]-direction) occurs by the movement of dislocations the Burgers vector of which is parallel to the [001]-direction.     

The plastic deformation behaviour of linear polyethylene and polyoxymethylene

The plastic deformation behaviour of linear polyethylene and polyoxmethylene has been studied as a function of strain rate over a very wide range of plastic strains (deformation ratios up to 25). The results have been examined in terms of the concept of a true stress-strain-strain rate relationship. It has been shown that results for drawn fibres and extruded rods are compatible on this basis, and true stress-strain-strain rate surfaces can be defined. The significance of these results with regard to both tensile drawing and hydrostatic extrusion processes is considered. A preliminary discussion of the rate dependence is also presented.     

The role of recovery forces in the deformation of linear polyethylene

In the light of the finding that the deformation of linear polyethylene is associated with the development of recoverable strain, a technique has been developed to determine the magnitude of the recovery forces. The difference between the applied force and the recovery force represents the effective force which acts on the anelastic processes and a consideration of the kinetics of deformation suggests that the anelastic process consists of the co-operative movement of a number of molecular segments. The extrapolated yield point appears to be associated with the effective force and has no particular structural significance, in that it corresponds merely to the point of maximum curvature in the relationship between effective stress and rate of deformation.     

Microstructural changes induced in linear polyethylene by plastic deformation (rolling)

Mechanical spectrometry and differential scanning calorimetry analysis completed by wideangle X-ray diffraction and density measurements were performed on linear polyethylene in order to characterize the influence of a plastic deformation (rolling) on the microstructure. Thus, rolling performed at room temperature results in the breaking of the thicker lamellae. The changes in the microstructure in polyethylene induced by rolling are in agreement with those suggested by other authors. A phenomenological model, applied to the relaxations exhibited by polyethylene, allows quantitative analysis of the microstructural changes due to plastic deformation. Thus, rolling induces an increase in the interactions between phases through an increase in the number of tie molecules connecting the broken crystallites. Moreover, rolling results in an increase of crystallographic defect concentration within the crystalline phase and this plastic deformation induces phase transformation from orthorhombic to monoclinic lattice, which is preferentially developed for thicker lamellae.     

Plastic deformation in a thick transversely isotropic layer containing a penny-shaped crack

Summary The width of a thin plastic annular zone formed during the deformation of a pennyshaped crack in a transversely isotropic layer of an ideal elasto-plastic material is determined. Considered are the cases where the penny-shaped crack is extended by normal stresses and by torsional stresses. The faces of the layer are shear free and deformation of the plastic zone around the penny-shaped crack occurs according to the Dugdale hypothesis. For each case, the solution of the problem is reduced to a Fredholm integral equation of the second kind. Iterative solutions are obtained for small values of the parameters and numerical results for the width of the plastic zone are determined. Graphical results showing the effect of transverse isotropy upon the width of the plastic zone are also presented.With 6 Figures     

Plastic deformation of polypropylene

In a wide range of drawing temperatures (20 to 150° C) and draw-ratios (5 to 20) the axial long periodL _T is a unique function of temperature. It is completely independent of the long period of the starting material which was varied within wide limits (125 to 350 Å). The gradual transformation of the long period observed at small draw-ratio ( between 1 and 5) from the valueL ₀ of an undrawn sample to the limiting valueL _T>L ₀ of highly drawn polypropylene ( 5) could be demonstrated on samples withL ₀ larger thanL _T. The change of long period is abrupt, indicating a discontinuous step in the transformation from the original microspherulitic into the fibre structure.These results may be interpreted in the same manner as in the case of polyethylene. During plastic deformation in the neck the lamellae are broken into small folded chain blocks which are then incorporated into the microfibrils, the basic building element of the fibre structure. The work of deformational forces must so mobilise the chains in the blocks that they get rearranged with a new long period, corresponding to the temperature of drawing.Paper I: The limiting axial long period of drawn polypropylene,J. Polymer Sci. A2 in press.On leave of absence from the Institute of Chemical Physics, Rocosalano, CSIC, Madrid, Spain.     

Plastic deformation of polyethylene and ethylene copolymers: Part I Homogeneous crystal slip and molecular mobility

The plastic behaviour of polyethylene and ethylene copolymers is studied under uniaxial tensile testing in parallel with the viscoelastic properties. Homogeneous plastic deformation is shown to take place at temperatures above the crystalline mechanical relaxation. The activation of homogeneous crystal slip is discussed in relation to the crystal lamella thickness and the molecular mobility of the crystalline chain stems. The thermally activated process of nucleation and propagation of screw dislocations that is proposed for the mechanism of the homogeneous crystal slip relies on the generation of 180° chain twists in the crystal stems of the sheared crystals. This kind of conformational chain defect is the basic link between the plastic and the viscoelastic properties of the materials. Homogeneous crystal slip can take place as long as the applied strain rate is consistent with the strain rate affordable by the screw dislocation propagation. The dependence on draw temperature of the crystal thickness in the fibre structure is ascribed to the stress-induced activation of 180° chain twists which allows an adjustment of the crystal thickness to the temperature of the experiment faster than an annealing treatment will. © 1998 Chapman & Hall     

Plastic deformation of polyethylene and ethylene copolymers: Part II Heterogeneous crystal slip and strain-induced phase change

The plastic behaviour of polyethylene and ethylene copolymers is studied under uniaxial tensile testing with particular attention to the development of plastic instability. Heterogeneous crystal slip is suggested to take place when homogeneous crystal slip either is not allowed at the temperature and strain rate of the experiment or is exhausted owing to extension of the chain folds. The chain unfolding concomitant to the fragmentation of the crystalline lamellae is suspected to have a low strain hardening that is responsible for the phenomenon. Partial screw dislocations with a shorter Burgers vector than in the case of homogeneous slip are proposed to become operative because of the activation of 90 ° chain twists in the crystalline stems. Dissociation of dislocations into partials involves stacking faults in the orthorhombic structure that may turn into monoclinic structure through a martensitic-like transformation. Crystal slip is likely to concentrate in these faulty regions owing to the reduced molecular interactions and lower density. Two types of correspondence of the transformed monoclinic phase with the parent orthorhombic structure are observed. The modification of the chain-folding macroconformation as a function of the crystallinity of the materials is suspected to influence the transformation shear mode. © 1998 Chapman & Hall     

Plastic deformation of invar alloys

Heavy cold-rolling (95% reduction) of a 64Fe36Ni Invar alloy results in a decrease in magnetization and magnetic hyperfine field at the Fe nucleus, and an increase in electrical resistivity and x-ray line width. These changes are explained by a transition of iron atoms from a high volume-high spin electronic configuration to a low volume-low spin configuration during the deformation process.     

Epitaxial crystallization of linear low-density polyethylene on high-density polyethylene

The oriented crystallization of linear low-density polyethylene (LLDPE) on high-density polyethylene (HDPE) has been investigated by transmission electron microscopy. From morphology and electron diffraction, it is confirmed that epitactic growth of LLDPE lamellae on the HDPE crystals takes place with an adoption of the HDPE crystal thickness at the interface and a continuous thinning of the LLDPE lamellae in the interface. This revised version was published online in November 2006 with corrections to the Cover Date.     

Structure and plastic deformation of polyethylene

A review is given of the structure of semi-crystalline polymers and the mechanisms of plastic deformation in them. High-density polyethylene (HDPE) is taken as the specific example because of the large number of detailed studies performed on this material. The early findings are also compared and contrasted with very recent detailed large-strain deformation studies and computer simulations of deformation-induced texture development in HDPE.     

Plastic deformation of CuAlMn shape-memory alloys

CuAlMn alloys transforming below room temperature were cold rolled after quenching up to 93% reduction. Alloys deformed only 10% were already completely martensitic and did not show reverse transformation to ₁ upon heating. With increasing deformation, refinement of martensitic plates occurred due to repeated internal twinning and injection of new orientation variants. This was accompanied by decreasing size of ordered domains down to that of shortrange ordered microdomains at 93% reduction. Deformation bands appearing at 60% reduction consisted of extremely small grains, giving an electron diffraction pattern resembling that of an amorphous structure.     

Plastic deformation of impact loaded frames

The deflections of an elastic-plastic portal frame subject to impact loading is investigated, both numerically and experimentally. The numerical transient analysis accounts for the influence of large deflections and rotations in addition to elastic-plastic material behaviour. The experimental method is used to measure the time-dependent deflections of the frame when impacted by a steel hammer. The numerical results show good agreement with the experimental observations.     

Plastic deformation of copper thin foils

According to one suggested model, bending of a single crystal introduces edge dislocations of the same sign. In the present study, this model is examined by computer simulation using molecular dynamics. When a notch is present on the tension surface, Heidenreich-Shockley partial dislocations are created near the tip of the notch. In the compression surface, partial dislocations are created due to wrinkling of the crystal plane. The results of simulation shows that dislocations are more easily created in a compressive bending region than in a tension bending region or simple tension region. For shear deformation, partial dislocations are created on the highest resolved shear stress slip plane {1 1 1} and slip in the direction of highest resolved shear stress.