P. Caton-Rose

Process structuring of polymers by solid phase orientation processing

Solid phase orientation of polymers is one of the most successful routes to enhancement of polymer properties. It unlocks the potential of molecular orientation for the achievement of a range of enhanced physical properties. We provide here an overview of techniques developed in our laboratories for structuring polymers by solid phase orientation processing routes, with a particular focus on die drawing, which have allowed control of significant enhancements of a single property or combinations of properties, including Young’s modulus, strength, and density. These have led to notable commercial exploitations, and examples of load bearing low density materials and shape memory materials are discussed.

The modelling of large deformations of pre-oriented polyethylene

Abstract High temperature reversion tests have revealed a state of pre-existing molecular orientation in extruded polyethylene sheet. This state is related to differences in stress-deformation behaviour when specimens of the sheet are stretched along different angles with respect to the extrusion direction. An established large deformation, rate-dependent constitutive equation has been developed to model this material, by incorporating the pre-orientation by the addition of a strained Gaussian network. The level of pre-orientation is deduced from the dimensional changes on shrinkage. The constitutive equation is incorporated into the finite element package abaqus , and the shapes and drawing forces of tensile specimens extended at various angles to the extrusion direction are modelled.

Finite element simulation of geogrid manufacture using large deformation elastic formulation

Abstract An elastic model of large solid polymer deformations has been used as a basis for numerical predictions of the shapes of polyethylene geogrids. These netlike products are made by solid phase deformation at elevated temperatures of extruded sheet containing arrays of suitably shaped holes. The elastic constitutive model is based on a theory of interacting polymer chains and is implemented within the finite element package ABAQUS in both two and three dimensions. Deformations correspond to extension ratios of up to 8. Good predictions of the final shapes of the geogrid products are obtained. It is concluded that the methods used are a valuable product development tool.

Fibre orientation structures and their effect on crack resistance of injection moulded transverse ribbed plate

Abstract An extensive study of the fibre orientation structures developed in a transverse ribbed plate during injection moulding, and the use of these structures to investigate the effect of local fibre orientation state on crack initiation resistance, is reported. The fibre orientation results for the ribbed plate, measured using large area image analysis system developed at Leeds University, showed that after an initial settling down period, the central core region, where the fibres are aligned perpendicular to the flow direction, decreased in size monotonically, with an associated monotonic increase in the outer shell regions, where the fibres are aligned preferentially along the injection direction. Interestingly, the level of orientation in the two regions remained almost constant: only the proportions of the two regions were found to change with flow length. Across the plate, close to the gate, the central core region was found to have a lens-like shape, while at the other end of the plate the core was thinner and also consistent in thickness across the sample width. The transverse rib was found to cause little disturbance to the fibre orientation of the base plate. The different proportions of the shell and core regions at different locations over the ribbed plate provided an ideal case to test the proposition of Friedrich that the crack resistance of a short fibre reinforced material depends on the number of fibres that are perpendicular to the crack tip. The impact test results gathered in this way confirmed this hypothesis of Friedrich.

Development of a Constitutive Model of Polypropylene for Thermoforming

In this paper the authors outline a constitutive model, implemented within finite element analyses, which was developed for large deformation, high temperature multi‐axial stretching of polypropylenes. The model has been generalised to a fully 3‐dimensional thermally coupled form. The paper describes how model parameters were characterised using constant width, biaxial and sequential stretching of polypropylenes at elevated temperature using a custom built flexible biaxial stretching machine developed at Queen’s University Belfast. The paper presents results of finite element model predictions of material stretching behaviour compared to range of physical experiments. The results presented in the paper confirm that this model is very effective in predicting the complex thermo‐mechanical behaviours of polypropylenes at elevated temperatures.

Application of activated barrier hopping theory to viscoplastic modeling of glassy polymers

An established statistical mechanical theory of amorphous polymer deformation has been incorporated as a plastic mechanism into a constitutive model and applied to a range of polymer mechanical deformations. The temperature and rate dependence of the tensile yield of PVC, as reported in early studies, has been modeled to high levels of accuracy. Tensile experiments on PET reported here are analyzed similarly and good accuracy is also achieved. The frequently observed increase in the gradient of the plot of yield stress against logarithm of strain rate is an inherent feature of the constitutive model. The form of temperature dependence of the yield that is predicted by the model is found to give an accurate representation. The constitutive model is developed in two-dimensional form and implemented as a user-defined subroutine in the finite element package ABAQUS. This analysis is applied to the tensile experiments on PET, in some of which strain is localized in the form of shear bands and necks. These deformations are modeled with partial success, though adiabatic heating of the instability causes inaccuracies for this isothermal implementation of the model. The plastic mechanism has advantages over the Eyring process, is equally tractable, and presents no particular difficulties in implementation with finite elements.

Fibre orientation: measurement, modelling and knowledge based design

Abstract In this paper the authors describe the results of a research programme which has investigated the links between the orientation distribution of short fibre reinforced composites produced during injection moulding and the mechanical properties of the resulting moulded components. A variety of injection moulded parts, including both model shapes (e.g., a transverse ribbed plate) and commercial products, has been manufactured and studied, both experimentally and using simulation. The fibre orientation distribution (FOD) has been characterised for each component at a number of chosen locations using an in-house developed image analyser. Measurement of the FOD for a range of different component shapes has led to the proposal of a number of preliminary design rules, which have been incorporated into a knowledge based engineering (KBE) design package. A crucial component of the KBE design optimisation is the use of a simulation package to predict the FOD for any component shape. Therefore, the accuracy of the principal commercial simulation package for FOD prediction, Moldflow, has been investigated by comparison with the experimentally measured FODs. Finally, the link between FOD and mechanical properties (both elastic modulus and fracture) has been studied by comparing analytical predictions with mechanical measurements.