Fionn P.E. Dunne

Computational modelling of large deformations in layered-silicate/PET nanocomposites near the glass transition

Layered-silicate nanoparticles offer a cost-effective reinforcement for thermoplastics. Computational modelling has been employed to study large deformations in layered-silicate/poly(ethylene terephthalate) (PET) nanocomposites near the glass transition, as would be experienced during industrial forming processes such as thermoforming or injection stretch blow moulding. Non-linear numerical modelling was applied, to predict the macroscopic large deformation behaviour, with morphology evolution and deformation occurring at the microscopic level, using the representative volume element (RVE) approach. A physically based elasto-viscoplastic constitutive model, describing the behaviour of the PET matrix within the RVE, was numerically implemented into a finite element solver (ABAQUS) using an UMAT subroutine. The implementation was designed to be robust, for accommodating large rotations and stretches of the matrix local to, and between, the nanoparticles. The nanocomposite morphology was reconstructed at the RVE level using a Monte-Carlo-based algorithm that placed straight, high-aspect ratio particles according to the specified orientation and volume fraction, with the assumption of periodicity. Computational experiments using this methodology enabled prediction of the strain-stiffening behaviour of the nanocomposite, observed experimentally, as functions of strain, strain rate, temperature and particle volume fraction. These results revealed the probable origins of the enhanced strain stiffening observed: (a) evolution of the morphology (through particle re-orientation) and (b) early onset of stress-induced pre-crystallization (and hence lock-up of viscous flow), triggered by the presence of particles. The computational model enabled prediction of the effects of process parameters (strain rate, temperature) on evolution of the morphology, and hence on the end-use properties.

Performance enhancement of polymer nanocomposites via multiscale modelling of processing and properties

Abstract This paper provides an overview of research on modelling of the structure–property interactions of polymer nanocomposites in manufacturing processes (stretch blow moulding and thermoforming) involving large-strain biaxial stretching of relatively thin sheets, aimed at developing computer modelling tools to help producers of materials, product designers and manufacturers exploit these materials to the full, much more quickly than could be done by experimental methods alone. The exemplar systems studied are polypropylene and polyester terephalate, with nanoclays. These were compounded and extruded into 2mm thick sheet which was then biaxially stretched at 155°C for the PP and 90 to 100°C for the PET. Mechanical properties were determined for the unstretched and stretched materials, together with TEM and XRD studies of structure. Multi-scale modelling, using representative volume elements is used to model the properties of these products.

A model for the consolidation of hexagonal array matrix coated fibre composites

Physically based constitutive equations for the consolidation of matrix coated fibres (MCFs) arranged in hexagonal arrays are presented in this paper. The constitutive equations have firstly been developed using a variational method for a unit cell representing hexagonal array packing of the coated fibres under symmetric in-plane compressive load. The resulting constitutive equations have subsequently been generalized to multiaxial stress states and implemented into finite element software within a finite deformation framework to enable simulations of practical manufacturing processes. The model has been validated by comparing predicted results with those obtained from independently developed explicit micromechanical finite element models. The ability of the model to predict the observed behaviour has been tested by comparing the results of simulations with those of experiments. The model predictions compare well with the measured results. In addition the results of simulations suggest that the densification curves for square and hexagonal array packing effectively provide the lower and upper bounds, respectively, for practical consolidation behaviour of MCFs under uniaxial constrained compression.

Multiscale modelling of layered-silicate/PET nanocomposites during solid-state processing

This work aims to develop a continuum, multi-scale, physically-based model of the forming process for layered-silicate nanocomposites based on poly(ethylene terephthalate) (PET) matrices, as might be used for packaging. This challenge is tackled using: (1) a physically-based model of PET implemented into the FEM-based code ABAQUS, (2) RVEs with prescribed morphologies reflecting TEM images, and (3) nonlinear computational homogenisation. As a result, 2-D two-scale FEM-based simulations under biaxial deformations (constant width) enabled the extraction of macroscopic stress-strain curves at different silicate contents and processing temperatures. In particular, interesting features in terms of morphology changes and its impact on the macroscopic stress response were captured: (a) morphology change by particle re-orientation and pronounced bending, (b) macroscopic strain hardening due to platelet re-orientation and local strain stiffening, (c) facilitated platelet alignment, and platelet delamination in tactoids through sheardominated deformations and increasing temperature.

Diffraction Post-Processor for Polycrystalline Plasticity Modelling

Microscopic and macroscopic deformation of a polycrystal due to an applied load can be modelled using crystal plasticity implemented within the Finite Element (FE) framework. However, while macroscopic predictions can readily be validated against conventional monotonic and cyclic stress-strain curves, verification at the microscopic level is harder to achieve, since it involves calibrating the predictions for stresses and strains in individual grains, or in grains grouped by certain criteria (e.g., orientation). In this paper an elasto-plastic polycrystal finite element model is introduced, and its calibration is performed at a mesoscopic level via comparison with neutron diffraction data obtained experimentally. Time-of-flight (TOF) neutron diffraction experiments carried out on ENGIN-X instrument at ISIS involved in situ loading of samples of C263 nickel-based superalloy. In order to compare the numerical predictions of the FE model with these experimental data, the corresponding mesoscale average elastic strains must be extracted from the results of the simulation by employing a ‘diffraction post-processor’. This provides a much improved technique for the calibration of FE formulation and enhances the confidence in the model. The FE diffraction post-processing procedures are discussed in detail, and comparison between the model predictions and experimental data are presented.

Modelling Heterogeneous Microstructures, Inhomogeneous Deformation and Failure in Superplasticity

A material model is presented for superplastic deformation, at the continuum level, that is coupled with the inhomogeneous microstructural evolution that occurs through normal grain growth and deformation enhanced growth. The model has been developed to enable finite element analyses at the process level, in which the evolution of experimentally observed distributions of grain size can be predicted, together with localisation and failure.