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This is a study of the modelling and prediction of strain recovery in a polylactide. Strain recovery near the glass transition temperature is the underlying mechanism for the shape memory in an amorphous polymer. The investigation is aimed at modelling such shape memory behaviour. A PLA-based copolymer is subjected to stress-strain, stress relaxation and strain recovery experiments at large strain at 60 °C just below its glass transition temperature. The material is 13% crystalline. Using published data on the mechanical properties of the crystals, finite element modelling was used to determine the effect of the crystal phase on the overall mechanical behaviour of the material, which was found to be significant. The finite element models were also used to relate the stress-strain results to the yield stress of the amorphous phase. This yield stress was found to possess strain rate dependence consistent with an Eyring process. Stress relaxation experiments were also interpreted in terms of the Eyring process, and a two-process Eyring-based model was defined that was capable of modelling strain recovery behaviour. This was essentially a model of the amorphous phase. It was shown to be capable of useful predictions of strain recovery.
John Sweeney; Paul Spencer; Karthik Nair; Phil Coates. Modelling the Mechanical and Strain Recovery Behaviour of Partially Crystalline PLA. Polymers 2019, 11, 1342 .
AMA StyleJohn Sweeney, Paul Spencer, Karthik Nair, Phil Coates. Modelling the Mechanical and Strain Recovery Behaviour of Partially Crystalline PLA. Polymers. 2019; 11 (8):1342.
Chicago/Turabian StyleJohn Sweeney; Paul Spencer; Karthik Nair; Phil Coates. 2019. "Modelling the Mechanical and Strain Recovery Behaviour of Partially Crystalline PLA." Polymers 11, no. 8: 1342.
Polypropylene has been oriented by solid-phase deformation processing to draw ratios up to ∼16, increasing tensile stiffness along the draw direction by factors up to 12. Nanoindentation of these materials showed that moduli obtained for indenter tip motion along the drawing direction (3) into to 1-2 plane (axial indentation) were up to 60% higher than for indenter tip motion along the 2 direction into the 1-3 plane (transverse indentation). In static tests, tensile and compressive determinations of elastic modulus gave results differing by factors up to ∼5 for strain along the draw direction. A material model incorporating both orthotropic elasticity and tension/compression asymmetry was developed for use with Finite Element simulations. Elastic constants for the oriented polypropylene were obtained by combining static testing and published ultrasonic data, and used as input for nanoindentation simulations that were quantitatively successful. The significance of the tension/compression asymmetry was demonstrated by comparing these predictions with those obtained using tensile data only, which gave predictions of indentation modulus higher by up to 70%.
D. Vgenopoulos; J. Sweeney; C.A. Grant; G.P. Thompson; P.E. Spencer; P. Caton-Rose; P.D. Coates. Nanoindentation analysis of oriented polypropylene: Influence of elastic properties in tension and compression. Polymer 2018, 151, 197 -207.
AMA StyleD. Vgenopoulos, J. Sweeney, C.A. Grant, G.P. Thompson, P.E. Spencer, P. Caton-Rose, P.D. Coates. Nanoindentation analysis of oriented polypropylene: Influence of elastic properties in tension and compression. Polymer. 2018; 151 ():197-207.
Chicago/Turabian StyleD. Vgenopoulos; J. Sweeney; C.A. Grant; G.P. Thompson; P.E. Spencer; P. Caton-Rose; P.D. Coates. 2018. "Nanoindentation analysis of oriented polypropylene: Influence of elastic properties in tension and compression." Polymer 151, no. : 197-207.
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.
J. Sweeney; P. E. Spencer; D. Vgenopoulos; M. Babenko; F. Boutenel; P. Caton-Rose; Philip Coates. Application of activated barrier hopping theory to viscoplastic modeling of glassy polymers. Mechanics of Time-Dependent Materials 2017, 22, 145 -165.
AMA StyleJ. Sweeney, P. E. Spencer, D. Vgenopoulos, M. Babenko, F. Boutenel, P. Caton-Rose, Philip Coates. Application of activated barrier hopping theory to viscoplastic modeling of glassy polymers. Mechanics of Time-Dependent Materials. 2017; 22 (2):145-165.
Chicago/Turabian StyleJ. Sweeney; P. E. Spencer; D. Vgenopoulos; M. Babenko; F. Boutenel; P. Caton-Rose; Philip Coates. 2017. "Application of activated barrier hopping theory to viscoplastic modeling of glassy polymers." Mechanics of Time-Dependent Materials 22, no. 2: 145-165.
In constitutive models of polymers, there has been a long history of the use of strain-rate dependent viscous processes, such as the Eyring and Argon models. These are combined with elastic elements to generate viscoplastic models that exhibit typical phenomena such as rate dependent yield, creep and stress relaxation. The Eyring process is one of the most frequently used such mechanisms. It has two significant drawbacks: it implies a temperature dependence of mechanical behaviour that is in an opposite sense to that observed; and it predicts a strain rate dependence of yield stress that is less complex than that observed, leading to the requirement for two or more Eyring processes. In recent years, new ideas for amorphous polymers have been developed that lead to an alternative plastic mechanism that addresses these concerns. In this paper a constitutive model that incorporates this mechanism is developed, and its effectiveness in modelling macroscopic mechanical behaviour of polymers is explored with respect to published data.
John Sweeney; Paul E. Spencer. The Use of a New Viscous Process in Constitutive Models of Polymers. Key Engineering Materials 2015, 651-653, 812 -817.
AMA StyleJohn Sweeney, Paul E. Spencer. The Use of a New Viscous Process in Constitutive Models of Polymers. Key Engineering Materials. 2015; 651-653 ():812-817.
Chicago/Turabian StyleJohn Sweeney; Paul E. Spencer. 2015. "The Use of a New Viscous Process in Constitutive Models of Polymers." Key Engineering Materials 651-653, no. : 812-817.
Thermoplastic polyurethane (TPU)/multi‐walled carbon nanotubes (CNT) nanocomposites were prepared by twin‐screw extrusion and micro injection molding. The electrical conductivity of micro injection molded polymer nanocomposites exhibits a low value and uneven distribution in the micromolded samples. Real‐time tracing of electrical conductivity was conducted to investigate the post thermal treatment on the electrical conductivity of microinjection molded composites. The results show that postmolding thermal treatment leads to a significant increase in the electrical conductivity by over three orders of magnitude for 5 wt % CNT‐filled TPU composites. In‐situ Transmission electron microscopy confirms the conductive CNT network does not change at the micron/sub‐micron scale during thermal treatment. TEM image analysis by a statistical method was used to determine the spatial distribution of CNT in the sample and showed that the average distance between adjacent CNT reduced slightly at the nanometer scale after postmolding thermal treatment. A new conductive mechanism is proposed to explain the enhancement of electrical conductivity after thermal treatment, i.e. micro‐contact reconstruction of adjacent CNT in the polymer matrix through annealing‐induced relaxation of interfacial residual stress and strain. Raman spectra and small angle X‐ray scattering curve of annealed samples provide supporting evidence for the proposed new conductive mechanism. The electron tunneling model was used to understand the effect of inter‐particle distance on the conductivity of polymer composites. © 2015 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2015, 132, 42416.
Dongxu Li; GuoXia Fei; Hesheng Xia; Paul E. Spencer; Philip Coates. Micro-contact reconstruction of adjacent carbon nanotubes in polymer matrix through annealing-Induced relaxation of interfacial residual stress and strain. Journal of Polymer Science 2015, 132, 1 .
AMA StyleDongxu Li, GuoXia Fei, Hesheng Xia, Paul E. Spencer, Philip Coates. Micro-contact reconstruction of adjacent carbon nanotubes in polymer matrix through annealing-Induced relaxation of interfacial residual stress and strain. Journal of Polymer Science. 2015; 132 (33):1.
Chicago/Turabian StyleDongxu Li; GuoXia Fei; Hesheng Xia; Paul E. Spencer; Philip Coates. 2015. "Micro-contact reconstruction of adjacent carbon nanotubes in polymer matrix through annealing-Induced relaxation of interfacial residual stress and strain." Journal of Polymer Science 132, no. 33: 1.
C.P.J. O’Connor; P.J. Martin; J. Sweeney; G. Menary; P. Caton-Rose; P.E. Spencer. Simulation of the plug-assisted thermoforming of polypropylene using a large strain thermally coupled constitutive model. Journal of Materials Processing Technology 2013, 213, 1588 -1600.
AMA StyleC.P.J. O’Connor, P.J. Martin, J. Sweeney, G. Menary, P. Caton-Rose, P.E. Spencer. Simulation of the plug-assisted thermoforming of polypropylene using a large strain thermally coupled constitutive model. Journal of Materials Processing Technology. 2013; 213 (9):1588-1600.
Chicago/Turabian StyleC.P.J. O’Connor; P.J. Martin; J. Sweeney; G. Menary; P. Caton-Rose; P.E. Spencer. 2013. "Simulation of the plug-assisted thermoforming of polypropylene using a large strain thermally coupled constitutive model." Journal of Materials Processing Technology 213, no. 9: 1588-1600.
J. Sweeney; C.P.J. O’Connor; P.E. Spencer; H. Pua; P. Caton-Rose; P.J. Martin. A material model for multiaxial stretching and stress relaxation of polypropylene under process conditions. Mechanics of Materials 2012, 54, 55 -69.
AMA StyleJ. Sweeney, C.P.J. O’Connor, P.E. Spencer, H. Pua, P. Caton-Rose, P.J. Martin. A material model for multiaxial stretching and stress relaxation of polypropylene under process conditions. Mechanics of Materials. 2012; 54 ():55-69.
Chicago/Turabian StyleJ. Sweeney; C.P.J. O’Connor; P.E. Spencer; H. Pua; P. Caton-Rose; P.J. Martin. 2012. "A material model for multiaxial stretching and stress relaxation of polypropylene under process conditions." Mechanics of Materials 54, no. : 55-69.
J. Sweeney; P. Caton-Rose; P. E. Spencer; H. Pua; C. O’Connor; P. J. Martin; G. Menary. The large strain response of polypropylene in multiaxial stretching and stress relaxation. International Journal of Material Forming 2012, 6, 519 -525.
AMA StyleJ. Sweeney, P. Caton-Rose, P. E. Spencer, H. Pua, C. O’Connor, P. J. Martin, G. Menary. The large strain response of polypropylene in multiaxial stretching and stress relaxation. International Journal of Material Forming. 2012; 6 (4):519-525.
Chicago/Turabian StyleJ. Sweeney; P. Caton-Rose; P. E. Spencer; H. Pua; C. O’Connor; P. J. Martin; G. Menary. 2012. "The large strain response of polypropylene in multiaxial stretching and stress relaxation." International Journal of Material Forming 6, no. 4: 519-525.
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.
C. O’Connor; P. Martin; G. Menary; J. Sweeney; P. Caton-Rose; P. Spencer. Development of a Constitutive Model of Polypropylene for Thermoforming. THE 14TH INTERNATIONAL ESAFORM CONFERENCE ON MATERIAL FORMING: ESAFORM 2011 2011, 1 .
AMA StyleC. O’Connor, P. Martin, G. Menary, J. Sweeney, P. Caton-Rose, P. Spencer. Development of a Constitutive Model of Polypropylene for Thermoforming. THE 14TH INTERNATIONAL ESAFORM CONFERENCE ON MATERIAL FORMING: ESAFORM 2011. 2011; ():1.
Chicago/Turabian StyleC. O’Connor; P. Martin; G. Menary; J. Sweeney; P. Caton-Rose; P. Spencer. 2011. "Development of a Constitutive Model of Polypropylene for Thermoforming." THE 14TH INTERNATIONAL ESAFORM CONFERENCE ON MATERIAL FORMING: ESAFORM 2011 , no. : 1.
In this study, a biaxial testing machine is used to stretch polypropylene sheet at elevated temperatures (150–160° C) in a number of modes, including simultaneous equibiaxial and a sequential mode that consists of planar extension followed by a second perpendicular extension to achieve a final equibiaxial state. On completion of the stretching process the applied forces continue to be monitored to give further data on stress relaxation. A constitutive model is devised that combines the Eyring process and large strain elastictheory. A Guiu‐Pratt model, including a single Eyring process, is fitted to the stress relaxation curves and produces values of Eyring parameters. Fits are improved if the initial part of the curve is excluded, indicating that the activation volume derived relates to a process in the relatively long term. In support of this, the strain rate dependence of the loading curves indicates a quite different value of activation volume. A two‐process model is derived with parameter values deduced from these two types of fit, and implemented in a finite element package. Its capability of modelling the whole of the stress relaxation curve is explored.
J. Sweeney; P. Caton-Rose; P. E. Spencer; C. O’Conner; P. J. Martin; G. Menary. The large strain response of polypropylene in multiaxial stretching and stress relaxation. THE 14TH INTERNATIONAL ESAFORM CONFERENCE ON MATERIAL FORMING: ESAFORM 2011 2011, 1 .
AMA StyleJ. Sweeney, P. Caton-Rose, P. E. Spencer, C. O’Conner, P. J. Martin, G. Menary. The large strain response of polypropylene in multiaxial stretching and stress relaxation. THE 14TH INTERNATIONAL ESAFORM CONFERENCE ON MATERIAL FORMING: ESAFORM 2011. 2011; ():1.
Chicago/Turabian StyleJ. Sweeney; P. Caton-Rose; P. E. Spencer; C. O’Conner; P. J. Martin; G. Menary. 2011. "The large strain response of polypropylene in multiaxial stretching and stress relaxation." THE 14TH INTERNATIONAL ESAFORM CONFERENCE ON MATERIAL FORMING: ESAFORM 2011 , no. : 1.
This work concerns the solid phase deformation processing of polypropylene/nanoclay composites, for which the materials are stretched to large tensile deformations at elevated temperatures. Under these conditions the polymer matrix is nonlinearly dependent on time and strain rate. A constitutive model that is a combination of an Eyring process and physically-based molecular chain models has been shown to give a good representation of the polymer behavior, which includes strain-rate dependent yielding and stress relaxation. In order to model the nanocomposite, platelike regions that are relatively stiff are introduced into a continuum of model polymer material. This is done using a Monte Carlo approach that sequentially places non-overlapping platelets in the matrix. The process for introducing the platelets has the potential to produce platelet orientation distributions that conform with prescribed statistics, such as may be deduced from observations on real nanocomposite.
P. E. Spencer; R. Spares; J. Sweeney; Philip Coates. Modelling the large strain solid phase deformation behaviour of polymer nanoclay composites. Mechanics of Time-Dependent Materials 2008, 12, 313 -327.
AMA StyleP. E. Spencer, R. Spares, J. Sweeney, Philip Coates. Modelling the large strain solid phase deformation behaviour of polymer nanoclay composites. Mechanics of Time-Dependent Materials. 2008; 12 (4):313-327.
Chicago/Turabian StyleP. E. Spencer; R. Spares; J. Sweeney; Philip Coates. 2008. "Modelling the large strain solid phase deformation behaviour of polymer nanoclay composites." Mechanics of Time-Dependent Materials 12, no. 4: 313-327.
Eileen Harkin-Jones; Lukasz Figiel; P. Spencer; Rund Abu-Zurayk; W. Al-Shabib; V. Chan; R. Rajeev; K. Soon; P. Buckley; J. Sweeney; G. Menary; C. Armstrong; H. Assender; Philip Coates; F. Dunne; T. McNally; P. Martin. Performance enhancement of polymer nanocomposites via multiscale modelling of processing and properties. Plastics, Rubber and Composites 2008, 37, 113 -123.
AMA StyleEileen Harkin-Jones, Lukasz Figiel, P. Spencer, Rund Abu-Zurayk, W. Al-Shabib, V. Chan, R. Rajeev, K. Soon, P. Buckley, J. Sweeney, G. Menary, C. Armstrong, H. Assender, Philip Coates, F. Dunne, T. McNally, P. Martin. Performance enhancement of polymer nanocomposites via multiscale modelling of processing and properties. Plastics, Rubber and Composites. 2008; 37 (2-4):113-123.
Chicago/Turabian StyleEileen Harkin-Jones; Lukasz Figiel; P. Spencer; Rund Abu-Zurayk; W. Al-Shabib; V. Chan; R. Rajeev; K. Soon; P. Buckley; J. Sweeney; G. Menary; C. Armstrong; H. Assender; Philip Coates; F. Dunne; T. McNally; P. Martin. 2008. "Performance enhancement of polymer nanocomposites via multiscale modelling of processing and properties." Plastics, Rubber and Composites 37, no. 2-4: 113-123.
A kinetic Monte Carlo (KMC) model for surface diffusion on a 2D lattice is proposed. An equivalent continuum cellular automaton (CA) model is derived from this. These models are shown to produce similar results at high temperatures. A hybrid KMC-CA model is derived which consistently allows material to transfer between a deterministic CA model and a stochastic KMC model concurrently embedded within it. The quality of the model is demonstrated by simulating the flattening of a sinusoidal surface profile and the evolution of an elliptical body into a circular one.
Simon P.A. Gill; Paul E. Spencer. Concurrent Multiscale Kinetic Monte Carlo-Continuum Models for the Evolution of Solids via Diffusion. Solid State Phenomena 2007, 129, 19 -24.
AMA StyleSimon P.A. Gill, Paul E. Spencer. Concurrent Multiscale Kinetic Monte Carlo-Continuum Models for the Evolution of Solids via Diffusion. Solid State Phenomena. 2007; 129 ():19-24.
Chicago/Turabian StyleSimon P.A. Gill; Paul E. Spencer. 2007. "Concurrent Multiscale Kinetic Monte Carlo-Continuum Models for the Evolution of Solids via Diffusion." Solid State Phenomena 129, no. : 19-24.
We present the numerically exact ground-state energy, effective mass, and isotope exponents of a one-dimensional lattice polaron, valid for any range of electron-phonon interaction, applying a continuous-time quantum Monte Carlo (QMC) technique in a wide range of coupling strength and adiabatic ratio. The QMC method is free from any systematic finite-size and finite-time-step errors. We compare our numerically exact results with analytical weak-coupling theory and with the strong-coupling 1∕λ expansion. We show that the exact results agree well with the canonical Fröhlich and Holstein-Lang-Firsov theories in the weak and strong coupling limits, respectively, for any range of interaction. We find a strong dependence of the polaron dynamics on the range of interaction. An increased range of interaction has a similar effect to an increased (less adiabatic) phonon frequency: specifically, a reduction in the effective mass.
P. E. Spencer; J. H. Samson; P. E. Kornilovitch; A. S. Alexandrov. Effect of electron-phonon interaction range on lattice polaron dynamics: A continuous-time quantum Monte Carlo study. Physical Review B 2005, 71, 184310 .
AMA StyleP. E. Spencer, J. H. Samson, P. E. Kornilovitch, A. S. Alexandrov. Effect of electron-phonon interaction range on lattice polaron dynamics: A continuous-time quantum Monte Carlo study. Physical Review B. 2005; 71 (18):184310.
Chicago/Turabian StyleP. E. Spencer; J. H. Samson; P. E. Kornilovitch; A. S. Alexandrov. 2005. "Effect of electron-phonon interaction range on lattice polaron dynamics: A continuous-time quantum Monte Carlo study." Physical Review B 71, no. 18: 184310.
An atomistic solid-on-solid (SOS) kinetic Monte Carlo (KMC) model for surface diffusion is found to exhibit curvature-driven decay. An equivalent continuum Mullins-Herring equation is derived and the two systems are shown to be compatible. A mixed KMC-continuum model is proposed that allows discrete atomistic simulations to be embedded within a smooth continuum. A blending region is introduced between the two model descriptions, which allow mass to be transferred between them in a consistent manner. The implementation of this multiscale modeling approach is illustrated with simulations of the decay of a sinusoid and a Gaussian peak
Simon P.A. Gill; Paul E. Spencer; Alan N. E. Cocks. Mixed KMC-Continuum Models for the Evolution of Rough Surfaces. International Journal for Multiscale Computational Engineering 2005, 3, 239 -256.
AMA StyleSimon P.A. Gill, Paul E. Spencer, Alan N. E. Cocks. Mixed KMC-Continuum Models for the Evolution of Rough Surfaces. International Journal for Multiscale Computational Engineering. 2005; 3 (2):239-256.
Chicago/Turabian StyleSimon P.A. Gill; Paul E. Spencer; Alan N. E. Cocks. 2005. "Mixed KMC-Continuum Models for the Evolution of Rough Surfaces." International Journal for Multiscale Computational Engineering 3, no. 2: 239-256.
S.P.A. Gill; P.E. Spencer; A.C.F. Cocks. A hybrid continuum/kinetic Monte Carlo model for surface diffusion. Materials Science and Engineering: A 2004, 365, 66 -72.
AMA StyleS.P.A. Gill, P.E. Spencer, A.C.F. Cocks. A hybrid continuum/kinetic Monte Carlo model for surface diffusion. Materials Science and Engineering: A. 2004; 365 (1-2):66-72.
Chicago/Turabian StyleS.P.A. Gill; P.E. Spencer; A.C.F. Cocks. 2004. "A hybrid continuum/kinetic Monte Carlo model for surface diffusion." Materials Science and Engineering: A 365, no. 1-2: 66-72.