Modeling anisotropic and rate-dependent plasticity in short-fiber reinforced thermoplastics

In this study, an anisotropic viscoelastic-viscoplastic macro-mechanical model is presented for short-fiber reinforced thermoplastics (SFRT). In injection molding of SFRT, the fiber orientation is influenced by the flow velocity profile which varies throughout the mold. The flow-induced orientation in the microstructure leads to anisotropy in the mechanical response. In addition to the mechanical anisotropy, SFRTs show time dependent behavior because of the thermoplastic matrix. The developed model captures the effects of both material orientation and loading rate on the yield behavior. In this study, uniaxial tests are performed at different strain rates and material orientations with samplescutfrominjectionmoldedplaques. Theexperimentalresultsshowthattheeffects of loading rate and material orientation on the yield are decoupled. The presented model takes advantage of this observation to simplify material characterization. An implicit integration scheme is used for the numerical implementation of the model as a UMAT in ABAQUS. Multiple relaxation times are used in order to capture the nonlinear pre-yield regime. An efficient method for obtaining the model parameters for different modes is proposed. Experimental results are used for validation of the model and a good agreement is observed for the prediction of viscoelastic and viscoplastic behavior

Yield stress distribution in injection-mouldedglassy polymers

A methodology for structural analysis simulations is presented that incorporates the distribution of mechanical propertiesalong the geometrical dimensions of injection-moulded amorphous polymer products. It is based on a previously developedmodelling approach, where the thermomechanical history experienced during processing was used to determine the yield stressat the end of an injection-moulding cycle. Comparison between experimental data and simulation results showed an excellentquantitative agreement, both for short-term tensile tests as well as long-term creep experiments over a range of strain rates,applied stresses, and testing temperatures. Changes in mould temperature and component wall thickness, which directly affectthe cooling profiles and, hence, the mechanical properties, were well captured by the methodology presented. Furthermore, itturns out that the distribution of the yield stress along a tensile bar is one of the triggers for the onset of the (strong) localizationgenerally observed in experiments.Spanish Government (Ministry of Sci-ence and Innovation, Ministry of Economy and Competitiveness)through grant numbers RYC-2010-07171 and DPI2011-25470This is the peer reviewed version of the following article: Verbeeten, W. M., Kanters, M. J., Engels, T. A. and Govaert, L. E. (2015), Yield stress distribution in injection-moulded glassy polymers. Polym. Int., 64(11): 1527–1536, which has been published in final form at http://dx.doi.org/10.1002/pi.4898. This article may be used for non-commercial purposes in accordance with Wiley Terms and Conditions for Self-Archivin

Lifetime Assessment of Load-Bearing Polymer Glasses: The Influence of Physical Ageing

The timescale at which ductile failure occurs in loaded glassy polymers can be successfully predicted using the engineering approach presented in a previous publication. In this paper the influence of progressive physical ageing on the plastic deformation behaviour of unplasticised poly(vinyl chloride) (uPVC) is characterised and incorporated in the existing approach. With the modification it is possible to quantitatively predict long-term failures which show a so-called endurance limit. The predictions are compared with failure data of uPVC specimens which were subjected to constant or dynamic loads. In dynamic loading conditions a second type of failure mode was observed: fatigue crack growth. A brief study on the influence of the frequency and stress ratio of the applied stress signal shows that crack growth failure is not expected to occur within experimentally reasonable timescales for constant loading conditions

An elasto-viscoplastic constitutive model for the rate-dependent behavior of polyvinylidene fluoride

To model the engineering performance of components made of polyvinylidene fluoride (PVDF), the 3D elasto-viscoplastic Eindhoven glassy polymer (EGP) model is extended to describe the rate-dependent behavior of PVDF. Careful analysis of the intrinsic behavior of PVDF revealed that the postyield compressive response shows a strain rate-dependence that evolves with increasing deformation. The extension of the constitutive model captures the deformation-dependent evolution of the activation volume and the rate-factor, which describes the driving stress. Given the significant temperature-dependent behavior, the model has been characterized for different temperatures (23, 55 and 75 °C). The accuracy of the model has been validated by means of tension and creep experiments at these temperatures. The constitutive model is implemented in finite element simulations and the results are compared with the experiments. It is shown that the proposed model allows for an accurate prediction of the short- and long-term rate-dependent behavior of PVDF.</p