Samenvatting
The process of plastic deformation in semicrystalline polymers is complicated due to the operation of a variety of mechanisms at different levels and is strongly dependent on their crystallinity level, the initial underlying microstructure, and the evolution of this structure during deformation. Any macroscopically homogeneous deformation is accommodated by various deformation mechanisms
in the heterogeneous microstructure. The objective of this work is to establish a quantitative relation between the microstructure and the mechanical performance of semicrystalline polymers, as characterized by elasto-viscoplastic deformation. In order to do that, a micromechanically based constitutive model is used. The model represents the microstructure as an aggregate of layered
composite inclusions, each consisting of a crystalline lamella, which is mechanically coupled to its adjacent amorphous layer. The crystalline phase is modeled as anisotropic elastic with plastic flow governed by crystallographic slip. The amorphous phase is assumed to be isotropic elastic with a rate dependent plastic flow and strain hardening resulting from molecular orientation. To relate the
volume-averaged mechanical behavior of each layered composite inclusion to the aggregate of composite inclusions, a hybrid local-global interaction law is used. The concept of a layered composite inclusion as a representative element is extended with a third phase, which is also referred to as the interphase or the rigid-amorphous phase. This phase represents the region between crystalline and amorphous domains, having a somewhat ordered structure and a fixed
thickness. The incorporation of the interphase in the composite inclusion model naturally leads to a dependence on the lamellar thickness, i.e. on an internal length scale. This rigid-amorphous phase is particularly relevant for quantitative modeling of the behavior of oriented semicrystalline structures. A comparison with experimental data shows a good prediction with the two-phase model for
isotropic material. A critical factor for adding quantitative predictive abilities to the micromechanical
model for prediction of the elasto-viscoplastic behavior in semicrystalline polymers is the stress-dependence of the rate of plastic deformation, the slip kinetics, which is the mechanism underlying time-dependent, macroscopic failure. The kinetics of the macroscopic plastic flow strongly depend on the slip kinetics of the individual crystallographic slip systems, accompanied by the yield kinetics
of the amorphous domain. To obtain an accurate quantitative prediction, the viscoplastic power law relation, normally used in micromechanical modeling, is replaced with an Eyring flow rule. The slip kinetics are then re-evaluated and characterized using a hybrid numerical/experimental procedure, and the results are validated for uniaxial compression data of HDPE. A double yield phenomenon is observed in the model prediction, and is found to be related to morphological changes during deformation, which induce a change of deformation mechanism.
Experimental data on the yield kinetics of polyethylene at different temperatures and strain rates reveals the contribution of two relaxation processes. Further experimental observations on the stress dependence of the time-to-failure show a linear relation in semi-logarithmic plots, with the same slope as that of the yield kinetics. This indicates that the kinetics of failure under applied strain-rate and applied stress are strongly related. To predict failure under both conditions and for different temperatures, the crystallographic slip kinetics and the amorphous yield kinetics were further refined, and the Eyring flow rule was modified by adding a temperature shift function. The creep behavior of polyethylene was then simulated directly using the multi-scale, micromechanical model, predicting the time-to-failure without any additional fitting parameter. To enable the prediction
of both tension and compression, a non-Schmid effect is added to the constitutive relation of each slip system. Injection molded or extruded polymers possess a different morphology than
isotropic polymers, due to the subjection to shear and elongational flow during processing. Therefore, their plastic deformation and failure behavior are anisotropic. The relation between the initially oriented microstructure and the deformation kinetics of oriented polyethylene tapes is investigated using the multi-scale micromechanical model. The initial orientation distribution for the
model is obtained based on wide angle X-ray scattering experiments. Due to the presence of oriented amorphous domains in the drawn samples, the macroscopic plastic flow is predominantly governed by the yield kinetics of the amorphous phase. The necessity of modeling the load angle dependence of the properties of the oriented amorphous domain for an accurate quantitative prediction is discussed. Furthermore, the possibilities for identifying the properties of distinct
crystallographic slip systems are investigated.
Originele taal-2 | Engels |
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Kwalificatie | Doctor in de Filosofie |
Toekennende instantie |
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Begeleider(s)/adviseur |
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Datum van toekenning | 17 sep. 2012 |
Plaats van publicatie | Eindhoven |
Uitgever | |
Gedrukte ISBN's | 978-90-386-3218-6 |
DOI's | |
Status | Gepubliceerd - 2012 |