Contact mechanics in glassy polymers

Polymers, primarily semi-crystalline, are widely used in applications where low friction is required; examples are cups in artificial hip joints, bearings and gears. Until now there is no clear indication why some polymers display low friction and others don’t. In this thesis a systematic identification of the role of the intrinsic properties of glassy polymers on singleasperity sliding friction experiments is performed. The problem is analysed using a hybrid numerical/experimental technique. In the numerical part the interaction between indenter and polymer is studied by means of a constitutive model capturing the intrinsic behaviour of glassy polymers, where the interaction between tip and polymer can be influenced by the incorporation of existing friction models. The experimental section concerns the development of reproducible sliding friction experiments, which in a later stage can be compared with simulations before conclusions can be drawn. Starting point is the constitutive model developed in our group over the last decade, which accurately captures the deformation response of glassy polymers, including strain localization phenomena as well as life time predictions. The choice for glassy polymers is, therefore, clearly not motivated by their relevance in low friction applications, but only because they represent a well-characterized class of polymers that allow quantitative predictions. First however some drawbacks of the existing model must be removed. The pre-yield regime itself is highly non-linear and thus correct modelling thereof is important in all simulations where non-homogeneous deformation is applied, like e.g. in indentation and sliding friction. Nevertheless, at present the pre-yield region is modelled as a compressible linear elastic solid and, as a result, details of indentation and unloading are not described quantitatively. The straightforward solution is to extend the existing model to include a spectrum of relaxation times in the pre-yield regime, via use of a multi-mode approach. The thus improved model now indeed also quantitatively predicts the indentation response of polycarbonate for different types of indenter geometries. A second drawback of the existing model is that it cannot deal with multiple relaxation mechanisms, as occur in cases where more than one molecular transition contributes to the stress. This behaviour typically manifests itself when high strain rates are applied, demonstrating a change in slope in the dependence of yield stress on the logarithm of strain rate. Solution of this problem requires a model extension by incorporation of a second, additional, flow process with its own non-linearity, that is, a multiprocess approach. A material which manifests this type of mechanical response is poly(methyl methacrylate); a quantitative prediction of its indentation response is achieved. Generally the friction force is regarded to be an additive composition of a deformationand an adhesion-related component, suggesting that components operate and contribute independently. Although decomposition in independent contributions is impossible to verify in an experimental set-up, it can be conveniently studied by using a numerical approach. Simulations with no adhesive interaction between tip and polymer show almost no influence of sliding velocity on friction force, whereas experiments show a significant influence. In case of an additive decomposition, this would imply a rate-dependence of the adhesive component. By inclusion of the Amontons-Coulomb friction law, which creates an interaction between tip and polymer, the suggested additive decomposition is proved not to be applicable and the large macroscopic deformation response proves to be the result of small changes in local processes. When interaction is taken into account, a bow wave is formed in front of the sliding tip, which leads to an increase in contact area between tip and polymer and results in an increase in friction force. As a consequence the experimentally observed time-dependent behaviour of the friction force can solely be attributed to a polymer’s intrinsic deformation response. Furthermore the influence of a polymer’s intrinsic material properties, such as strain hardening and the thermodynamic state, on the friction force can be studied conveniently

Finite strain viscoplastic modeling of polymer glasses: application to contact phenomena

There are several techniques to probe local mech. properties of polymer systems. Two frequently used techniques are indentation and scratching, also known as sliding friction. The first is used to det. material parameters such as Young's modulus and yield strength, the later to resolve issues concerning friction and wear properties. Both techniques are based on contact of a specimen with a well-defined indentation/scratching geometry. If we take a closer look at an indentation expt., an indenter is pressed into the material and a force, the so called normal force, and penetration into the surface are measured. For the scratching expt. an extra sliding dimension is added and besides the normal force and penetration depth, a lateral force and sliding distance are measured. The first step of a scratching expt. is indentation; this implies that before we can start with investigation of sliding phenomena, all the phenomena governing indentation have to be captured. For polymers this technique should be used with great care, this because of the strong non-linearity and rate dependence of polymer systems. To understand both contact phenomena a combination of expts. and numerical techniques are used. To comprehend macroscopic polymer deformation a polymers' intrinsic deformation should be captured accurately. This deformation behavior is used as input for our constitutive model and subsequently the model is used for finite element calcns. (c) 2008 American Institute of Physics. [on SciFinder (R)

Condition monitoring of uPVC gas pipes

No abstract

Multiscale modelling of the mechanical behaviour of oriented semicrystalline polymers

A multiscale numerical model is used to investigate the structure–property relationship for oriented semicrystalline polymers. The basic element in this model is a layered two-phase composite inclusion, comprising both a crystalline and an amorphous domain. An aggregate of preferentially oriented composite inclusions is used in a macroscopic finite element model

Modelling large-strain deformation of thermo-rheologically complex materials : characterisation and validation of PMMA and iPP

In this study an attempt is made to describe the thermorheologically complex deformation behaviour of the glassy polymer PMMA and semi-crystalline polymer iPP, by using a constitutive modelling approach [1]. For both polymers, it is shown that this approach successfully captures the thermorheologically complex behaviour of PMMA and iPP. Moreover, the model is capable of predicting yield stress and creep lifetime of PMMA using only one parameter set