Kaiserslautern - Fachbereich Maschinenbau und Verfahrenstechnik
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- Finite-Elemente-Methode (3)
- Aerosol (1)
- Aerosol Particles (1)
- Aerosol Partikeln (1)
- Bifurkation (1)
- Cauchy-Born Regel (1)
- Cauchy-Born Rule (1)
- Continuum-Atomistic Multiscale Algorithm (1)
- EPDM (1)
- Elastomer (1)
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Nanotechnology is now recognized as one of the most promising areas for technological
development in the 21st century. In materials research, the development of
polymer nanocomposites is rapidly emerging as a multidisciplinary research activity
whose results could widen the applications of polymers to the benefit of many different
industries. Nanocomposites are a new class of composites that are particle-filled
polymers for which at least one dimension of the dispersed particle is in the nanometer
range. In the related area polymer/clay nanocomposites have attracted considerable
interest because they often exhibit remarkable property improvements when
compared to virgin polymer or conventional micro- and macro- composites.
The present work addresses the toughening and reinforcement of thermoplastics via
a novel method which allows us to achieve micro- and nanocomposites. In this work
two matrices are used: amorphous polystyrene (PS) and semi-crystalline polyoxymethylene
(POM). Polyurethane (PU) was selected as the toughening agent for POM
and used in its latex form. It is noteworthy that the mean size of rubber latices is
closely matched with that of conventional toughening agents, impact modifiers.
Boehmite alumina and sodium fluorohectorite (FH) were used as reinforcements.
One of the criteria for selecting these fillers was that they are water swellable/
dispersible and thus their nanoscale dispersion can be achieved also in aqueous
polymer latex. A systematic study was performed on how to adapt discontinuousand
continuous manufacturing techniques for the related nanocomposites.
The dispersion of nanofillers was characterized by transmission, scanning electron
and atomic force microcopy (TEM, SEM and AFM respectively), X-ray diffraction
(XRD) techniques, and discussed. The crystallization of POM was studied by means
of differential scanning calorimetry and polarized light optical microscopy (DSC and
PLM, respectively). The mechanical and thermomechanical properties of the composites
were determined in uniaxial tensile, dynamic-mechanical thermal analysis
(DMTA), short-time creep tests, and thermogravimetric analysis (TGA).
PS composites were produced first by a discontinuous manufacturing technique,
whereby FH or alumina was incorporated in the PS matrix by melt blending with and
without latex precompounding of PS latex with the nanofiller. It was found that direct melt mixing (DM) of the nanofillers with PS resulted in micro-, whereas the latex mediated
pre-compounding (masterbatch technique, MB) in nanocomposites. FH was
not intercalated by PS when prepared by DM. On the other hand, FH was well dispersed
(mostly intercalated) in PS via the PS latex-mediated predispersion of FH following
the MB route. The nanocomposites produced by MB outperformed the DM
compounded microcomposites in respect to properties like stiffness, strength and
ductility based on dynamic-mechanical and static tensile tests. It was found that the
resistance to creep (summarized in master curves) of the nanocomposites were improved
compared to those of the microcomposites. Master curves (creep compliance
vs. time), constructed based on isothermal creep tests performed at different temperatures,
showed that the nanofiller reinforcement affects mostly the initial creep
compliance.
Next, ternary composites composed of POM, PU and boehmite alumina were produced
by melt blending with and without latex precompounding. Latex precompounding
served for the predispersion of the alumina particles. The related MB was produced
by mixing the PU latex with water dispersible boehmite alumina. The composites
produced by the MB technique outperformed the DM compounded composites in
respect to most of the thermal and mechanical characteristics.
Toughened and/or reinforced PS- and POM-based composites have been successfully
produced by a continuous extrusion technique, too. This technique resulted in
good dispersion of both nanofillers (boehmite) and impact modifier (PU). Compared
to the microcomposites obtained by conventional DM, the nanofiller dispersion became
finer and uniform when using the water-mediated predispersion. The resulting
structure markedly affected the mechanical properties (stiffness and creep resistance)
of the corresponding composites. The impact resistance of POM was highly
enhanced by the addition of PU rubber when manufactured by the continuous extrusion
manufacturing technique. This was traced to the dispersed PU particle size being
in the range required from conventional, impact modifiers.
Sublimation (Evaporation) is widely used in different industrial applications. The important applications are the sublimation (evaporation) of small particles (solid and liquid), e.g., spray drying and fuel droplet evaporation. Since a few decades, sublimation technology has been used widely together with aerosol technology. This combination is aiming to get various products with desired compositions and morphologies. It can be used in the fields of nanoparticles generation, particle coating through physical vapor deposition (PVD) and particle structuring. This doctoral thesis deals with the experimental and theoretical investigations of sublimation (evaporation) kinetics of fine aerosol particles (droplets). The experimental study was conducted in a test plant including on-line control of the most important paramters, such as heating temperature, gas flow and pressure. On-line and in-line particle measurements (Optical sensor, APS) were employed. Relevant parameters in sublimation (evaporation) such as heating temperature, particle concentration and aerosol residence time were investigated. Polydispersed particles (droplets) were introduced into the test plant as precursor aerosols. Two kinds of materials were used as test materials, including inorganic particles of NH4Cl and organic particles of DEHS. NH4Cl particles with smooth surface and porous structure were put into the experiments, respectively. The influence of the particle morphology on the sublimation process was studied. Basing on the experiments, different theoretical models were developed. The simulation results under different parameters were compared with experimental results. The change of concentration of particles was specially discussed. The discussion was focused on the relationship of the total particle concentration and the change of single particles with diverse initial diameters. The study of the sublimation kinetics of particles with different morphologies and different specific surface areas was carried out. The factor of increased surface area on the sublimation process was taken into the simulation and the results were compared with experimental results. A sublimation (evaporation) kinetics was investigated in this thesis. Basing on the property of a material, such as molecular weight, molecular size and vapor pressure, the sublimation (evaporation) kinetics was described. The optimum sublimation (evaporation) conditions with respect to the material properties were advanced. A Phase Transition Effect during the sublimation (evaporation) was found, which describes the increase of the large particles on the cost of small particles. A similar effect is observed in crystal suspension (called Ostwald ripening) but with another physical background. In order to meet the need of in-line particle measurement, a hot gas sensor (O.P.C.) was developed in this study, for measuring the particle size and the size distribution of an aerosol. With the newly developed measuring cell, the operating conditions of the aerosol could be increased up to 500°C.
Dry Sliding and Rolling Tribotests of Carbon Black Filled EPDM Elastomers and Their FE Simulations
(2008)
Unlubricated sliding systems being economic and environmentally benign are already realized in bearings, where dry metal-plastic sliding pairs successfully replace lubricated metal-metal ones. Nowadays, a considerable part of the tribological research concentrates to realize unlubricated elastomer-metal sliding systems, and to extend the application field of lubrication-free slider elements. In this Thesis, characteristics of the dry sliding and friction are investigated for elastomer-metal sliding pairs. In this study ethylene-propylene-diene rubbers (EPDM) with and without carbon black (CB) filler were used. The filler content of the EPDMs was varied: EPDMs with 0-, 30-, 45- and 60 part per hundred rubber (phr) CB amount were investigated. Quasistatic tension and compression tests and dynamic mechanical thermal analysis (DMTA) were carried out to analyze the static a viscoelastic behavior of the EPDMs. The tribological properties of the EPDMs were investigated using dry roller (metal) – on – plate (rubber) type tests (ROP). During the ROP tests the normal load was varied. The coefficient of friction (COF) and the temperature were registered online during the tests, the loss volumes were determined after certain test durations. The worn surfaces of the rubbers and of the steel counterparts were analyzed using scanning electron microscope (SEM) to determine the wear mechanisms. Because possible chemical changes may take place during dry sliding due to the elevated contact temperature the chemical composition of the surfaces was also analyzed before and after the tribotests. For the latter investigations X-ray photoelectron spectroscopy (XPS), sessil drop tests and Raman spectroscopy were used. In addition, the dry sliding tribotests were simulated using finite element (FE) codes for the better understanding of the related wear mechanisms. Finally, as the internal damping effect of the elastomers plays a great role in the sliding wear process, their viscoelasticity has been taken into account. The effect of viscoelasticity was shown on example of rolling friction. To study the rolling COF for the EPDM with 30 phr CB (EPDM 30) an FE model was created which considered the viscoelastic behavior of the rubber during rolling. The results showed that the incorporated CB enhanced the mechanical and tribological properties (both COF and wear rate have been reduced) of the EPDMs. Further on, the CB content of the EPDM influences fundamentally the observed wear mechanisms. The wear characteristics changed also with the applied normal load. In case of the EPDM 30 a rubber tribofilm was found on the steel counterpart when tests were performed at high normal loads. Analysis of the chemical composition of the surfaces before and after the wear tests does not result in notable changes. It was demonstrated, that the FE method is powerful tool to model both, the dry sliding and rolling performances of elastomers.
The main motivation of this contribution is to introduce a computational laboratory to analyse defects and fractures at the sub--micro scale. To this end, we have attempted to present a continuum--atomistic multiscale algorithm for the analysis of crystalline deformation, i.e. we have combined the above--mentioned Cauchy--Born rule within a finite element approximation (FEM) on the continuum region with a molecular dynamics (MD) resolution on the atomistic domain. The aim is twofold: on the one hand the stability, i.e. validity of the Cauchy--Born rule and its transition to non--affine deformation at the micron--scale is studied with the help of molecular dynamics approach to capture fine--scales features; on the other hand a horizontal FEM/MD, i.e. continuum atomistic coupling, is envisaged in order to study representative cases of crystalline defects. To cope with the latter we have introduced a horizontal coupling method for continuum--atomistic analysis.
The present thesis is concerned with the simulation of the loading behaviour of both hybrid lightweight structures and piezoelectric mesostructures, with a special focus on solid interfaces on the meso scale. Furthermore, an analytical review on bifurcation modes of continuum-interface problems is included. The inelastic interface behaviour is characterised by elastoplastic, viscous, damaging and fatigue-motivated models. For related numerical computations, the Finite Element Method is applied. In this context, so-called interface elements play an important role. The simulation results are reflected by numerous examples which are partially correlated to experimental data.
In the present contribution, a general framework for the completely consistent integration of nonlinear dissipative dynamics is proposed, that essentially relies on Finite Element methods in space and time. In this context, fully flexible structures as well as hybrid systems which consist of rigid bodies and inelastic flexible parts are considered. Thereby, special emphasis is placed on the resulting algorithmic fulfilment of fundamental balance equations, and the excellent performance of the presented concepts is demonstrated by means of several representative numerical examples, involving in particular finite elasto-plastic deformations.
The main goal of this work is to model size effects, as they occur in materials with an intrinsic microstructure at the consideration of specimens that are not by orders larger than this microstructure. The micromorphic continuum theory as a generalized continuum theory is well suited to account for the occuring size effects. Thereby additional degrees of freedoms capture the independent deformations of these microstructures, while they provide additional balance equation. In this thesis, the deformational and configurational mechanics of the micromorphic continuum is exploited in a finite-deformation setting. A constitutive and numerical framework is developed, in which also the material-force method is advanced. Furthermore the multiscale modelling of thin material layers with a heterogeneous substructure is of interest. To this end, a computational homogenization framework is developed, which allows to obtain the constitutive relation between traction and separation based on the properties of the underlying micromorphic mesostructure numerically in a nested solution scheme. Within the context of micromorphic continuum mechanics, concepts of both gradient and micromorphic plasticity are developed by systematically varying key ingredients of the respective formulations.