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Organisationseinheit der BAM
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Eingeladener Vortrag
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The time-dependent sorption of biodiesel in a typical polyethylene for container applications is investigated in comparison to conventional diesel fuel at three different temperatures. In this context, the desorption behavior is also addressed. Subsequently, the effects of both penetrants on mechanical properties are characterized in terms of impact strength and dynamic-mechanical analysis. The discussion of property changes is firstly based on the sorption kinetics of biodiesel and diesel, which is determined by immersion experiments allowing for the calculation of respective diffusion coefficients. Changes in impact strength as determined by the Charpy method are further characterised in more detail by analyzing the fracture surfaces, and correlated with results of dynamic-mechanical analysis.
In a previous paper we investigated the influence of sorbed biodiesel or diesel on mechanical properties of a typical polyethylene grade for tank applications. Besides the basic sorption and desorption behavior of these two fuels, the study addressed the concentration-dependent mechanical properties as revealed by a non-instrumented Charpy impact test and dynamic mechanical analysis (DMA). In the present paper we extend this investigation focusing on the temperature-dependent impact fracture behavior. Therefore, an instrumented Charpy impact test was employed, allowing a more detailed analysis of the fracture behavior. Furthermore, from the load-time-diagrams obtained from the instrumented impact test, corresponding fracture times can be calculated, allowing a clear correlation of the fuel sorption induced changes in fracture toughness with the enhanced ß-relaxation observed by DMA. As in the previous study, the fracture surfaces of the impact tested specimens were analyzed in order to confirm the brittle or ductile character of the fracture indicated by impact strength and the corresponding load-deflection diagrams.
The aim of this research is to study the influence of moisture absorption at low moisture contents on the creep behaviour of an epoxy adhesive in steel bonded joints. Single lap joints were manufactured using high strength steel adherends and a two-component epoxy adhesive. The single lap joints were tested at load Levels corresponding to average lap shear stresses of±5%, 15%, 30% and 45% of the dry lap shear strength in both 40 °C air and 40 °C distilled water. Specimens were not pre-aged to be able to analyse the coupled effect of moisture and loading. The test results show that an increase in the load level resulted in an increase in the instantaneous strain and in the creep strain rate. The creep strain of single lap joints loaded in water was generally larger than for the ones loaded in air. For joints loaded in water the creep behaviour was found to be dependent on the moisture concentration in the adhesive. At low moisture percentages creep was suppressed, resulting in a lower instantaneous strain. At higher moisture percentages creep was promoted, resulting in a larger strain rate. The suppression of creep at low moisture percentages is attributed to water molecules bonding to the epoxy macromolecules, resulting in a reduction in molecular mobility and a smaller creep strain. At higher moisture percentages the plasticizing effect of the water dominates, resulting in a larger creep strain. The Maxwell threeelement solid model and Kelvin-Voigt three-element solid model were used to simulate the creep behaviour of the single lap joints loaded in air and water. The models gave good representations of the creep Response across the different load levels in both water and air, they were however unable to give a correct representation of the instantaneous strain of the single lap joints loaded in water. This is attributed to the models being unable to account for the present short-term relaxation process that is dependent on the moisture concentration.
The physical and chemical effects of diesel and biodiesel fuels on two high-density polyethylene (PE-HD) types were investigated. Both semi-crystalline PE-HD are common thermoplastic materials for container and storage tank applications. Biodiesel, a composition of unsaturated fatty acid esters from renewable resources, was chosen as it is regarded a possible green alternative to fossil fuels. The study aims at identifying significant differences between biodiesel and conventional diesel fuels based on the differences in the chemical nature of the two. The physical effects of the fuels on the polymer at first comprises the sorption behavior, i.e. kinetics and final equilibrium concentration. Not only are both fuels absorbed by the amorphous phase of the semi-crystalline PE-HD, they also induce a plasticization effect that modifies the molecular mobility and therefore also the characteristic yielding properties, manifest in the obtained stress-strain curves. The chemical effects related to degradation phenomena is investigated by a long-term storage scenario using partially immersed tensile test specimens in diesel and biodiesel. We were able to confirm the proposed co-oxidation mechanism by Richaud et al. for polyethylene-unsaturated penetrant systems on a larger scale based on practical tensile tests. One of the investigated polyethylene grades subjected to tensile drawing showed a significant loss of plastic deformation and the onset of premature failure after 150 days of storage in biodiesel. Further biodiesel storage showed a systematically reduced elongation at break before necking. None of these effects were observed in diesel. Oxidation of fuels and polymer after progressing storage times were analyzed by the evolution of carbonyl species in FT-IR/ATR spectroscopy.
In the present study, a simple approach was used to investigate the effect of UV-exposure on two high density Polyethylene materials (PE-HD), commonly used for storage tanks, on fuel sorption behavior and colonization by microorganisms. The aim was to investigate whether the sorption behavior of the fuels (diesel/biodiesel) and the colonization by microorganisms, frequently occurring in the fuel, is affected and may lead to undesirable or safety-relevant material changes. We showed that the UV-irradiation leads to significant changes of the sorption behavior due to chemi-crystallization and crosslinking. The fuel Sorption is affected by the UV-induced formation of polar carbonyl and hydroxyl groups predominantly occurring at the surface. With respect to microbial colonization behavior for Bacillus subtilis and Pseudomonas aeruginosa, isolated from a contaminated diesel sample, differences of the initial adhesion could be shown depending on the initial type of polyethylene as well as on the degree of UV-induced degradation.
High-density polyethylene becomes optically transparent during tensile drawing when previously saturated with diesel fuel. This unusual phenomenon is investigated as it might allow conclusions with respect to the material behavior. Microscopy, differential scanning calorimetry, density measurements are applied together with two scanning X-ray scattering techniques: wide angle X-ray scattering (WAXS) and X-ray refraction, able to extract the spatially resolved crystal orientation and internal surface, respectively. The sorbed diesel softens the material and significantly alters the yielding characteristics. Although the crystallinity among stretched regions is similar, a virgin reference sample exhibits strain whitening during stretching, while the diesel-saturated sample becomes transparent. The WAXS results reveal a pronounced fiber texture in the tensile direction in the stretched region and an isotropic orientation in the unstretched region. This texture implies the formation of fibrils in the stretched region, while spherulites remain intact in the unstretched parts of the specimens. X-ray refraction reveals a preferred orientation of internal surfaces along the tensile direction in the stretched region of virgin samples, while the sample stretched in the diesel-saturated state shows no internal surfaces at all. Besides from stretching saturated samples, optical transparency is also obtained from sorbing samples in diesel after stretching.
The impact fracture behavior of two common high-density polyethylene grades for container applications were intensively studied by the instrumented Charpy impact test after well-defined exposure to UV-irradiation. Individual stages of the impact event, such as crack initiation and crack propagation energy as well as maximum impact load, were investigated from the recorded load–deflection curves. UV-induced material property changes were further investigated by infrared spectroscopy, differential scanning calorimetry, and dynamic-mechanical analysis as well as density measurements. Based on the results of the Charpy impact test, three indicators were identified to describe the extend of photooxidation on high-density polyethylene: (a) a reduced Charpy impact strength—at least to half of its initial value for a distinctly brittle impact fracture, (b) a marked decrease in the crack propagation contribution to the impact strength, and (c) an increase of the brittle features of the fracture surface.
Einleitung
Bei der Detektion von Mikrorissen und der Charakterisierung spezifischer Schädigungen von Polymer- und Kompositwerkstoffen kommt es neben quantitativen Bewertungen auch auf die Analyse von orientierungsabhängigen Struktureffekten an. Am Beispiel typischer Vertreter ausgewählter Materialien (PP/PE, CFK) wird dargestellt, inwieweit das Potential der Röntgen-Refraktions-Topographie vorteilhaft zur umfassenden Materialcharakterisierung genutzt werden kann. Die korrosiven Eigenschaften nichtmetallischer Werkstoffe werden maßgeblich vom mikrostrukturellen Aufbau der verwendeten Materialien bestimmt. Typische Versagens-mechanismen von Faser-Kompositen sind neben der Faserenthaftung vor allem durch Mikrorisse gekennzeichnet, die sich teils parallel, teils senkrecht zu den Fasern in der Matrix ausbreiten. Eine wichtige Anwendung der Röntgen-Refraktions-Topographie umfaßt die orientierungsselektive Detektion von Mikroriß-Oberflächen. Die Nachweisgrenze derartiger Schädigungen erstreckt sich bis zum Nanometerbereich. Definiert herbeigeführte Ermüdungsschäden an Kompositmaterialien lassen sich mit dieser Methode ausgezeichnet erfassen und gestatten eine quantitative Korrelation zu mechanischen Kenngrößen. Mittels schrittweiser Probenabtastung und der ortskorrelierten Speicherung von simultan erfassten Refraktions- bzw. Absorptionszählraten wird ein zweidimensionales Röntgen-Refraktions-Topogramm rekonstruiert, das u.a. die präzise Information der integralen Rissdichte enthält.
Studienziel: Zwei Nagelsysteme, zum einen der unaufgebohrte Tibianagel mit 3fach-Verriegelung in 2 Ebenen und großem Bolzenspiel (UTN® der Fa. Mathys®) mit der Möglichkeit von medial zweifach und von ventral einfach zu verriegeln und zum anderen der unaufgebohrte Tibianagel mit 3fach-Verriegelung in einer Ebene mit geringem Bolzenspiel (Targon®-Nagel der Fa. Aesculap®), der 3 mediale Verriegelungsmöglichkeiten besitzt, wurden im Biegeversuch statisch geprüft. Anatomisch besteht bei der ventralen Verriegelung immer die Verletzungsmöglichkeit von Sehnen, Nerven und Gefäßen. Andererseits besteht bei einem höheren Bolzenspiel immer ein höherer Instabilitätsgrad. Inwieweit ist die Verriegelung in zwei Ebenen notwendig?
Methode: Wir prüften statisch zwei intramedulläre Verriegelungssysteme an 8 Leichenknochen (matched pairs) in zwei Ebenen an einer Universalprüfmaschine der Firma Schenk-Trebel im Biegeversuch.
Ergebnisse: Die Untersuchungsergebnisse zeigen, dass nicht die Bolzenlage, sondern das so genannte Bolzenspiel für die Stabilität der distalen Fragmente entscheidend ist. Die Verriegelung sollte somit prinzipiell durch ein formschlüssiges winkelstabiles Bolzensystem möglich sein.