Modellierung quellungsbedingter Spannungen in PMMA mittels FEM unter Berücksichtigung von Sorptionsisothermen

Ein verbesserter Festigkeitsnachweis für Kunststoffkomponenten wird angestrebt, der umgebungsbedingte Einflüsse auf die Beanspruchbarkeit und auf die Beanspruchungen unter Berücksichtigung ihrer Geometrie abbildet, und zwar von Beginn des Entwicklungsprozesses an. Das Werkstoffverhalten eines Modellsystems (PMMA unter dem Einfluss von Umgebungsfeuchte bei Raumtemperatur) wird in Diffusionsexperimenten und mechanischen Prüfungen untersucht. Das Sorptionsverhalten des Modellsystems wird mit dem Dual-Stage-Modell beschrieben, woraus lineare quellungsbedingte Ausdehnungskoeffizienten folgen. Das auf diese Weise parametrisierte Werkstoffverhalten dient als Eingangsdaten für gekoppelte Computersimulationen der mechanischen Spannungen in einer dünnen PMMA-Platte, die durch das Zusammenspiel zwischen Diffusion, Weichmachung und Quellung bzw. Schwindung entstehen (isotherm). Experimentelle Spannungszustände durch überlagerte Trocknung und Drei-Punkt-Biegung bestätigen die Größenordnung simulierter Trocknungsspannungen. Abschließend wird die entwickelte Methodik in einer Fallstudie zur Ermittlung von Quellspannungen in Platten aus einem Epoxid- und einem Acrylharz eingesetzt

Nonlinear material behaviour and failure of closed-cell polymer foams

Classical strength criteria, like the von Mises criterion, are used to postulate the failure of ductile materials like steel or brass. It is known that for the application of foams in modern lightweight structures extended criteria are required, since foams are sensitive to hydrostatic stress. This observation on the macroscale can be explained by the deformation mechanisms of one single foam cell. Under hydrostatic stress, the deformation of the cell causes a non-uniform stress state of the cell walls. To understand the mechanism on the microlevel, a finite element model on the basis of a tetrakaidecahedron as unit cell was implemented. Utilising a strain energy-based homogenisation concept, the effective properties of the foam can be obtained. To adapt the geometric properties of the model to the real microstructure of the foam, results of a computer tomography image analysis were used by considering several imperfections in the cell geometry. For the analysis of the stress state on the microlevel, different load cases were applied to the unit cell. By means of these simulations, the geometrically nonlinear stress–strain curves on the macrolevel were deduced. Furthermore, the analysis of the finite element model provides an insight into the deformation mechanism on the microscale and allows the prediction of failure as well. Finally, the predicted failure points are represented in the Burzyński plane and compared with experimental results. The current paper focuses on the hard foam ROHACELL® IG-series (industrial grade), which is a closed-cell PMI foam produced by Evonik Industries AG, Germany

Strategies to Cope with Inferior Long-Term Photostability of Bentonite Polyolefin Nanocomposites

This study provides insight into the causes of inferior long-term stability of nanocomposites based on organic layered silicates (OLSs) used for cable mantles. A hierarchy was established by analyzing bentonite products and their respective polyolefin nanocomposites. Thermogravimetric analysis (TGA), X-ray diffraction (XRD), gas adsorption, energy-dispersive spectroscopy (SEM-EDX), and infrared spectroscopy (IR) provided evidence for the adsorption of stabilizers onto the filler surface and thus their reduction in activity, promoting polymer oxidation. This behavior corresponds to the specific surface area of the incorporated OLS. Therefore, it can be stated that gas adsorption and XRD are especially useful for the evaluation of long-term photostability. It was revealed that photocatalytically active iron is of secondary importance since iron-rich bentonites still formed the most stable nanocomposite. This also applies to the Hofmann elimination products of the modifying agent, where higher contents do not accelerate the degradation process. No elimination products could be traced within the composites. Due to the polymer-filler interface being essential for long-term photostability, prior analysis of the filler surface properties can be used to estimate the stability of the respective nanocomposite as a rationale for product selection in the early stages of development. The reasons identified in this work for decreasing the long-term photostability of OLS nanocomposites compared with unfilled formulations is an important step toward increasing their stability

Ageing study of different types of long-term pressure tested PE pipes by IR-microscopy

Although the lifetime of plastic pipes is commonly determined by hydrostatic pressure tests which measure the mechanical resistance, thermo-oxidative degradation plays an important role in practice. It is, therefore, an important question to monitor the elemental processes occurring during ageing of polymers, namely the loss of stabilising additives and the degradation of the polymer. The diffusion of the phenolic antioxidant AO-18, the processing stabiliser, and changes in polymer morphology during hydrostatic pressure testing, have been systematically studied for pipes made of PE 80 and PE 100 using infrared microscopy in a quantitative manner. For both materials a parabolic concentration profile of AO-18 develops over the pipe wall upon hydrostatic pressure testing. Under comparable conditions of testing the loss rate was larger for the PE 80 than for the PE 100. An annealing of the polymer occurs upon ageing with a similar increase in degree of crystallinity for bot h materials. However, the PE 100 has an overall higher level of crystallinity, and it may be speculated that this is one of the reasons for the slower loss of antioxidant. The results from IR-microscopy are in excellent agreement with analyses of samples taken over the pipe wall by oxidative induction time (OIT) and extraction HPLC. The conversion of the phosphite additive PS-2 to the corresponding phosphate as a result of oxidation has been quantitatively assessed. The total content of phosphate and phosphite was found to be constant, and almost no extraction is seen before 5000 h. GPC analysis of samples taken proves that no significant reduction in average molar mass of the polymer takes place