Volume 51, Issue 4

The oscillation behaviour of the Schenck and Roell-Amsler servohydraulic dynamic tensile testing machines used at the RWTH IEHK has been investigated [1]. The dynamic longitudinal and bending oscillations, propagating through the test set-up, have been characterised with a Fast Fourier Transformation (FFT) and a simple analytical model. When increasing the strain rate, the Eigen frequency of the test set-up becomes a limiting factor. With higher testing speed the oscillations amplitude and inertia effects also increase, which makes it difficult to determine accurately the mechanical properties as usually done with quasistatic tensile tests. A set-up optimisation can be reached by decreasing the mass between the load cell and the sample, either by moving the load cell closer to the sample, or with a lighter machine design and grip shape. A combination of both possibilities has been followed at IEHK. A global piezoquartz load cell, a quasilocal calibrated strain gage load cell on the grip, and a local calibrated strain gage load cell on the sample can be used as load measurement systems. The corresponding load signals have been compared with regard to initial oscillation amplitude, frequency, and damping factor for different strain rates. The positive effects of a lightweight sample grip design are also shown and quantified. The initial amplitude can be reduced easily by using damping rubbers. Advantages and draw backs of this method are shown. Short sample geometries are also beneficial to increase the vibration frequency. Knowing the vibration behaviour of the machine, it is then easier to classify the materials to be tested dynamically, based on the quasistatic yield ratio and fracture elongation values. Since vibrations cannot be avoided with conventional servohydraulic machines, an adequate smoothing method must still be chosen to determine the mechanical properties and flow curves. Low pass filtering results are compared with analytical polynoms and the traditional spline smoothing method used at IEHK.

Many methods are currently used to measure fracture toughness of ceramic materials. Due to its simplicity, its non-destructive nature, and the fact that minimal machining is required to prepare the sample, the use of the Vickers hardness indentations to measure fracture toughness (K I C ) has become quite popular. The aim of the present work is to investigate the hardness from Vickers indentations and the fracture toughness based on crack-length measurements of cracks introduced into the sample surface of cold isostatically pressed (CIP)-Al 2 O 3 with 99.8% purity.

Polymethylmethacrylate (PMMA) also known as bone cement is a frequently used material for medical applications. This study investigated the material properties of the bone cement Palacos R. Flexural, tensile, and compression tests on Palacos R, following common standard, revealed typical brittle material behaviour with high compression (83 MPa) by rather low tension stress (35 MPa). No significant difference was measured between dry stored and 37°C saline solution stored specimens regarding tensile, compression and flexural properties. As bone cement can be considered as a visco-elastic material, additional tests under varied strain rates revealed an increasing Young's modulus and ultimate strength of Palacos R with increasing strain rate. To evaluate long-term load-cycling behaviour an S-N curve was determined. The material parameters of Palacos R may help to predict and evaluate failure of bone cement and improve its medical application.

It is reasonable to assume that the existence of inherent anti-oxidizing activity of nanotubes and nanofibers (CNT, CNF) would be a good tool for solving the problem of oxidative stability of polymer composites. CNT/CNFs have electron affinities similar to those of fullerenes and are therefore capable of acting as radical traps in free radical chain reactions, we found good evidence in our earlier works. Here, the anti-oxidizing activity of commercial carbon nanofibers of herringbone structure CNF-HB was examined with the aim to quantitatively assess the interference of Ni impurities. The model initiating the aerobic oxidation of cumene was used to determine the kinetic parameters of anti-oxidizing activity of commercial and purified CNF-HBs. CNF-HB inhibits the model reaction according to the mechanism of trapping of peroxy radicals: the oxidation proceeds with pronounced induction period and kinetic curves linearly at the semi-logarithmic coordinates (especially in the case of purified samples). For the first time effective concentration of inhibiting centres and inhibition rate constants for the CNF-HB have been determined. The inhibition rate constant is equal to k 7 = (6 ± 1.0)×10 3 l mol −1 s -1 that is typical for the anti-oxidants of average force.

Polymer matrix composites have a great importance and a wide range of fields of application due to their superior mechanical, physical, and chemical properties. Polymer matrix composites are generally used in aerospace, aircraft, chemical, defence, automotive, and medical industries. Matrix and fiber technologies have had a great development in the last 30 years. The major disadvantage of composite materials are the high production costs. Injection moulding techniques are very useful for production of short fiber reinforced polymer matrix composites. In this study, production of short fiber reinforced polymer matrix composites has seen examined and miscallenous reinforced and matrix materials are described.

Kurzfassung In-situ-Neutronenradiografie wird erfolgreich eingesetzt, um flüssiges Wasser in Niedertemperatur-PEM-Brennstoffzellen während des Betriebs zerstörungsfrei abzubilden und das sogenannte Wassermanagement — eine der größten Herausforderungen der Entwicklung — durch Anpassung der Materialeigenschaften zu optimieren. Dabei werden die besonderen Eigenschaften der Neutronen genutzt, die metallische Bauteile relativ leicht durchdringen, während selbst kleine Wassermengen durch ihre starke Streuung leicht nachweisbar sind. Die relativ langen Messzeiten für die Neutronentomografie werden durch den Betrieb der Zelle im „angehaltenen‟ Zustand realisierbar, sodass die dreidimensionale Wasserverteilung darstellbar wird.

Kurzfassung Bauteile können stoßartigen Beanspruchungen, z.B. durch den Aufprall von anderen Bauelementen und Bruchstücken, ausgesetzt sein. Ob die Bauteile solchen Aufprallbeanspruchungen stand- halten können, ohne dass Schäden auftreten, die die Funktionsfähigkeit und Sicherheit beeinträchtigen, hängt einerseits von den mechanischen Eigenschaften der hierfür verwendeten Werkstoffe ab, andererseits aber auch von der Festigkeit und Geometrie der aufprallenden Teile. Beschussversuche mit zylindrischen Projektilen aus gebräuchlichen Schleifkörperkeramiken auf Stahlblech und Polycarbonat zeigen, dass die Aufprallfestigkeit von Werkstoffen mit steigender Scharfkantigkeit, abnehmendem Durchmesser und zunehmender Festigkeit der aufprallenden Teile abnimmt.

In this work, changes in mechanical properties of aluminum after continuous casting has been studied. Molten aluminum (Al 1235) was converted directly into an endless coiled strip, suitable for cold rolling, using a twin drum continuous casting process. After hot forming, industrial pure aluminum Al 1235 was rolled at room temperature to different reduction ratios. Tensile strength, elongation, and Vickers hardness of the aluminum were determined experimentally. The theoretical part comprises also analytical modeling. The results revealed the formability characteristics of material depending on the process. The mechanical properties of the rolled sheets in the hot forming process are poorer than those in the cold rolled case.

Kurzfassung Im vorliegenden Beitrag wird dargestellt, dass in Mehrstoffaluminiumbronzen vom Typ CuAl9Fe5Ni4Mn1 mit einem α + χ-Gleichgewichtsgefüge, die vergütet oder thermomechanisch durch Warmverformung, Abschrecken, gefolgt von Anlassen wärmebehandelt wurden, eine Anlasssprödigkeit im Temperaturbereich 300—400°C auftritt. Die Untersuchungen, hauptsächlich mithilfe eines Transmissionselektronenmikroskops durchgeführt, zeigen, dass im besagten Temperaturbereich zuerst kohärente, sehr feine χ-Ausscheidungen in den beim Aufheizen nicht vollständig gelösten α-Kristallen ausgeschieden werden. Diese Ausscheidungen blockieren die Versetzungen und verursachen die Anlasssprödigkeit. Eine nennenswerte Präsenz oder ein bemerkenswerter Einfluss der geordneten β 1 -Phase konnte nicht festgestellt werden, nicht einmal in den thermomechanisch behandelten Proben. Insgesamt führt die Vermeidung des kritischen Temperaturbereichs zu guten mechanischen Eigenschaften.