Dielectric Properties of Graphite Nanocomposites

Polymer nanocomposites are an emerging class of multifunctional materials that have not been optimized for their functional potential. In this study, the dielectric properties of graphite polymer nanocomposites were evaluated. The objective was to develop for composite applications, an alternative cost-effective nanoscale carbon material with properties comparable to those of carbon nanotubes

X-ray microtomographic characterization and quantification of the strain rate dependent failure mechanism in cenosphere epoxy syntactic foams

This work investigates the failure mechanism in cenosphere epoxy syntactic foams at the quasi-static and dynamic strain rates. Split-Hopkinson pressure bar experiments are controlled to stop dynamic deformation of the foams at various strain stages. The internal microstructure at each strain is characterized in the x-ray microtomography and compared to the microstructure in the foams deformed quasi-statically. The microscopic observations reveal that the failure process in syntactic foams at the low and high rates is dominated by the crushing of cenospheres and the cracking of the epoxy matrix. However, the mechanism of failure in the foam is significantly affected by the strain rate. Compared to quasi-static compression, macro-cracks form earlier in the matrix at dynamic rates and can propagate to split cenospheres. The volume of the damage as defined by the failure of both cenospheres and the matrix is calculated from the x-ray microtomographic images. It is found that the damage can be quantitatively related to the strain and the strain rate using an empirical equation

In-situ solvothermal processing of polycaprolactone/hydroxyapatite nanocomposites with enhanced mechanical and biological performance for bone tissue engineering

The interest in biodegradable polymer-matrix nanocomposites with bone regeneration potential has been increasing in recent years. In the present work, a solvothermal process is introduced to prepare hydroxyapatite (HA) nanorod-reinforced polycaprolactone in-situ. A non-aqueous polymer solution containing calcium and phosphorous precursors is prepared and processed in a closed autoclave at different temperatures in the range of 60–150 °C. Hydroxyapatite nanorods with varying aspect ratios are formed depending on the processing temperature. X-ray diffraction analysis and field-emission scanning electron microscopy indicate that the HA nanorods are semi-crystalline. Energy-dispersive X-ray spectroscopy and Fourier transform infrared spectrometry determine that the ratio of calcium to phosphorous increases as the processing temperature increases. To evaluate the effect of in-situ processing on the mechanical properties of the nanocomposites, highly porous scaffolds (>90%) containing HA nanorods are prepared by employing freeze drying and salt leaching techniques. It is shown that the elastic modulus and strength of the nanocomposites prepared by the in-situ method is superior (∼15%) to those of the ex-situ samples (blended HA nanorods with the polymer solution). The enhanced bone regeneration potential of the nanocomposites is shown via an in vitro bioactivity assay in a saturated simulated body fluid. An improved cell viability and proliferation is also shown by employing (3-(4,5- dimethylthiazol-2-yl)-2, 5-diphenyl tetrazolium bromide) (MTT) assay in human osteosarcoma cell lines. The prepared scaffolds with in vitro regeneration capacity could be potentially useful for orthopaedic applications and maxillofacial surgery

Large strain compressive response of 2-D periodic representative volume element for random foam microstructures

A numerical investigation has been conducted to determine the influence of Representative Volume Element (RVE) size and degree of irregularity of polymer foam microstructure on its compressive mechanical properties, including stiffness, plateau stress and onset strain of densification. Periodic two-dimensional RVEs have been generated using a Voronoi-based numerical algorithm and compressed. Importantly, self-contact of the foam’s internal microstructure has been incorporated through the use of shell elements, allowing simulation of the foam well into the densification stage of compression; strains of up to 80 percent are applied. Results suggest that the stiffness of the foam RVE is relatively insensitive to RVE size but tends to soften as the degree of irregularity increases. Both the shape of the plateau stress and the onset strain of densification are sensitive to both the RVE size and degree of irregularity. Increasing the RVE size and decreasing the degree of irregularity both tend to result in a decrease of the gradient of the plateau region, while increasing the RVE size and degree of irregularity both tend to decrease the onset strain of densification. Finally, a method of predicting the onset strain of densification to an accuracy of about 10 per cent, while reducing the computational cost by two orders of magnitude is suggested

Three-dimensional in situ observations of compressive damage mechanisms in syntactic foam using X-ray microcomputed tomography

Syntactic foams with hollow glass microspheres embedded in an epoxy matrix are used in marine, aerospace and ground transportation vehicle applications. This work presents an in situ experimental study of failure mechanisms in syntactic foam based on X-ray microcomputed tomography with uniaxial compression. Under different levels of compressive strain, the material was scanned using X-ray microcomputed tomography to obtain three-dimensional (3D) images of its internal microstructure. Experiments with the same parameters were carried out to investigate repeatability. The microscopic observations have suggested that damage nucleation occurs at the weakest microspheres. When applied strain increases, shear collapse bands (SCBs) develop local to the fragmented microspheres due to stress concentration and bending deformation around SCBs occurs. After significant strain, the thickness of the SCBs increases owing to the accumulation of the broken microspheres. The relationship between the volume fraction of microspheres and applied bulk strain has been characterised

Toughening of low-weight foam-based composites

Various industries, incl. defense, aerospace and automotive, are spending large budgets on the development of lightweight materials. Especially, sandwich composites gain increasing attention due to their high strength-to-weight which allows for a structural weight reduction compared to conventional metallic structures. Sandwich composites comprise of fibrous outer layers and a core material. Syntactic foam, comprising of a binder and hollow microspheres, would be an attractive material to be used as the core material since it is a lightweight material with tremendous compressive strength, low moisture absorption, and low radar detectability. However, the major disadvantage of syntactic foam is its brittle behavior under mechanical loading. Due to its brittleness, it has a low resistance to the formation and propagation of cracks, limiting the number of advanced applications of syntactic foam.DOCTOR OF PHILOSOPHY (MSE

De Market to Book-ratio als voorspeller van toekomstige herwaarderingen bij Europese beursgenoteerde vastgoedbeleggingsinstellingen

In dit artikel onderzoeken wij de vraag of de Market to Book-ratio van vastgoedbeleggingsinstellingen een goede voorspeller is van toekomstige afwaarderingen van de vastgoedportefeuilles van deze instellingen. Dit is onderzocht aan de hand van multi-level regressieanalyses onder 135 Europese beursgenoteerde vastgoedbeleggingsinstellingen over de periode vanaf 2005 tot en met 2013. Uit ons onderzoek blijkt dat in het geval de Market to Book-ratio kleiner (groter) is dan één, en de markt een toekomstige negatieve (positieve) herwaardering verwacht, deze herwaardering geheel of ten dele met een vertraging van tenminste één boekjaar in de jaarrekening wordt verwerkt