Abstract: In this paper, the geometrical effective thermal conductivity of porous materials is investigated based on two different approaches: the finite element method as a representative for numerical approximation methods and an analytical method for 2D homogenised models based on a solution of the respective boundary value problem. It is found that the relative conductivity is practically independent of the specific shape or topology of the inclusions. Only the morphology (closed-cell or open-cell) of the structure slightly influences the conductivity. Furthermore, it is shown that a small perturbation of the circular inclusions of 2D models increases the effective conductivity.
Abstract: The objective of this work was to evaluate the creep behaviour of Si3N4 based ceramics obtained by uniaxial hot-pressing. As sintering additive, an yttrium-rare earth oxide solid solution, designed RE2O3, that shows similar characteristics to pure Y2O3, was used. Samples were sintered using high-purity α-Si3N4 powder, with additive mixtures based on RE2O3/Al2O3 or RE2O3/AlN, at 5 and 20 vol.%, respectively. The sintered samples were characterized by X-ray diffractometry,
scanning electron microscopy and density. Specimens of 3x3x6 mm3 were submitted to creep tests, under compressive stresses between 100 and 350 MPa at temperatures ranging from 1250 to 13750C in air. Samples with RE2O3/Al2O3 showed β-Si3N4 as crystalline phase, with grains of high aspect
ratio, and a relative density around 99% of the theoretical density. The Si3N4/RE2O3/AlN samples presented α-Si3N4 solid solution, designed α-SiAlON, with a more equiaxed microstructure and slightly lower relative density (96-98%). The results of creep tests indicated that these ceramics containing α-SiAlON are the more creep resistant, with steady-state creep rates around 10-4 h-1,
with stress exponents (n) in the range 0.67-2.53, indicating grain boundary sliding as the main creep mechanism.
Abstract: Open-cell ceramic foams are being considered for a variety of applications owing to their high permeability and low weight, including molten metal filters, catalytic substrates and radiant burners. In these applications, foams are exposed to high temperatures and thermal gradients and thus the materials used require resistance to severe thermal stresses during service. Unfortunately, little is known about the thermal shock behaviour of cordierite foams when subjected to sudden
changes in temperature. The objective of the present study was to investigate the thermal shock behaviour of two kinds of cordierite-based ceramic foams manufactured by the replication process. Thermal shock experiments were carried out by rapidly transferring the heated samples from a resistance furnace to
a quenching bath containing distilled water followed by measuring of the retained flexural strength. Unlike dense cordierite samples that showed a sudden decrease in flexural strength at quench temperature differences above 325 K, the strength retained after thermal shock by open-cell cordierite foams decreased gradually with increasing quench temperatures. This suggests a cumulative damage mechanism reflecting an increase in damage throughout the material rather than sudden failure owing to propagation of pre-existing cracks.
Abstract: In this work the reliability of the hole-drilling technique (HDT) for measuring welding residual stresses was analysed. HDT residual stress results were systematically compared with those determined by X-ray diffraction. A systematic overestimation of the residual stresses determined by HDT was observed, which was mainly attributed to the possibility of the so-called plasticity effect occurring. Experimental results were discussed taking the measurement principles of both
techniques into consideration. In addition, preliminary results of a numerical study, using the finite element method, will be presented for a better understanding of the plasticity effect on HDT residual stress results.
Abstract: Mechanically alloyed aluminium, MA Al, powder is difficult to consolidate.
Consolidation often involves complex processing that includes a hot extrusion stage. An alternative consolidation method consisting of a press-and-sinter process has been developed at the University of Seville. Nevertheless, sintered MA Al compacts have a low ductility. In the present work, to improve the ductility of consolidated compacts, hard MA Al powder was blended prior to the consolidation processing with different amounts (10, 20, 30 wt%) of soft unmilled Al powder. The bimodal microstructure (hard/soft) of the final compacts makes it possible to balance strength and ductility values.
Abstract: In the present paper, work of a preliminary nature pertaining to the development of a
simple tool for easy identification of reinforcement clustering in particle-reinforced MMCs is reported. The tool consists of an image processing and analysis routine performed with the aid of a public-domain software. Results obtained were in the form of distribution curves pertaining to the Euclidian distances between particle centroids within sample images. Analysis of that distribution allowed the identification of cluster occurrence. The technique was successfully applied to computer-generated images of virtual MMCs, as well as to micrographs of rheocast MMCs. Apart from particle clustering, the distribution curves were found useful to roughly assess the matrix cell size, for cases where this is significantly higher that the size of the reinforcing particles.
Abstract: A continuous effort is being devoted to study the feasibility of improving the wear
resistance of ductile iron components through an insertion of a hard metal powder layer during the casting process. Tests have been run to evaluate the wear resistance of the inserted material, compared to a non inserted ductile iron. The tests have been run in accordance with ASTM G65-94, which specifies
the so-called rubber wheel test. Three different kinds of material have been tested: a non inserted ductile iron, an inserted ductile iron in the as-cast state and an inserted ductile iron with a rectified surface. The tests have been run gradually; each test piece has been taken out from the test apparatus and weighted at different
intervals, from 200 to 6400 wheel revolutions (or 0.1 to 4.4 km of sliding distance); the results obtained show that the non inserted ductile iron presents a loss of weight fifty percent higher than the inserted material. The analysis of the surface of the test pieces after the wear tests has been run under stereo and scanning electron microscopy. The non inserted ductile iron test piece surface has shown an
uniform waviness with a wavelength around half a millimeter and under higher magnification much finer wear marks have been observed. The inserted ductile iron test piece surface has shown the same relief with half a millimeter wavelength but a significant difference has been noticed: the fine wear marks have been observed in the metallic matrix of the insert but they stop in the neighborhood of the tungsten carbide particles, leading the authors to propose that these hard particles remained unchanged until the metallic matrix supporting them has been sufficiently
removed to cause the respective extraction; this behavior should explain the smaller weight loss rate of the inserted material.
Abstract: In this study is proposed a procedure for damage discrimination based on acoustic
emission signals clustering using artificial neural networks. An unsupervised methodology based on the self-organizing maps of Kohonen is developed considering the lack of a priori knowledge of the different signal classes. The methodology is described and applied to a cross-ply glassfibre/
polyester laminate submitted to a tensile test. In this case, six different AE waveforms were identified. The damage sequence could so be identified from the modal nature of those waves.
Abstract: In this study an optic fibre system for health monitoring of fibre reinforced plastics was developed. It is based on the detection of acoustic emission (AE) waves in a loaded material. A low-finesse Fabry-Pérot interferometer sensor is used as alternative to the conventional piezoelectric transducers for AE waves sensing. An original procedure for optical fibre sensor interrogation is proposed.
Abstract: Glass can be considered to be a high-technology engineering material with a
multifunctional potential for structural applications. However, the conventional approach to the use of glass is often based only on its properties of transparency and isolation. It is thus highly appropriate and necessary to study the mechanical behaviour of this material and to develop adequate methods and models leading to its characterisation. It is evident that the great potential of growth for structural glass applications is an important opportunity of development for the glass
industry and the building/construction sectors. The work presented in this paper is a reflection of this conclusion. The authors shortly present the state-of-the-art on the application of glass as a structural element in building and construction, and refer to other potential fields of application and available glass materials. The experimental procedures and methods adopted in three-point bending tests performed on 500 × 100 [mm2] float, laminated and tempered glass specimens with thicknesses between 4 and 19 mm are thoroughly described. The authors evaluated the mechanical
strength and stiffness of glass for structural applications. This work contributes to a deeper knowledge of the properties of this material.