Porous silicon samples have been reduced in nanometric particles by a well known industrial mechanical process, the ball grinding in a planetary mill; the process has been extended to crystalline silicon for comparison purposes. The silicon nanoparticles have been studied by X-ray diffraction, infrared spectroscopy, gas porosimetry and transmission electron microscopy. We have estimated crystallites size from about 50 nm for silicon to 12 nm for porous silicon. The specific surface area of the powders analyzed ranges between 100 m2/g to 29 m2/g depending on the milling time, ranging from 1 to 20 h. Electron microscopy confirms the nanometric size of the particles and reveals a porous structure in the powders obtained by porous silicon samples which has been preserved by the fabrication conditions. Chemical functionalization during the milling process by a siloxane compound has also been demonstrated.
High stress regions around corrosion pits can lead to crack nucleation and propagation. In fact, in many engineering applications, corrosion pits act as precursor to cracking, but prediction of structural damage has been hindered by lack of understanding of the process by which a crack develops from a pit and limitations in visualization and measurement techniques. An experimental approach able to accurately quantify the stress and strain field around corrosion pits is still lacking. In this regard, numerical modeling can be helpful. Several numerical models, usually based on finite element method (FEM), are available for predicting the evolution of long cracks. However, the methodology for dealing with the nucleation of damage is less well developed, and, often, numerical instabilities arise during the simulation of crack propagation. Moreover, the popular assumption that the crack has the same depth as the pit at the point of transition and by implication initiates at the pit base has no intrinsic foundation. A numerical approach is required to model nucleation and propagation of cracks without being affected by any numerical instability and without assuming crack initiation from the base of the pit. This is achieved in the present study, where peridynamics (PD) theory is used in order to overcome the major shortcomings of the currently available numerical approaches. Pit-to-crack transition phenomenon is modeled, and nonconventional and more effective numerical frameworks that can be helpful in failure analysis and in the design of new fracture-resistant and corrosion-resistant materials are presented.
Aluminium silicon alloys have shown favourable properties when used as the matrix for abradable coatings in low pressure compressors of gas turbines [1 and 2]. This paper aims to describe the wear mechanisms found in aluminium silicon based abradables. To this end three thermally sprayed coatings are investigated. Aluminium silicon polyester, aluminium silicon-graphite and the most recently developed, aluminium silicon-hexagonal boron nitride (hBN) examined here are amongst a few of these materials. To be able to design materials to functi?n in as wide a parameter range as possible, a test ng simulating engine mechanisms is required. Tests were conducted using titanium blades at velocities ranging from 250 - 450 m/s, temperatures of ambient to 450°C and controlled incursion rate of 5, 50 and 500 µm/s. The data obtained from these tests is best interpreted in the form of wear maps which characterise the seal performance and therefore are of use to engine and material designers.
Air separation plants use centrifugal compressors where air and electrical energy are the only raw materials used in the production process. So energy costs play a crucial role and the compressors are heavily penalized when guaranteed performance levels are not achieved. In order to better generate performance, abradable coatings, previously used in the gas turbine industry, have been designed into turbocompressors. This paper will show the optimization and performance improvements of a new aluminium silicon-boron nitride material.
Even though the failure mechanisms in aerospace structures are mainly governed by the incidence of fatigue loading, environmentally assisted fracture related pit-to-crack transition can still occur in many aluminum alloys, stainless steels and high strength low alloy steels. Despite of the relative rarity of this phenomenon, the consequent failures can be extremely destructive and lead to the loss of an aircraft. Prediction of damage evolution starting at corrosion pits acting as precursor to cracking has been hampered by a lack of insight into the process, as well as limitations in visualization and measurement techniques. In this regard, numerical modeling can be beneficial. The current study presents numerical predictions of pit evolution from a flat metal surface to realistic pit morphologies by using a new continuum mechanics formulation, peridynamics. Intergranular/transgranular pit-tocrack transition under different loading conditions and microstructural features are investigated without imposing any limiting assumption on the site of the crack initiation. Based on the numerical results, it can be concluded that microstructure has a significant effect for the prediction of pit-to-crack transition phenomenon.
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