The Effect of Sequence of Operations on Fatigue Life of LSP Treated Open-hole Aluminium Specimens

Fastener holes in aeronautical structures are typical sources of fatigue cracks due to their induced local stress concentration. A very efficient solution to this problem is to establish compressive residual stresses around the fastener holes that retard the fatigue crack nucleation and its subsequent local propagation. Previous work done on the subject of the application of LSP treatment on thin, open-hole specimens [1] has proven that the LSP effect on fatigue life of treated specimens can be detrimental, if the process is not properly optimized. In fact, it was shown that the capability of the LSP to introduce compressive residual stresses around fastener holes in thin-walled structures representative of typical aircraft constructions was not superior to the performance of conventional techniques, such as cold-working.

High-temperature mechanical properties of aluminium alloys reinforced with titanium diboride (TiB2) particles

The physical and mechanical properties of metal matrix composites were improved by the addition of reinforcements. The mechanical properties of particulate-reinforced metal-matrix composites based on aluminium alloys (6061 and 7015) at high temperatures were studied. Titanium diboride (TiB2) particles were used as the reinforcement. All the composites were produced by hot extrusion. The tensile properties and fracture characteristics of these materials were investigated at room temperature and at high temperatures to determine their ultimate strength and strain to failure. The fracture surface was analysed by scanning electron microscopy. TiB2 particles provide high stability of the aluminium alloys (6061 and 7015) in the fabrication process. An improvement in the mechanical behaviour was achieved by adding TiB2 particles as reinforcement in both the aluminium alloys. Adding TiB2 particles reduces the ductility of the aluminium alloys but does not change the microscopic mode of failure, and the fracture surface exhibits a ductile appearance with dimples formed by coalescence.

Influence of aluminium on dimensional change of sintered 430 Ferritic Stainless Steel

Aluminium is added to decrease matrix chromium losses on 430 stainless steel sintered on nitrogen atmosphere. Three different ways were used to add a 3% (in weight) aluminium: as elemental powder, as prealloyed powder, and as intermetallic Fe-AI compound. After die pressing at densities between 6.1-6.5 g/cm3, samples were sintered on vacuum and on N2-5%H2 atmosphere in a dilatometric furnace. Therefore, dimensional change was recorded during sintering. Weight gain was obtained after nitrogen sintering on all materials due to nitrides formation. Sample expansion was obtained on all nitrogen sintered steels with Al additions. Microstructure showed a dispersion of aluminium nitrides when pre-alloyed powders are used. On the contrary, aluminium nitride areas can be found when aluminium is added as elemental powders or as Fe-AI intermetallics. Also nitrogen atmosphere leads to austenite formation and hence, on cooling, dilatometric results showed a dimensional change at austenitic-ferritic phase transformation temperature.

Properties of aluminium nodules foamed with concentrated solar energy

Commercial aluminium foam filled structures and sandwich panels are available for structural applications. As alternative to these materials, small granular foamed pieces are proposed to fill structures as well as sandwich panels. On the present work, foam precursors are obtained by Powder Metallurgy (PM) route, using natural calcium carbonate as foaming agent instead of titanium hydride. Extruded precursor bars were cut into small pieces (around 4.5 mm long and 5mm in diameter). Foaming treatment was carried out on two different ways: electrical preheated furnace and by solar furnace. Foamed nodules presented a low cell size, density e.g. 0.67 g/cm3 to 0.88 g/cm3 and a height/diameter ratio between 0.72 and 0.84 as a function of precursor size. These properties depend on the foaming particle size, foaming cycle and precursor dimensions. Carbonate precursors are easily foamed by concentrated solar energy, due to the lower risk of cell collapse than with hydride precursors, resulting from cell stabilization by oxide skin formation into cells and a low degree of foamed nodules bonding.

Intensive use of aluminium in car body construction

The developments in materials over the last decade have been considerable within the automotive industry, being one of the leaders in innovative product applications. Sustainable product development of an automotive structure requires a balanced approach towards technological, economical and ecological aspects. The introduction of new materials and processes is dependent on satisfying different factors. Competitive and legislative pressures, creating the need for change, affect these factors considerably. The process, direction and speed of change are often reactive. Current paper shows the application of aluminium alloys, for the use in the bottom structure of a car to face the problem for the weight of the entire bottom structure under static load conditions, including stiffness, strength and buckling constraints. In addition to minimized mass and materials' price, the assessment of an environmental impact of materials-candidates during the entire life cycle of the structure is considered.

Influence of the Lubrication Conditions on the Surface Finish of Turned Aeronautical Aluminium Alloys. A preliminary Study

Nowadays, one of the main objectives that affects the development of any new product is the respect for the environment. Until the late 80's, the development and manufacture of the most of the product were aimed to achieve maximum quality in time and costs with environmental issues relegated to secondary importance. On the other hand, in the 90's, the pressure from factors such as markets, financial and legislative factors, led to environmental considerations being taken into account. In this context, the current aeronautical industry strategies are based on the search for economic, environmental and energy efficiency considerations for all the processes involved in the aircraft manufacturing.

Mode II fracture energy in the adhesive bonding of dissimilar substrates: carbon fibre composite to aluminium joints

The end-notched flexure (ENF) test calculates the value of mode II fracture energy in adhesive bonding between the substrates of same nature. Traditional methods of calculating fracture energy in the ENF test are not suitable in cases where the thickness of the adhesive is non-negligible compared with adherent thicknesses. To address this issue, a specific methodology for calculating mode II fracture energy has been proposed in this paper. To illustrate the applicability of the proposed method, the fracture energy was calculated by the ENF test for adhesive bonds between aluminium and a composite material, which considered two different types of adhesive (epoxy and polyurethane) and various surface treatments. The proposed calculation model provides higher values of fracture energy than those obtained from the simplified models that consider the adhesive thickness to be zero, supporting the conclusion that the calculation of mode II fracture energy for adhesives with non-negligible thickness relative to their adherents should be based on mathematical models, such as the method proposed in this paper, that incorporate the influence of this thickness.

Laser ablation modelling of aluminium, silver and crystalline silicon for applications in photovoltaic technologies

Laser material processing is being extensively used in photovoltaic applications for both the fabrication of thin film modules and the enhancement of the crystalline silicon solar cells. The two temperature model for thermal diffusion was numerically solved in this paper. Laser pulses of 1064, 532 or 248 nm with duration of 35, 26 or 10 ns were considered as the thermal source leading to the material ablation. Considering high irradiance levels (108–109 W cm−2), a total absorption of the energy during the ablation process was assumed in the model. The materials analysed in the simulation were aluminium (Al) and silver (Ag), which are commonly used as metallic electrodes in photovoltaic devices. Moreover, thermal diffusion was also simulated for crystalline silicon (c-Si). A similar trend of temperature as a function of depth and time was found for both metals and c-Si regardless of the employed wavelength. For each material, the ablation depth dependence on laser pulse parameters was determined by means of an ablation criterion. Thus, after the laser pulse, the maximum depth for which the total energy stored in the material is equal to the vaporisation enthalpy was considered as the ablation depth. For all cases, the ablation depth increased with the laser pulse fluence and did not exhibit a clear correlation with the radiation wavelength. Finally, the experimental validation of the simulation results was carried out and the ability of the model with the initial hypothesis of total energy absorption to closely fit experimental results was confirmed.

Fatigue in laser shock peened open-hole thin aluminium specimens

An experimental study was performed in order to determine the influence of the sequence of operations on the effectiveness of Laser Shock Peening (LSP) treatment in increasing the fatigue performances of open-hole aluminium specimens. Residual stress measurements, fractographic analysis and FEM analysis were performed, indicating the presence of compressive residual stresses on the surface of the treated specimens and tensile residual stresses in the mid-section along the thickness of the specimens. Negative effects on fatigue lives were encountered on the specimens with the hole already present, while positive effect were observed in specimens in which the hole was drilled after LSP treatment. These results indicate that LSP can be a good solution for “in production” application, in which open holes are to be drilled after the LSP treatment. The application in which LSP is used “in service” on structures with pre-existing cut-outs, has proven to be impracticable in the investigated configuration.

Measurements and Prediction of Pedestrian Walking Loads on an Aluminium Catwalk

During the last two decades the topic of human induced vibration has attracted a lot of attention among civil engineering practitioners and academics alike. Usually this type of problem may be encountered in pedestrian footbridges or floors of paperless offices. Slender designs are becoming increasingly popular, and as a consequence, the importance of paying attention to vibration serviceability also increases. This paper resumes the results obtained from measurements taken at different points of an aluminium catwalk which is 6 m in length by 0.6 m in width. Measurements were carried out when subjecting the structure to different actions:1)Static test: a steel cylinder of 35 kg was placed in the middle of the catwalk; 2)Dynamic test: this test consists of exciting the structure with singles impulses; 3)Dynamic test: people walking on the catwalk. Identification of the mechanical properties of the structure is an achievement of the paper. Indirect methods were used to estimate properties including the support stiffness, the beam bending stiffness, the mass of the structure (using Rayleigh method and iterative matrix method), the natural frequency (using the time domain and frequency domain analysis) and the damping ratio (by calculating the logarithmic decrement). Experimental results and numerical predictions for the response of an aluminium catwalk subjected to walking loads have been compared. The damping of this light weight structure depends on the amplitude of vibration which complicates the tuning of a structural model. In the light of the results obtained it seems that the used walking load model is not appropriate as the predicted transient vibration values (TTVs) are much higher than the measured ones.

Influence of high aluminium content on the mechanical properties of directionally solidified multicrystalline silicon

The purpose of this research is the mechanical characterisation of multicrystalline silicon crystallised from silicon feedstock with a high content of aluminium for photovoltaic applications. The mechanical strength, fracture toughness and elastic modulus were measured at different positions within the multicrystalline silicon block to quantify the impact of the segregation of impurities on these mechanical properties. Aluminium segregated to the top of the block and caused extensive micro-cracking of the silicon matrix due to the thermal mismatch between silicon and the aluminium inclusions. Silicon nitride inclusions reduced the fracture toughness and caused failure by radial cracking in its surroundings due to its thermal mismatch with silicon. However, silicon carbide increased the fracture toughness and elastic modulus of silicon.

Compact discs as versatile cost-effective substrates for releasable nanopatterned aluminium films

We demonstrate that standard polycarbonate compact disk surfaces can provide unique adhesion to Al films that is both strong enough to permit Al film nanopatterning and weak enough to allow easy nanopatterned Al film detachment using Scotch tape. Transferred Al nanohole arrays on Scotch tape exhibit excellent optical and plasmonic performance.