Ciência & Tecnologia dos Materiais最新文献

In a close future, it will be possible to print any kind of food through a nozzle, with the desired nutritive quantity, originating richer, healthier and more controlled meals. The exponential growth of this new market led to intensive research worldwide; however, dealing with a low cost printer is not an easy task for the general consumer. This paper presents the difficulties that a common user might face using a 3D food printer, and explains the assembly, configuration and modification of a RepRap Prusa I3 3D printer. An extrusion equipment capable of extruding materials in paste form, focused in the cake industry with sugar pastes, was developed. This extruder allowed the deposition of new materials that couldn’t previously be used since they have a solid consistency at low temperatures. The new extruder was tested with two different types of pastes: Nutella® chocolate and sugar pastes. The sugar paste prototypes already produced opened different opportunities to improve the surface quality, use of pastes with different colours, more than one extruder and others that in the limit contribute to the possibility of a small business in the cake design area.

This work studies plastic behaviour and its influence on the abrasive wear resistance of a group of high chromium white cast irons. The irons were poured in metallic moulds and heat treated. The studied alloys have 3% C, 12% Cr, as well as 0.6% and 2.4% Si. The plastic characteristics are evaluated through the parameters obtained from Meyer's test: the strain hardening capacity (n) and the constant of penetration resistance (k).

The influence of silicon content and applied heat treatments on Meyer's test parameters was determined. The heat treated samples showed values between n=2.3 and n=2.5. These values confirm a hardening capacity greater than the cast specimens. The high silicon alloy specimens show greater n-values than the low silicon alloy ones. This tendency is remarked when the complete treatments (austenitizing, cooling and holding) are applied. Correlation between n, k and the relative abrasive wear show good values for (k), but too much dispersion for the Meyer's Index (n).

The mechanical characterization of mortars is problematic due to their sensitivity to stress concentration defects, namely pores and flaws. In this paper, data on three-point bending, Young's modulus and fracture toughness of mortar beams are presented and discussed. Here, Weibull statistics is applied to analyse flexural strength of the developed mortar. Based on data obtained according to the EN196-1 standard and corrected values taking into account the actual position where failure origin took place, Weibull modulus was found to decrease from 28 to 22, respectively. In addition, fracture toughness was determined using Griffith approach based on critical crack sizes measured by fractography in fracture surfaces. A value of 0.37±0.04 MPa m^½ was obtained, which is typical of very fragile brittle materials.

In this study, in order to obtain a functionally graded material, NiTi strips were annealed at 350 °C, 450 °C and 550 °C in a furnace using an assembly that allowed a temperature gradient along them, and their transformation temperatures were studied by Differential Scanning Calorimetry (DSC). Furthermore, the strips were bent at both ends and dipped into a water bath at room temperature which was then heated to 61 °C in order to observe the influence of the gradient annealing on their strain recovery. It was found that the strips’ coolest regions presented the greatest strain recovery, particularly the strips annealed at 350 °C and 450 °C, although any strip exhibited a full strain recovery, due to plastic deformation during bending. These results, together with the DSC analysis at both regions (coolest and hottest), allow us to conclude that the graded annealing was successful for the intended functional gradient, as a gradient of transformation temperatures along the strips has been obtained, despite the primitive assembly, thus presenting an interesting result for a first approach. Further tests will be performed with a new experimental procedure especially designed for this purpose.

Metal matrix composites (MMCs) are a new class of materials that exhibit good wear resistance and high hardness. Since the wear resistance and hardness are surface properties, if the reinforcing particles are only added to the surface layer instead of bulk, the wear resistance and surface hardness can be improved without sacrificing the bulk properties.

In this study, was attempted to incorporate micro-sized SiC particles into an AA2024-T351 aluminium alloy by a friction stir processing (FSP) to form surface composite layer. The SiC particles (average particle size of 22.7 μm) were packed into a groove of 1.5 mm width and 1.5 mm depth cut on the aluminium plate. The influence to probe several strategies for reinforcement (number and direction of passes) on the particle distribution and homogeneity was studied.

Microstructural observations were carried out by employing both optical and scanning electron microscopy. In addition, the Electron Backscatter Diffraction (EBSD) technique was used to obtain crystallographic data as crystal orientation, grain size distribution and texture. The results have confirmed the refinement of grain produced in the nugget region of the processed alloy. On the other hand, although there is not an increase of hardness, surface composite layer presents better wear resistance than the aluminium base alloy as indicated by a lower specific wear rate (27%).

The design of a self-contained breathing apparatus (SCBA), which is an extremely important device for firefighting on board of ships, covers not only in-service loadings, but also a proper selection of materials, manufacturing processes, as well as the specification of other construction details, such as hydrostatic test parameters, among others.

The work developed and presented in this manuscript sought to verify the feasibility and the advantages of using composite materials in a SCBA subjected to high internal pressure (300 bar, 30 MPa), instead of other metallic material currently in use. Hence, a carbon fibre reinforced polymer (CFRP) and the filament winding technique were considered in the SCBA redesign. Analytical formulas, as well as engineering standards and advanced finite element analysis (FEM) were used to analyse two types of SCBA commonly used on board of ships.

Five valid axial tensile tests were performed in samples of CFRP using a DARTEC 100 kN servo hydraulic machine according with standard ASTM D3039/D 3039 M (2002) in order to determine the mechanical properties of the material, and a displacement transducer or strain gages were used to determine strains induced in the composite during experimental axial tensile tests, Then, several FE numerical simulations were carried out in order to verify the compliance of a redesigned SCBA with the functional requirements, and the main manufacturing parameters were also determined. Furthermore, experimental internal pressure tests were carried out in a pressure vessel similar to the redesigned SCBA. The results of FE simulations and experimental tests were compared and conclusions could be drawn.

Vacuum Insulation Panels (VlP) are presently regarded as one of the most promising state-of-the-art building insulation solutions. Based on their thermal conductivities of about 4 mW/(m K), with a thickness below 40 mm, they have a great potential for near zero-energy buildings (nZEB) and for applications where high insulation standards and living space savings are crucial. However, VIP are still unaffordable for the majority of homeowners and contractors (up to 100 €/m²), mostly due to the cost of the conventional fumed silica used as core material to secure the long service life requirements of building applications. This study presents the early developments of alternative cores engineered for VIP targeting the building market. The adopted strategy is to replace fumed silica with cheaper natural inorganic/organic lightweight materials or, alternatively, by creating multimaterial nanostructured composite matrices. The different compositions were analysed according to their physical, chemical and morphological characteristics and their respective thermal conductivity ranks. Promising lambda values as low as 5.3 mW/(m K) have been achieved for gas pressures below 10 mbar (1 kPa). It is expected that these novel core systems will be capable of suppressing the different heat transfer mechanisms at more reasonable costs than the current VIP fumed silica ones.

The literature presents several results referring to multiaxial tests of stress concentrators specimens. Different types of these specimens are presented and they were used accordingly to the stress triaxiality as follows: butterfly specimen for high degree of stress triaxiality, plane specimen for medium state of triaxiality and cylindrical specimen for low degree of triaxiality. This article shows the results of experimental determinations, finite element analysis and theoretical study of two types of plate specimens with stress concentrators and it aims to obtain high stress state in failure section (volume), more uniform stress distribution and stress ratio to be constant until failure.

Solidification thermal parameters, such as growth rate, cooling rate and dendrite arm spacing (λ), have been measured in a hypoeutectic Al-Mg alloy directionally solidified under upward and downward transient heat flow conditions. The experimental setup used in this work consists of a water-cooled mould with heat being extracted from the bottom or the top, promoting upward and downward directional solidification, respectively. It is shown that the dendritic arm spacing are not significantly affected by interdendritic convection for both solidification configurations and single growth laws are proposed for both cases. The Bouchard- Kirkaldy model is shown to overestimate the experimental primary dendritic arm spacing, despite fitting properly the secondary dendrite arm spacing.

With the constant need to improve and make the production of asphalt mixtures more sustainable, new production techniques have been developed, the implementation of which implies the correct knowledge of their performance. One of the most promising asphalt production techniques is the use of foamed bitumen. However, it is essential to understand how this binder will behave when subjected to the expansion process. The loss of volume of the foamed bitumen could be translated by a decay curve, which allows to determine the ideal temperature and water content added to the bitumen in order to assure adequate conditions to the mix the bitumen with the aggregates. On the present study, a conventional 160/220 pen grade bitumen was tested by using different temperatures and water contents, and it was concluded that the optimum temperature for the production of foamed bitumen (with the studied bitumen) is 150 °C, which corresponds to a viscosity of 0.1 Pa.s. The water content mostly influence the half-life of the bitumen foam, resulting in quicker volume reductions for higher water contents.