Ciência & Tecnologia dos Materiais最新文献

The hydraulic bulge test can be used as a means to achieve large uniform plastic strains under biaxial stress conditions, applied to hardening curve determination of sheet metal materials. The larger information on hardening behaviour is the reason why a mechanical characterization system has been developed to determine stress-strain curve by reading bulge test variables: bulge pressure (p), radius of curvature (ρ) and pole thickness (t). The determination of stress-strain curve may be based on continuous data acquisition from bulge test results (p, ρ, t) or from the use of analytical equations relating these variables with dome height. In this paper it is presented a study comparing different methodologies to obtain stress-strain curve by means of analytical methodologies relating “dome height with pole thickness” evolution. It is shown that these existent methodologies to determine pole thickness don’t apply when compared with experimental values and they show an evident dispersion among them. Therefore a better analytical methodology is proposed and tested. The study and corresponding material characterization is applied to two aluminium alloys, AA5754-T4 and AA6061-T6, currently used in automotive industry.

The potential of a hydrophobic aerogel-like material for phenol adsorption from aqueous solutions was studied. Firstly, the effects of phenol concentration and adsorbent dosage on the adsorption equilibrium were investigated. The adsorption capacity of phenol at equilibrium increased with the increase of initial phenol concentration and decreased by increasing the adsorbent dosage. The optimum adsorbent dosage was 0.45 g/45 mL, corresponding to a removal percentage of 54% and a liquid-solid ratio of 100. Langmuir and Freundlich isotherm models fitted fairly the experimental equilibrium data, but the Freundlich model was adopted as the more suitable for describing the studied system. Secondly, the performance of the adsorbent was investigated by using two different system layouts: perfectly stirred tank in batch conditions, and fixed bed column tested in continuous regime. Pseudo-first and pseudo-second-order kinetic models were fitted to have an assessment on the kinetic parameters and adsorption mechanism. Finally, feasibility of chemical regeneration of the adsorbent, using an acid solution, and its effect on the phenol adsorption capacity were also examined. The changes in the chemical structure of the adsorbent after 4 regeneration cycles were assessed by Fourier transform infrared spectroscopy (FTIR). High regeneration efficiency of the adsorbent was observed in all performed cycles.

Nowadays there is a growing interest in studying the influence of primary austenite structure and the formation of graphite particles during solidification to correlate with the formation of microshrinkage or even shrinkage defects on castings.

In order to obtain information on the development of the dendritic structure, advanced thermal analysis techniques were applied in the study of hypoeutectic and close eutectic melt compositions to determine the occurrence of the dendritic coherency point (DCP). The occurrence of the DCP has a major importance on metal yield through the design of the in-gate and feeding system of a casting as it determines the available time for feeding and take advantage of the internal expansion inside the mould cavity due to graphite precipitation.

To correlate the thermal analysis results and the solidification sequence of the metallographic constituents to identify the occurrence of the DCP, quenching of the thermal analysis sample during solidification was performed to freeze the solidification process at a given characteristic stage of the solidification curve. This has allowed the observation of the stable solidification structure that has developed until the time of quenching.

The work performed allowed a better understanding of the solidification structure of hypoeutectic nodular cast iron melts and the occurrence of the dendritic coherency point.

The present paper will make a brief review on some of the most relevant through-thickness reinforcement technologies for CFRP/Metal and CFRP/CFRP joints developed so far. A distinction between the traditional (“z-pinning” and “stitching and tufting”) and novel (“COMELD^TM”, “CMT”, “HYPER” and “RHEA”) 3D-reinforcement techniques will be made and the most relevant characteristics of each one will be pointed out.

Micro powder injection moulding-μPIM is a powder injection moulding-PIM variant for microparts/devices. This study is about the optimization and respective production of nanocomposites feedstocks suitable to μPIM process. The optimization of MWCNT content in metal matrix composites-MMC feedstock were performed using torque rheometry until achieve homogeneous feedstocks with suitable flowability to μPIM process (60:40 vol.%). During debinding step, it is likely loss of MWCNT, forcing a new stage where physical connection between powders and MWCNTs should be established. Mechanical milling seems the suitable technique to be adopted to overcome this major problem. In preliminary route, the binder M1 (with or without SA addition) is mixed with copper or 316L steel powder. Therefore, two different routes were selected: route 1 – the addition of different MWCNT contents is done during the preparation of feedstock; route 2 – a mechanical milling of metallic powders with MWCNT precedes the conventional production of feedstocks, but with SA addition to improve the nanoreinforced content. The first route impairing its processability by μPIM. The other route revealed to induce homogeneity mixing and torque values suitable to be used as feedstock for μPIM. The route 2 allowed to manufacture with high quality microparts up to 2% of MWCNT addition.

The influence of the tension-compression asymmetry of the plastic flow, due to intrinsic single-crystal deformation mechanisms, on porosity evolution and the overall ductility of voided metallic polycrystals is assessed. To this end, detailed micromechanical finite-element analyses of three-dimensional unit cells containing a single initially spherical cavity are carried out. The plastic flow of the matrix (fully-dense material) is described by a criterion that accounts for strength-differential effects induced by deformation twinning of the constituent grains of the metallic polycrystalline materials. The dilatational response of porous polycrystals are calculated for macroscopic axisymmetric tensile loadings corresponding to a fixed value of the stress triaxiality and the two possible values of the Lode parameter. It is shown that damage accumulation, and ultimately ductility of the porous polycrystals are markedly different as compared to the case when the matrix is governed by von Mises criterion. Most importantly, a direct correlation is established between the macroscopic material parameter k that is intimately related to the particularities of the plastic flow of the matrix and the rate of damage accumulation.

In the present study thin films of MgO – modified Ba0.6Sr0.4TiO3 (BST60/40) solid solution were prepared by the sol-gel-type chemical solution deposition method on stainless steel substrates. A multilayer spin-coating approach was utilized for the Ba0.6Sr0.4TiO3-MgO thin film deposition with subsequent thermal annealing at T=650-750°C. Dried BST60/40-MgO gel powders were studied with thermogravimetric and differential thermal analysis to determine their thermochemical properties. X-ray diffraction analysis was utilized for thin film characterization in terms of its phase composition and crystal structure. The influence of y=1, 3 and 5 mol.% MgO doping on nanomechanical properties of BST60/40 thin films was studied with nanoindentation. It was found that BST6040 thin films adopted the tetragonal P4mm (99) structure. The volume of the BST60/40 elementary cell, the average hardness as well as the mean value of the Young modulus decreased with an increase in amount of MgO.

The installation procedures (which induce mechanical damage) and abrasion can cause unwanted changes on the properties of the geotextiles. In this work, a nonwoven polypropylene geotextile was subjected to three degradation tests: (1) mechanical damage, (2) abrasion and (3) mechanical damage followed by abrasion (successive exposure). The damage caused by the degradation tests was evaluated by tensile, tearing and static puncture tests. Based on the changes occurred in the mechanical properties, reduction factors were determined. Results showed that the degradation tests provoked relevant reductions in the mechanical strength of the geotextile (higher reductions in the successive exposure to both degradation tests). The reduction factors for the combined effect of mechanical damage and abrasion obtained in the successive exposure to both degradation tests were different (slightly higher) than those obtained by the traditional methodology (determination of reduction factors separately for each degradation test and further multiplication).

Supermartensitic stainless steels with 12-13% Cr show higher corrosion resistance than conventional grades such as UNS S42000 and S41000. The reduction of carbon content to less than 0.03%, and the addition of Ni and Mo are the most important compositional changes which enhances mechanical properties and corrosion resistance. Ti addition is used to combine with C and N, and avoid Cr carbides precipitation and to improve mechanical properties. In general, SMSS steels are quenched and tempered or double tempered. The purpose of this work was to investigate how the microstructure and the corrosion decay by sensitization are influenced by quenching and tempering heat treatments in a novel supermartensitic 13% Cr stainless steel with Ti addition. DL-EPR (double loop potentiokinetic electrochemical reactivation) test was used to obtain the degree of sensitization (DOS). The results show that, despite the extra low carbon content, and the stabilization with Ti, the material can become sensitized with heat treatments. The sensitization is rather related to Ti(C, N) precipitation and reverse austenite than to Cr carbides.

Adhesive bonding is a process of permanent union between the components of a structure, which is used to manufacture complex shape structures, which could not be manufactured in one piece, aiming to provide a structural joint that theoretically should be at least as resistant as the base material. Composite materials reinforced with fibres are becoming increasingly popular in many applications, as a result of a number of competitive advantages over conventional materials. Regarding the manufacture of composite structures, although the currently used techniques reduce to the maximum the connections, these are still necessary due to the typical size of the components and design, technological and logistical limitations. Moreover, it is known that in many high performance structures, it is necessary to join components in composite materials with other light metals such as aluminium, for the purpose of structural optimization. This work aims to experimentally and numerically study single-L adhesive joints between aluminium components and carbon-fibre reinforced composite structures under peeling loads, considering different geometric conditions and adhesives. It was found that the adhesive ductility and aluminium plate thickness are highly relevant parameters to improve the joints strength.