Corrosion behaviour of aluminium syntactic functionally graded composites

Syntactic Functionally Graded Metal Matrix Composites (SFGMMC) are a type of composites reinforced by microballoons exhibiting a graded reinforcement distribution. These materials constitute a promising new generation of lightweight structural materials for aerospace, marine and shielding/insulation applications. In this work, A356 alloy reinforced with silica-alumina microballoons (SiO2-Al2O3) was processed by casting techniques. The influence of the microballoon distribution gradient on the corrosion behaviour of the composite was investigated by potentiodynamic polarisation and Electrochemical Impedance Spectroscopy (EIS). Composite surfaces were analysed before and after testing by Optical Microscopy (OM) and Scanning Electron Microscopy (SEM) to determine the influence of microstructural changes.

Tribocorrosion behaviour of hot pressed CoCrMo−Al2O3 composites for biomedical applications

Alumina/alumina wear couple can lower the wear rates and thus metallic ion releasing on load bearing metallic implant materials. However, the low fracture toughness of ceramics is still a major concern. Therefore, the present study aims to process and to triboelectrochemically characterise the 5 and 10 vol.-%Al2O3 reinforced CoCrMo matrix composites. Corrosion and tribocorrosion behaviour of the composites were investigated in 8 g L−1 NaCl solution at body temperature. Corroded and worn surfaces were investigated by a field emission gun scanning electron microscope equipped with energy dispersive X-ray spectroscopy. After tribocorrosion experiments, wear rates were calculated using a profilometer. Results suggest that Al2O3 particle addition decreased the tendency of CoCrMo alloy to corrosion under both static and tribocorrosion conditions. However, no significant influence on the corrosion and wear rates was observed in composites mainly due to increased porosity and insufficient matrix/reinforcement bonding.

Effective Properties Evaluation for Smart Composite Materials

The purpose of this article is to present a method which consists in the development of unit cell numerical models for smart composite materials with piezoelectric fibers made of PZT embedded in a non-piezoelectric matrix (epoxy resin). This method evaluates a globally homogeneous medium equivalent to the original composite, using a representative volume element (RVE). The suitable boundary conditions allow the simulation of all modes of the overall deformation arising from any arbitrary combination of mechanical and electrical loading. In the first instance, the unit cell is applied to predict the effective material coefficients of the transversely isotropic piezoelectric composite with circular cross section fibers. The numerical results are compared to other methods reported in the literature and also to results previously published, in order to evaluate the method proposal. In the second step, the method is applied to calculate the equivalent properties for smart composite materials with square cross section fibers. Results of comparison between different combinations of circular and square fiber geometries, observing the influence of the boundary conditions and arrangements are presented.

LYOCELL AND COTTON FIBERS AS REINFORCEMENTS FOR A THERMOSET POLYMER

Cellulose fibers obtained from the textile industry (lyocell) were investigated as a potential reinforcement for thermoset phenolic matrices, to improve their mechanical properties. Textile cotton fibers were also considered. The fibers were characterized in terms of their chemical composition and analyzed using TGA, SEM, and X-ray. The thermoset (non-reinforced) and composites (phenolic matrices reinforced with randomly dispersed fibers) were characterized using TG, DSC, SEM, DMTA, the Izod impact strength test, and water absorption capacity analysis. The composites that were reinforced with lyocell fibers exhibited impact strengths of nearly 240 Jm(-1), whereas those reinforced with cotton fibers exhibited impact strengths of up to 773 Jm(-1). In addition to the aspect ratio, the higher crystallinity of cotton fibers compared to lyocell likely plays a role in the impact strength of the composite reinforced by the fibers. The SEM images showed that the porosity of the textile fibers allowed good bulk diffusion of the phenolic resin, which, in turn, led to both good adhesion of fiber to matrix and fewer microvoids at the interface.

Evaluation of the flexural strength and elastic modulus of resins used for temporary restorations reinforced with particulate glass fibre

Objective: The flexural strength and the elastic modulus of acrylic resins, Dencor, Duralay and Trim Plus II, were evaluated with and without the addition of silanised glass fibre. Materials and methods: To evaluate the flexural strength and elastic modulus, 60 test specimens were fabricated with the addition of 10% ground silanised glass fibres for the experimental group, and 60 without the incorporation of fibres, for the control group, with 20 test specimens being made of each commercial brand of resin (Dencor, Duralay and Trim Plus II) for the control group and experimental group. After the test specimens had been completed, the flexural strength and elastic modulus tests were performed in a universal testing device, using the three-point bending test. For the specimens without fibres the One-Way Analysis of Variance and the complementary Tukey test were used, and for those with fibres it was not normal, so that the non-parametric Mann-Whitney test was applied. Results: For the flexural strength test, there was no statistical difference (p > 0.05) between each commercial brand of resin without fibres [Duralay 84.32(+/- 8.54), Trim plus 85.39(+/- 6.74), Dencor 96.70(+/- 6.52)] and with fibres (Duralay 87.18, Trim plus 88.33, Dencor 98.10). However, for the elastic modulus, there was statistical difference (p > 0.01) between each commercial brand of resin without fibres [Duralay 2380.64 (+/- 168.60), Trim plus 2740.37(+/- 311.74), Dencor 2595.42(+/- 261.22)] and with fibres (Duralay 3750.42, Trim plus 3188.80, Dencor 3400.75). Conclusion: The result showed that the incorporation of fibre did not interfere in the flexural strength values, but it increased the values for the elastic modulus.

Compressive strength of dental composites photo-activated with different light tips

The aim of this study was to evaluate the compressive strength of microhybrid (FiltekTM Z250) and nanofilled (FiltekTM Supreme XT) composite resins photo-activated with two different light guide tips, fiber optic and polymer, coupled with one LED. The power density was 653 mW cm-2 when using the fiber optic light tip and 596 mW cm-2 with the polymer. After storage in distilled water at 37± 2 °C for seven days, the samples were subjected to mechanical testing of compressive strength in an EMIC universal mechanical testing machine with a load cell of 5 kN and speed of 0.5 mm min-1. The statistical analysis was performed using ANOVA with a confidence interval of 95% and Tamhane’s test. The results showed that the mean values of compressive strength were not influenced by the different light tips (p > 0.05). However, a statistical difference was observed (p < 0.001) between the microhybrid composite resin photo-activated with the fiber optic light tip and the nanofilled composite resin. Based on these results, it can be concluded that microhybrid composite resin photo-activated with the fiber optic light tip showed better results than nanofilled, regardless of the tip used, and the type of the light tip did not influence the compressive strength of either composite. Thus, the presented results suggest that both the fiber optic and polymer light guide tips provide adequate compressive strength to be used to make restorations. However, the fiber optic light tip associated with microhybrid composite resin may be an interesting option for restorations mainly in posterior teeth.

Evaluation of degree of conversion and hardness of dental composites photo-activated with different light guide tips

Objective: The aim of this study was to evaluate the degree of conversion and hardness of different composite resins, photo-activated for 40 s with two different light guide tips, fiber optic and polymer. Methods: Five specimens were made for each group evaluated. The percentage of unreacted carbon double bonds (% C=C) was determined from the ratio of absorbance intensities of aliphatic C=C (peak at 1637 cm-1) against internal standard before and after curing of the specimen: aromatic C-C (peak at 1610 cm-1). The Vickers hardness measurements were performed in a universal testing machine. A 50 gf load was used and the indenter with a dwell time of 30 seconds. The degree of conversion and hardness mean values were analyzed separately by ANOVA and Tukey's test, with a significance level set at 5%. Results: The mean values of degree of conversion for the polymer and fiber optic light guide tip were statistically different (P<.001). The hardness mean values were statistically different among the light guide tips (P<.001), but also there was difference between top and bottom surfaces (P<.001). Conclusions: The results showed that the resins photo-activated with the fiber optic light guide tip promoted higher values for degree of conversion and hardness.

Preparation and characterisation of refractory whiskers and selected alumina composites

A whisker is a common name of single crystalline inorganic fibre of small dimensions, typically 0.5-1 μm in diameter and 20-50 μm in length. Whiskers are mainly used as reinforcement of ceramics. This work describes the synthesis and characterisation of new whisker types. Ti0.33Ta0.33Nb0.33CxN1-x, TiB2, B4C, and LaxCe1-xB6 have been prepared by carbothermal vapour–liquid–solid (CTR-VLS) growth mechanisms in the temperature range 900-1800°C, in argon or nitrogen. Generally, carbon and different suitable oxides were used as whisker precursors. The oxides reacted via a carbothermal reduction process. A halogenide salt was added to form gaseous metal halogenides or oxohalogenides and small amount of a transition metal was added to catalyse the whisker growth. In this mechanism, the whisker constituents are dissolved into the catalyst, in liquid phase, which becomes supersaturated. Then a whisker could nucleate and grow out under continuous feed of constituents. The syntheses of TiC, TiB2, and B4C were followed at ordinary synthesis conditions by means of mass spectrometry (MS), thermogravimetry (TG), differential thermal analysis (DTA) and quenching. The main reaction starting temperatures and reaction time for the different mixtures was revealed, and it was found that the temperature inside the crucible during the reactions was up to 100°C below the furnace set-point, due to endothermic nature of the reactions. Quench experiments showed that whiskers were formed already when reaching the temperature plateau, but the yield increased fast with the holding time and reached a maximum after about 20-30 minutes. Growth models for whisker formation have been proposed. Alumina based composites reinforced by (2-5 vol.%) TiCnano and TiNnano and 25 vol.% of carbide, and boride phases (whiskers and particulates of TiC, TiN, TaC, NbC, (Ti,Ta)C, (Ti,Ta,Nb)C, SiC, TiB2 and B4C) have been prepared by a developed aqueous colloidal processing route followed by hot pressing for 90 min at 1700°C, 28 MPa or SPS sintering for 5 minutes at 1200-1600°C and 75 MPa. Vickers indentation measurements showed that the lowest possible sintering temperature is to prefer from mechanical properties point of view. In the TiNnano composites the fracture mode was typically intergranular, while it was transgranular in the SiCnano composites. The whisker and particulate composites have been compared in terms of e.g. microstructure and mechanical properties. Generally, additions of whiskers yielded higher fracture toughness compared to particulates. Composites of commercially available SiC whiskers showed best mechanical properties with a low spread but all the other whisker phases, especially TiB2, exhibited a great potential as reinforcement materials.

Steel and macro-synthetic self-compacting fibre reinforced concrete, experimental study on the long-term deformations

Experimental study on the long-term deformations of the fibre reinforced concrete. Steel and macro-synthetic fibers were used to evaluate the shrinkage, creep, mid-span deflection, cracking and rupture analysis of three different types of samples. At the end the main topics of ACI guidelines were analyzed in order to perform an overview of design.

High temperature, low cycle fatigue of a hybrid particulate/fiber aluminum metal-matrix composite

The effect of shot particles on the high temperature, low cycle fatigue of a hybrid fiber/particulate metal-matrix composite (MMC) was studied. Two hybrid composites with the general composition A356/35%SiC particle/5%Fiber (one without shot) were tested. It was found that shot particles acting as stress concentrators had little effect on the fatigue performance. It appears that fibers with a high silica content were more likely to debond from the matrix. Final failure of the composite was found to occur preferentially in the matrix. SiC particles fracture progressively during fatigue testing, leading to higher stress in the matrix, and final failure by matrix overload. A continuum mechanics based model was developed to predict failure in fatigue based on the tensile properties of the matrix and particles. By accounting for matrix yielding and recovery, composite creep and particle strength distribution, failure of the composite was predicted.

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.

A methodology to measure the interface shear strength by means of the fiber push-in test

A methodology is presented to measure the fiber/matrix interface shear strength in composites. The strategy is based on performing a fiber push-in test at the central fiber of highly-packed fiber clusters with hexagonal symmetry which are often found in unidirectional composites with a high volume fraction of fibers. The mechanics of this test was analyzed in detail by means of three-dimensional finite element simulations. In particular, the influence of different parameters (interface shear strength, toughness and friction as well as fiber longitudinal elastic modulus and curing stresses) on the critical load at the onset of debonding was established. From the results of the numerical simulations, a simple relationship between the critical load and the interface shear strength is proposed. The methodology was validated in an unidirectional C/epoxy composite and the advantages and limitations of the proposed methodology are indicated.

Rheological and tribological properties of poly(phenylene sulphide) and poly(ether ether ketone) composites incorporating carbon nanotubes and inorganic nanoparticles: a comparative study

The rheological and tribological properties of single-walled carbon nanotube (SWCNT)-reinforced poly(phenylene sulphide) (PPS) and poly(ether ether ketone) (PEEK) nanocomposites prepared via melt-extrusion were investigated. The effectiveness of employing a dual-nanofiller strategy combining polyetherimide (PEI)-wrapped SWCNTs with inorganic fullerene-like tungsten disulfide (IF-WS2) nanoparticles for property enhancement of the resulting hybrid composites was evaluated. Viscoelastic measurements revealed that the complex viscosity ?, storage modulus G?, and loss modulus G? increased with SWCNT content. In the low-frequency region, G? and G? became almost independent of frequency at higher SWCNT loadings, suggesting a transition from liquid-like to solid-like behavior. The incorporation of increasing IF-WS2 contents led to a progressive drop in ? and G? due to a lubricant effect. PEEK nanocomposites showed lower percolation threshold than those based on PPS, ascribed to an improved SWCNT dispersion due to the higher affinity between PEI and PEEK. The SWCNTs significantly lowered the wear rate but only slightly reduced the coefficient of friction. Composites with both nanofillers exhibited improved wear behavior, attributed to the outstanding tribological properties of these nanoparticles and a synergistic reinforcement effect. The combination of SWCNTs with IF-WS2 is a promising route for improving the tribological and rheological performance of thermoplastic nanocomposites.

Análisis de elementos de madera reforzados con materiales compuestos

Gran parte del patrimonio construido cuenta con edificios cuya estructura está compuesta por elementos de madera. El volumen económico que supone el mantenimiento y renovación de dicho patrimonio es considerable, por ello, es de especial interés el estudio de las diferentes técnicas de refuerzo aplicables a este tipo de estructuras. Las estructuras de madera han sido tradicionalmente reforzadas con piezas del mismo material, aumentando la sección de los elementos dañados, o con acero. La aparición de los materiales compuestos de polímeros reforzados con fibras, y su progresiva aplicación en obras de construcción, hizo que a principios de la década de los noventa se comenzara a aplicar este material en refuerzos de estructuras de madera (Puente de Sins, 1992). La madera es un material natural con una excelente relación entre sus características mecánicas y su peso. Con el uso de materiales compuestos como refuerzo ésta característica se mantiene. En cuanto a su modelo constitutivo, se admite un comportamiento elástico lineal a tracción paralela a la fibra hasta la rotura, mientras que a compresión, se considera un comportamiento lineal elástico inicial, seguido de un tramo plástico. En vigas de madera aserrada sometidas a flexión predomina el modo de fallo por tracción localizándose la fractura frecuentemente en el canto inferior. Los FRP tienen un comportamiento elástico lineal a tracción hasta la rotura y cuentan con excelentes propiedades mecánicas en relación a su peso y volumen. Si se refuerza la viga por el canto inferior se aumentará su capacidad de absorber tracciones y por tanto, es previsible que se produzca un incremento en la capacidad de carga, así como un aumento de ductilidad. En este trabajo se analizan los beneficios que aportan distintos sistemas de refuerzos de materiales compuestos. El objetivo es contribuir al conocimiento de esta técnica para la recuperación o aumento de las propiedades resistentes de elementos de madera sometidos a flexión. Se ha llevado a cabo un estudio basado en datos obtenidos experimentalmente mediante el ensayo a flexión de vigas de madera de pino silvestre reforzadas con materiales compuestos. Las fibras que componen los tejidos utilizados para la ejecución de los refuerzos son de basalto y de carbono. En el caso de los compuestos de fibra de basalto se aplican en distintos gramajes, y los de carbono en tejido unidireccional y bidireccional. Se analiza el comportamiento de las vigas según las variables de refuerzo aplicadas y se comparan con los resultados de vigas ensayadas sin reforzar. Además se comprueba el ajuste del modelo de cálculo no lineal aplicado para predecir la carga de rotura de cada viga reforzada. Con este trabajo queda demostrado el buen funcionamiento del FRP de fibra de basalto aplicado en el refuerzo de vigas de madera y de los tejidos de carbono bidireccionales con respecto a los unidireccionales. ABSTRACT Many of the buildings of the built heritage include a structure composed by timber elements. The economic volume involved in the maintenance and renewal of this built heritage is considerable, therefore, the study of the different reinforcement techniques applicable to this type of structure is of special interest. The wooden structures have traditionally been reinforced either with steel or with pieces of the same material, increasing the section of the damaged parts. The emergence of polymer composites reinforced with fibers, and their progressive use in construction, started to be applied as reinforcement in timber structures at the beginning of the nineties decade in the 20th century (Sins Bridge, 1992). Wood is a natural material with an excellent ratio between its mechanic characteristics and its weight. This feature is maintained with the use of composites as reinforcement. In terms of its constitutive model, linear elastic behavior parallel to the fiber up to fracture is admitted when subjected to tensile stress, while under compression, an initial linear elastic behavior, followed by a section plasticizing, is considered. In sawn timber beams subjected to bending, the predominant failure is mainly due to tensile stress; and frequently the fracture is located at the beam lower face. The FRP have a linear elastic behavior until fracture occurs, and have excellent mechanical properties in relation to their weight and volume. If the beam is reinforced by its lower face, its capacity to absorb tensile stresses will increase, and therefore, an increase in its carrying capacity is likely to be produced, as well as an increase in ductility. This work analyzes the benefits different reinforcement systems of composite materials provide, with the aim of contributing to the knowledge of this technique for recovering or increasing the strength properties of timber elements subjected to bending loads. It is a study based on data obtained experimentally using bending tests of pine timber beams reinforced with composite materials. Fibers used for the execution of the reinforcement are basalt and carbon. Basalt fiber composites are applied in different grammages, whereas with carbon composites, unidirectional and bidirectional fabrics are used. The behavior of the beams was analyzed regarding the reinforcement variables applied, and the results are compared with those of the tested beams without reinforcement. Furthermore it has been proved adjunting the nonlinear calculation model applied to predict the failure load of each reinforced beam. This work proves the good behavior of fiber reinforce plastic (FRP) with basalt fiber when applied to timber beams, and that of bidirectional carbon fabrics as opposed to the unidirectional ones.