Laís Vasconcelos da Silva

Short beam strength of curaua, sisal, glass and hybrid composites

In this study, the short beam strength characteristics of randomly oriented composites were comprehensively investigated. The following parameters were varied: fiber used (curaua or sisal), fiber washing or surface chemical treatment with sodium hydroxide/sodium borohydride, fiber length (from 5 to 60 mm), hybridization with glass fiber and the pre-processing of the polyester resin. The overall fiber volume fraction was kept constant (30 vol.%). In all configurations, the composites containing curaua fiber obtained higher short beam strength than those with sisal. An increase in fiber length yielded higher short beam strength. The optical and electronic micrographs showed mostly horizontal cracks, typical of shear failure. In addition, it was carried out a study of the ASTM D2344 standard regarding the span-to-thickness ratio recommended for testing, the measured strength decreased for higher span-to-thickness ratio for specimens with higher length and width, and the failure mode changed to bending around span-to-thickness >12.

Glass fiber hybrid composites molded by RTM using a dispersion of carbon nanotubes/clay in epoxy

Nanocomposites based on epoxy and a mixture of clays and multi-walled carbon nanotubes (MWCNT) were produced by casting, and also molded by RTM using glass fibers as reinforcement, yielding a hybrid multi-scale micro/nanocomposite material. Two types of montmorillonite clays were used, natural (MMT-Na) and organophilic (MMT-30B). Higher viscosity was obtained for the mixture with MMT-30B and it was observed that this clay did not perform as well as the MMT-Na in helping the dispersion of the carbon nanotubes (CNT). The glass transition temperature (T g ) of the nanocomposites increased in up to 6 °C with the addition of MWCNT and up to 10 °C with the addition of MMT-30B, differently from the MMT-Na which did not alter the T g of the material. By transmission electron microscopy, it was verified that more homogeneous dispersions and more intercalated structures were obtained with the MMT-30B than with the MMT-Na. Finally, the low clay content used and, especially, the very low MWCNT content, did not significantly alter the studied flexural properties.

Composites of polyester + glass fiber residues vs. composites with mineral fillers

Polymer composites, such as those composed of a polyester, glass fibers (GFs), and mineral fillers (e.g. CaCO3), pose a threat to the environment because of the growing amount of residues and due to difficulties in their recycling. Therefore, we have studied effects of incorporation of (polyester + GFs) waste material as a filler into virgin composites. Two types of polyester + glass fiber composites were developed using hot compression molding, one of them with recycled (polyester + glass fiber) material obtained via knife or ball milling; the other, a control group, contained CaCO3, a traditional filler in this field. Dynamic friction and wear rate were determined using a pin-on-disk tribometer and a stylus profilometer, respectively. As expected, the presence of the residues significantly decreases dynamic friction and wear rate when compared to CaCO3, since the main constituent of the residues is a polymeric material. Thus, polyester + glass fiber composite residues are a candidate for a partial substitution of CaCO3. This should lower the environmental contamination caused by discarding the residues as well as provide composites with lower wear rates.

Tribology of composites produced with recycled GFRP waste

Nowadays, is expected that for most materials to be environmentally friendly. Besides, waste from end-of-life products may be considered a secondary source of materials with an energetic advantage due to its high energy content. This paper deals with the study of friction and wear characteristics of Glass fibre-reinforced polymer (GFRP) composites with polyester/glass fiber (P/GF) waste as filler, replacing the widely used calcium carbonate (CaCO3). Polyester composites based on two or three components, using a combination of polyester, CaCO3, GF, and GF waste, were produced. Pin-on-disc sliding wear test was performed using a polished stainless steel counterface. Roughness, surface energy, and hardness of the composites were characterized before the tests. The GF content (15, 25, 35, and 50 wt.%), the sliding velocity (0.021 and 0.042 m/s), and the normal load (1, 5, and 10 N) were varied. Based on the experimental results, it was observed that the friction coefficient and wear rate were influenced by material composition, surface roughness and energy, adhesive, and abrasive contact mechanisms. P/GF composites having P/GF waste presented enhanced performance considering friction and wear in relation to those with CaCO3 in their composition.

Strength analysis of composite cables

Carbon Fiber Reinforced Polymer (CFRP) cables, due to their outstanding performance in terms of specific stiffness and strength, are usually found in civil construction applications and, more recently, in the Oil & Gas sector. However, experimental data and theoretical solutions for these cables are very limited. On the contrary, several theoretical and numerical approaches are available for isotropic cables (metallic wire ropes), some of them with severe simplifications, nonetheless showing good agreement with experimental data. In this study, experimental tensile results for 1×7 CRFP cables were compared to a simplified analytical model (assumed transversally isotropic) and to a 3D finite element model incorporating the experimental uncertainty in important input parameters: longitudinal elastic modulus, Poisson’s ratio, static friction coefficient and ultimate tensile strain. The average experimental breaking load of the cable was 190.25 kN (coefficient of variation of 1.74%) and the agreement with the numerical model predictions were good, with an average-value deviation of –1.15%, which is lower than the experimental variations. The simplified analytical model yielded a discrepancy above 10%, indicating that it needs further refinement although much less time consuming than the numerical model. These conclusions were corroborated by statistical analyses (i.e. Kruskal–Wallis and Mann-Whitney).

Metallic and composite cables: a brief review

For cable-moored offshore tension-lag platforms in ultra-deep waters (2000 m), the usage of metallic cables is impractical, making carbon fibre reinforced polymer (CFRP) natural substitutes. CFRP cables have been applied in different situations, such as in cable-stayed bridges, in order to take advantage of its outstanding fatigue behaviour, higher specific stiffness and strength, and good corrosion resistance. However, there are not much experimental data available in the literature for these cables and theoretical solutions still need to be further developed. On the other hand, several theoretical approaches have already been developed for isotropic (metallic) cables, some of them with many simplifications nonetheless still showing good agreement compared to experimental data. On this context, this paper aims to report recent advances on composite cables, comparing previous research results on both composite and isotropic cables on the experimental, analytical and numerical fields.

Numerical and Experimental Analysis of the Tensile and Bending Behaviour of CFRP Cables

Due to their high fatigue life, specific strength and specific stiffness in comparison with steel, carbon-fibre reinforced polymer (CFRP) cables have attracted the infrastructure industry interest in recent years, primarily for use as structural tendons. Particularly the oil and gas industry showed interest for application in offshore platform anchorage systems, because of their exceptional corrosion and creep/relaxation behaviour. In such applications, the cables need to be tensioned in service and to be bent around relatively small-diameter spools for transportation and maintenance. Therefore, their tensile and bending behaviour is a subject of great concern. The aim of this work was to perform a test program on 1 × 19 CFRP cables in two different situations: tensile loading and four-point bending loading. Finite element models were developed to simulate both conditions, including frictional contact between the cable wires. A simplified analytical model was also used to predict the cable behaviour in tension. Numerical predictions were compared to experimental data showing relatively good accuracy, unlike the verified analytical model. CFRP cables presented outstanding tensile behaviour, but bending over small radius spools could not reach the performance of steel wire ropes. Furthermore, simulation could only fairly predict bending below strains of μ1,000 μe for the external rods, beyond which the cable presented highly non-linear behaviour that could not be simulated by the numerical model.

Numerical model updating applied to the simulation of carbon fiber–reinforced polymer cables under bending and tensile stress

Wire ropes are widely used in applications where the axial stress is high and flexural and torsional stresses are relatively low. Study of their mechanical behavior encompasses many factors, bringing considerable complexity to the construction of numerical or analytical models that suitably represent their behavior, including contact stresses between rods, helical geometry, rotation of wires when extended and also, in the case of carbon fiber–reinforced polymer (composite) cables, their anisotropic behavior. In view of the lack of suitable analytical solutions, this work focuses on the updating of a finite element model by incorporating factors commonly neglected by simplified analytical approaches. The carbon fiber–reinforced polymer cable was modeled under tensile stress and under four-point bending. After that a sensitivity analysis of the main parameters governing the problem was conducted. The updating process minimized the deviation between numerical and experimental data, and the model was able to reproduce the tensile and bending behavior with deviations smaller than 1%. The adopted methodology can be extended to similar cases.