Resumo em Inglês:

Abstract This work describes proposed experimental methods for the characterization of concrete in fresh and hardened state, with the purpose of contributing to the experimental characterization of concrete. The main objective was to identify the rheological behavior of high performance (HP) and self-compacting (SC) concretes. Initially, a ordinary concrete composition was proposed, from which, silica fume (5, 10 and 15% in mass) and superplasticizer (0.6% in mass) were added to obtain HPC and SCC. Furthermore, for the self-compacting concretes, a modification in granular skeleton was proposed, with a reduction of total aggregate percentage by 22.73% in relation to the other compositions. The interference of these modifications was evaluated on the behavior of fresh concrete though the flow and slump test and hardened concrete through mechanical properties. The results showed that the modification in the granular skeleton of ordinary concrete contributed directly to the reduction of inertia factors of SCC, favoring the mobility conditions. For both self-compacting and high performance concretes, consistency and mechanical strength gain over ordinary concrete were influenced when the silica fume and superplasticizer concentration was increased, limiting your properties gains to these additions.

Resumo em Inglês:

Abstract Ionomeric Polymer-Metal Composites (IPMC) are smart materials whose electromechanical behavior depends on the electrical stimulus intensity, membrane hydration level, and ionic migration. This paper investigates the effects of the voltage, relative humidity, and counterion type (Li+ and K+) on a Nafion-based IPMC performance. Instrumentation capable of applying an electrical stimulus and measuring the electromechanical response was developed. The ionic conductivity was obtained using Electrochemical Impedance Spectroscopy. Complementary SEM analyses were performed before and after actuation cycles. The IPMC performance improved when the electrical stimulus was 5.00 V, RH = 90%, and Li+ was used. The IPMC-Li sample is an excellent candidate to be used as an actuator since it exhibited fast actuation movement, considerable displacement, and no evident back-relaxation. However, its mechanical performance decreased over time because of a progressive increase in platinum electrode crack density and dehydration. The video analysis technique is an efficient, effective, and low-cost technique.

Resumo em Inglês:

Abstract The society’s development provides consequences such as the increasing generation of industrial waste. In the city of Franca, São Paulo’s inland city, there are footwear industries that generate a large amount of leather waste during their production process. In most cases, there’s no use of this waste and they’re disposed of in landfills. A reuse alternative is the production of particleboards used in construction sectors. Therefore, one of the biggest challenges is the use of this leather waste adding value to the product and enabling the production of sustainable material. This work proposes the production of particleboards from Eucalyptus wood and leather fibers using castor oil polyurethane adhesive. In addition, the assessment of the leather fiber influence on modulus of rupture and elasticity obtained from a static bending test. The study investigated the proportion of 10%, 25% e 50% leather waste in relation to Eucalyptus particles and 10% castor oil polyurethane adhesive. The mechanical performance of the panels was evaluated based on ABNT NBR 14810-1 and 2 (2013; 2018). The results allowed us to ascertain that the incorporation of 10% leather waste in particleboard rated them, according to NBR 14810-1 and 2 (2013; 2018), as structural panels of type P4 for use in dry conditions.

Resumo em Inglês:

Abstract Adhesively bonded repair patches are an excellent approach for repairing locally damaged composite components. If correctly applied, fiber-reinforced patches may restore/increment the mechanical response of damaged laminates without significantly increasing the structure’s mass or altering its geometry. However, in order to take full advantage of this repairing technique, one must employ patches with a minimal surface area and maximum efficiency in incrementing the strength of the component. The present work aims to study optimum-based patch shapes for conventional repair geometries, namely rectangular and elliptical. Shell Finite Elements models were used to simulate a parent plate, which is a rectangular flat laminate with a central trespassing damage region. Unbalanced single-ply patches were modeled on the upper surface of the damaged laminate. The patches' efficiency was computed as its capability in restoring the modal response of the repaired component to its undamaged configuration. Sequential linear programming was employed alongside shell finite element models to obtain optimal geometrical parameters for the patches' shape. The study cases comported two different boundary conditions and two stacking sequences. The optimum-base repair patches were defined regarding size and fiber orientation angle.

Resumo em Inglês:

The objective of the present paper is to propose a framework, utilizing the OpenCV library, for post-processing infrared images obtained using Long-Pulse Thermography (LPT), with the goal of segmenting the images into defective and sound areas. A series of thermograms of a carbon fiber/epoxy specimen, containing precision milled flat-bottom holes, was acquired using an LPT system comprised of an uncooled microbolometer imager and halogen optical sources. Flaw detectability and planar size estimation were used to evaluate the results obtained with the proposed post-processing framework, in comparison to raw images, and images subjected only to pre-processing algorithms.

Resumo em Inglês:

Abstract Natural fibres have many advantages over synthetic ones, making them attractive for reinforcing polymer materials. This work evaluates the use of an Amazonian plant, namely Curauá (Ananas erectifolius), as a reinforcement phase of biocomposites fabricated by cold pressing. Curauá fibres have been shown to be a promising fibre for composite materials, especially due to their higher elastic modulus than other plant species. An L9 Taguchi design is used to investigate the effect of fibre fraction, NaOH concentration and immersion time on the tensile properties of biocomposites. Statistical models are able of predicting and revealing the optimal composition of the biocomposites. The tensile strength of Curauá biocomposites is significantly affected by the fibre fraction, followed by the factors of immersion time and NaOH concentration. High tensile strength is obtained by adding 25 wt.% of Curauá fibres treated under different conditions. There is an interaction between NaOH concentration and the immersion time; a higher concentration requires less time or vice versa to achieve ideal roughness, promoting strong fibre/matrix adhesion.

Resumo em Inglês:

Abstract This work presents an efficient approach to quantify uncertainties in composite laminates using the interval analysis, anti-optimization technique, and the α-cut procedure. The solutions are compared with the traditional and robust Monte Carlo method in 3 cases scenarios: natural frequencies, buckling, and strength safe factor. For natural frequencies and buckling loads, the presented Interval based methodology showed 2.5% to 4.5% larger error values when compared to the Monte Carlo method using the same number of function calls. This implies a larger uncertain area, and hence, a better solution. For the strength test using Tsai-Wu failure theory, the error values are even greater: 22% to 46%. A violation of the failure limit was detected by the proposed Interval based approach, but not detected by Monte Carlo method. The solutions show that the presented methodology yields a safer and more precise analysis when compared to the traditional Monte Carlo approach.

Resumo em Inglês:

Non-destructive tests are of pivotal importance in the industry, as they contribute to the decreasing of maintenance costs, reducing downtime and accidents. Ultrasound is a non-destructive method that can be used with several arrangements to identify faults in engineering materials. This work evaluated the use of three ultrasonic methods applied in FRP composites to detect manufacturing defects. The critically refracted longitudinal wave (LCR), the B-Scan images, and the signal-to-noise ratio of TFM (Total Focusing Method) images were compared to detect the delamination and the fiber waviness in unidirectional composite of carbon/epoxy. Rectangular samples without defects and with defects were used in the tests. To obtain the delamination in the samples, pieces of Teflon were placed between layers and for the samples with waviness, a silica sphere was used. The results showed that LCR wave is not recommended to detect these kinds of defects, the B-Scan image is efficient in detecting delamination, and the SNR variation of TFM images allows to identify the waviness and delamination in unidirectional FRP composites.

Resumo em Inglês:

Abstract Civil structures are, normally, subjected to gravitational and thermal loads. The association of the effects of these loads should be the object of analysis when the loss of stability of the slender columns was verifying. In the case of reinforced concrete structures, temperature variations induce internal stresses in addition to gravitational ones since they are caused by the difference in the thermal properties of steel and concrete. Since the structure is a slender system, its stiffness was divided into two parts. In the first one, the properties of the concrete were introduced, including cracking and creep, doing the equations obtained at the time-dependent mathematical process. In the second, the geometric, the construction imperfections and the internal efforts mobilized by the temperature variation were considered. Additionally, strains and thermal loads were considered together. At the end, the values of critical buckling loads for different moments of interest were determined.

Resumo em Inglês:

Abstract This study aimed to evaluate the physical, chemical, and mechanical properties of different Amazonian vegetable fibers to produce cementitious mortars reinforced with different types of vegetable fibers from the Amazon. Also, to analyze the influence of different types of chemical and physical treatments, on the direct tensile strength of vegetable fibers, water absorption of vegetable fibers and on the mechanical properties of compressive strength, flexural strength, and water absorption of mortars. Four vegetable fibers from the Amazon rainforest, from the Upper Rio Negro region (piassava, jute, tucum and razor grass) were used to produce cement composites with 50% Portland CPII-F cement and additions of supplementary cement materials (40% metakaolin and 10% fly ash), with cement mortar and sand trace 1:2:0.60. The cementitious composites were subjected to two types of curing: initial curing in air followed by final curing in water and curing in a pressurized autoclave with CO2, both for 28 days. The treatments applied to the fibers were: washing with hot water; hornification; chemical treatment with sodium hydroxide and hybridization (hot water washing coupling, hornification, sodium hydroxide (NaOH) and application of hydrogen peroxide (H2O2)). As a result, the hybridization treatment increased the mechanical strength of the tucum fiber from 67.20 MPa (untreated fiber) to 318.80 MPa (treated), corroborating the increase in the crystallinity index from 59.84% to 66.73%. The flexural strength of cementitious composites reinforced with 4.5% tucum fibers and razor grass submitted to curing in an autoclave with CO2 was, respectively, 49.61% and 61.75% higher than the reference composite (without fibers) in the water curing. Therefore, both the autoclave cure and the hybridization treatment proved to be viable for its application in composites with vegetable fibers.

Resumo em Inglês:

This works presents free vibration analysis of laminated composite beam problems using Timoshenko beam finite elements formulated in strain gradient notation. The formulation in physically interpretable notation identifies precisely the one parasitic shear term present in the in-plane, two-node, six-degree-of-freedom Timoshenko beam element. The spurious term can be eliminated a-priori of implementation and analysis. An assessment of the deleterious effects of parasitic shear in the computation of natural frequencies and mode shapes of laminated composite beams is performed via convergence studies. Beams with different boundary conditions and lamination schemes are analyzed. Results from models containing parasitic shear and from models corrected for it are compared. It is seen that parasitic shear affects significantly the convergence characteristics of the model as it retards convergence. It is also observed that parasitic shear affects the shapes of vibration modes. After elimination of parasitic shear, convergence of natural frequencies is attained quite rapidly. Such result is most pronounced when computing the fundamental frequency. Further, convergent results are compared to results presented in the literature for accuracy assessment, and very good agreement is found.

Resumo em Inglês:

Abstract Composite materials are employed as an alternative of conventional materials due their attractive properties, such as, environmental resistance and high strength-to-weight ratio. However, a limitation to the application of the composites is the low resistance to high temperatures, due their polymeric matrix. Alternatively, geopolymers are inorganic polymers used mainly instead Ordinary Portland Cement (OPC) in civil engineering. They combine good mechanical properties, corrosion resistance and thermal stability, allowing the use as a thermal barrier. This work regards the combined application of composites and geopolymers through adhesive bonding. The main objective is to evaluate the effect of adhesive type and surface treatment on the adhesion between geopolymer and composite pultruded substrates. Pull off tests were carried out in composite-adhesive-geopolymer sandwich specimens. Different surface treatments were examined: unidirectional abrasion and bidirectional abrasion. Specimens were bonded either with an epoxy or a polyurethane adhesive. The influence of the geopolymer manufacturing process on the performance of the structure was also observed.

Resumo em Inglês:

Abstract Hydrotalcite-type materials, or layered double hydroxide (LDH), are very promising in applying slow-release drug systems through ion exchange and delamination characteristics at acidic pH. In this paper, Zn/Al LDHs in 2:1 and 3:1 molar ratios intercalated with the drug ibuprofen (IBU) were added to chitosan/alginate membranes. In the release tests, an increase in the solubility of the drug in the membranes could be observed, and the percentage of drug release is lower than the percentage of drug release in the free samples. The following release percentages were obtained: 15.48% and 17.5% of the drug in water, and 23.22% and 26.25% of the drug in PBS buffer solution, respectively, for membrane samples containing Zn2Al-IBU and Zn3Al-IBU LDHs. Thus, the synthesized hybrid membranes (chitosan/alginate-LDH-IBU) are promising in the application as resorbable membranes for bone grafting with drug release.

Resumo em Inglês:

Abstract This article seeks to characterize the seed husk fiber of Moringa oleifera and understand its influence when added to a cementitious composite, in terms of mechanical performance. Moringa fibers were chemically and physically tested and were added to a cementitious composite. Specimens were molded for Ultrasonic Speed Pulse test and Uniaxial Compression Strength test, and subsequent observation in SEM. The results show a fiber with high lignin content and high absorption of water. Adding fiber to the composite, the water in the mixture is absorbed, which reduces the formation of hydrated cement compounds over time. Consequently, it results in a composite with low mechanical strength. The fiber/matrix interface analyzed in the micrographs is porous, has microcracks and a high concentration of calcium hydroxide. Despite this, the same lignin content that impairs mechanical strength in this composite is what makes the fiber resistant to weathering. More studies regarding the effectiveness of this quality should be carried out.

Resumo em Inglês:

This work shows the estimation of the volumetric fiber content (Vf) using image processing techniques. The Vf is needed to establish mechanical properties of composites using the rule of mixtures or other more advanced prediction models for the mechanical properties. The proposed method is tested with images acquired for carbon, Kevlar, and fiberglass fiber bundles used in the fiber deposition modeling (FDM) in the additive manufacturing process. The fiber bundles are provided by Markforged ® and are used in the Markforged Two ® printer. Then, different gray threshold and segregation algorithms are convolved with the digitized images to to isolate matrix from fibers such that a separate fiber and matrix area counting can be done and an appropriate Vf can be established. Results obtained with image analysis are close to values reported using technical standards, hence validating the proposed method.

Resumo em Inglês:

Abstract Carbon fiber reinforced plastics (CFRP) offer several advantages in the aeronautical and automotive industry due to their combination of lightweight, high strength, and corrosion resistance. CFRP parts are usually produced in near-net-shape; however, additional machining processes are often required for achieving desired dimensional accuracy and surface finish. Thus, this work evaluates the influence of the cutting parameters in CFRP end milling to generate a better surface finish. The experiment was designed using a three-factor, three-level Box-Behnken considering feed rate (f), axial depth of cut (ap), and cooling conditions (cc) as controllable factors, and roughness parameters (Ra, Rq, Rz, Rt) as response variables (the occurrence of defects was evaluated qualitatively). Results indicated a strong influence of the quadratic effect of axial depth of cut and its interactions with feed rate and cooling condition on the roughness values and a milder but significant influence of the feed rate and cooling conditions. Multivariate analysis returned the optimum level of input parameters (f= 0.21 mm/rev and ap= 0.8 mm with cooled compressed air), resulting in Ra= 1.58 µm, Rq= 1.98 µm, Rz= 9.39 µm, Rt= 13.63 µm. Also, no defects were observed after machining under the optimum conditions.